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Träfflista för sökning "L773:1089 7690 srt2:(1990-1999)"

Sökning: L773:1089 7690 > (1990-1999)

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1.
  • Al-Abdalla, A, et al. (författare)
  • Ab initio model potential embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6
  • 1998
  • Ingår i: Journal of Chemical Physics. - : Springer Science and Business Media LLC. - 0021-9606 .- 1089-7690. ; 108, s. 2005-2014
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we present the results of an ab initio model potential (AIMP) embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6; complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on CrCl63- and CrBr63- clusters embedded in ab initio model potential representations of the surrounding lattices Cs2NaYCl6 and Cs2NaYBr6. The experimental structural data are revisited and some new results are found which differ significantly from those available in the literature. The calculated local structure parameters and electronic transition energies which can be compared to experiments are found to be very good; new structural and spectroscopic results are produced which have been neither measured nor calculated, which are complementary to the available ones, and whose quality is expected to be high as well. In particular, the question of the competition of the excited-state absorptions with the potential vibronic laser emission has been adressed: A considerable overlap between the broad E-2(g) –> (2)A(1g) excited-state absorption and (4)A(2g) <– T-4(2g) emission bands is predicted in both materials, which must result in a reduction in the emission efficiency. Finally, it is shown that the quantum mechanical embedding effects due to the fact that the external Cs+, Na+, Y3+, Cl-, and Br- ions are not point charges, are non-negligible; lacking of these effects must be one of the reasons which make previous Density Functional Theory calculations show significantly larger discrepancies with the available experiments.
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2.
  • Andrews, L, et al. (författare)
  • Infrared spectra of cis and trans-(NO)(2)(-) anions in solid argon
  • 1998
  • Ingår i: Journal of Chemical Physics. - 1089-7690 .- 0021-9606. ; 109:1, s. 177-185
  • Tidskriftsartikel (refereegranskat)abstract
    • Laser-ablation of over 20 different metal targets with concurrent 10 K codeposition of Ar/NO mixtures produces metal independent infrared bands at 1589.3 cm−1 due to (NO)2+, a new absorption at 1221.0 cm−1, and a band set at 1300.3, 1222.7, 884.4 cm−1. The latter bands decrease more on annealing than the 1221.0 cm−1 band. Isotopic substitution (14NO,15NO, 15N18O, and mixtures) shows that these new vibrations involve two equivalent N–O oscillators, which identifies two new (NO)2 species. The excellent agreement with frequencies, intensities, and isotopic frequency ratios from density functional theory calculations substantiates assignment of the 1221.0 cm−1 band to trans-(NO)2− and the three band set to cis-(NO)2−. The observation of a weak combination band at 2492.0 cm−1 further substantiates assignment of the two N–O stretching modes in cis-(NO)2−
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3.
  • Antzutkin, Oleg, et al. (författare)
  • High-order multiple quantum excitation in 13C nuclear magnetic resonance spectroscopy of organic solids
  • 1999
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 110:6, s. 2749-2752
  • Tidskriftsartikel (refereegranskat)abstract
    • Excitation and detection of high-order multiple quantum (MQ) coherences among 13C nuclear spins in singly-13C-labeled organic solids is demonstrated experimentally. MQ signals involving at least ten quanta of spin angular momentum are observed in nuclear magnetic resonance (NMR) measurements on polycrystalline L-methionine-methyl-13C and L-alanine-1-13C, using a time-reversible multiple pulse excitation sequence modified specifically for experiments on systems with weak homonuclear dipole-dipole couplings and strong inhomogeneous interactions such as anisotropic chemical shifts. The feasibility of high-order MQ excitation and detection in 13C-labeled organic solids promises to expand significantly the range of applications of MQ NMR as a structural tool, to include such systems as 13C-labeled synthetic polymers and biopolymers.
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4.
  • Antzutkin, Oleg, et al. (författare)
  • Suppression of sidebands in magic-angle-spinning nuclear magnetic resonance : general principles and analytical solutions
  • 1994
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 100:1, s. 130-140
  • Tidskriftsartikel (refereegranskat)abstract
    • Several theoretical and experimental aspects of sideband suppression in the nuclear magnetic resonance (NMR) spectra of rotating solids are considered. The principles of sideband suppression are explored using general symmetry arguments and previous treatments are examined critically. Analytical solutions are given for sideband suppression pulse sequences employing four, five, six, and nine pulses. The analytical solutions for four pulses are complete. Experimental demonstrations are given. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.
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5.
  • Barnes, Leslie A, et al. (författare)
  • Bond length, dipole moment, and harmonic frequency of CO
  • 1993
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 98:5, s. 3972-3977
  • Tidskriftsartikel (refereegranskat)abstract
    • A detailed comparison of some properties of CO is given, at the modified coupled-pair functional, single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), using a variety of basis sets. With very large one-particle basis sets, the CCSD(T) method gives excellent results for the bond distance, dipole moment, and harmonic frequency of CO. In a [6s 5p 4d 3f 2g 1h] + (1s 1p 1d) basis set, the bond distance is about 0.005a0 too large, the dipole moment about 0.005 a.u. too small, and the frequency about 6 cm-1 too small, when compared with experimental results.
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6.
  • Barnes, Leslie A, et al. (författare)
  • Structure and energetics of Cr(CO)6 and Cr(CO)5
  • 1993
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 98:5, s. 3978-3989
  • Tidskriftsartikel (refereegranskat)abstract
    • The geometric structure of Cr(CO)6 is optimized at the modified coupled-pair functional (MCPF), single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), and the force constants for the totally symmetric representation are determined. The geometry of Cr(CO)5 is partially optimized at the MCPF, CCSD, and CCSD(T) levels of theory. Comparison with experimental data shows that the CCSD(T) method gives the best results for the structures and force constants, and that remaining errors are probably due to deficiencies in the one-particle basis sets used for CO. The total binding energies of Cr(CO)6 and Cr(CO)5 are also determined at the MCPF, CCSD, and CCSD(T) levels of theory. The CCSD(T) method gives a much larger total binding energy than either the MCPF or CCSD methods. An analysis of the basis set superposition error (BSSE) at the MCPF level of treatment points out limitations in the one-particle basis used here and in a previous study. Calculations using larger basis sets reduce the BSSE, but the total binding energy of Cr(CO)6 is Still Significantly smaller than the experimental value, although the first CO bond dissociation energy of Cr(CO)6 is well described. An investigation of 3s3p correlation reveals only a small effect. In the largest basis set, the total CO binding energy of Cr(CO)6 is estimated to be 140 kcal/mol at the CCSD(T) level of theory, or about 86% of the experimental value. The remaining discrepancy between the experimental and theoretical value is probably due to limitations in the one-particle basis, rather than limitations in the correlation treatment. In particular, an additional d function and an f function on each C and 0 are needed to obtain quantitative results. This is underscored by the fact that even using a very large primitive set (1042 primitive functions contracted to 300 basis functions), the superposition error for the total binding energy of Cr(CO)6 is 22 kcal/mol at the MCPF level of treatment.
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7.
  • Barta, P., et al. (författare)
  • Temperature evolution of the electronic band structure of the undoped and doped regioregular analog of poly(3‐alkylthiophenes) : A spectroscopic and theoretical study
  • 1994
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 100:2, s. 1731-1741
  • Tidskriftsartikel (refereegranskat)abstract
    • The undoped and ferric chloride p‐doped ‘‘head‐to‐head’’ ‘‘tail‐to‐tail’’ analog of poly(3‐decylthiophene) obtained from substituted bithiophenes, i.e., poly(4,4’didecyl‐2,2’bithiophene)‐PDDBT has been investigated. The samples were studied by means of ultraviolet photoelectron spectroscopy,optical absorption measurements, and polymerbands structure calculations. Experiments were carried out at different temperatures between 100 °C and −180 °C. The results indicate that PDDBT is nonplanar in the undoped state. No temperature effects have been observed in the π‐electron properties. In the doped state, however, lowering of the temperature results in an increase of the system planarity that modifies the electronic band structure. This effect has been shown to be fully reversible with temperature.
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8.
  • Bishop, D. M., et al. (författare)
  • Effects of vibration on the polarizability and the first and second hyperpolarizabilities of HF, HCl, and HBr
  • 1999
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 111:7, s. 3042-3050
  • Tidskriftsartikel (refereegranskat)abstract
    • We report calculations, using electron uncorrelated and correlated wave functions, of the electronic and vibrational properties which pertain to certain nonlinear optical properties for HF, HCl, and HBr. Our main focus is on vibrational effects (zero-point-vibrational averaging and pure vibration). Analysis of the results obtained at various levels of approximation indicates that first-order perturbation theory is generally adequate for finding the zero-point-vibrational-averaging corrections for these molecules and that complete second-order perturbation theory nearly always gives reliable results for the pure vibrational corrections. Attention is drawn to some differences with previously published results for these properties. © 1999 American Institute of Physics.
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9.
  • Björling, Mikael, et al. (författare)
  • On the application of Flory-Huggins and integral equation theories to asymmetric hard sphere mixtures
  • 1999
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 111:15, s. 6884-6889
  • Tidskriftsartikel (refereegranskat)abstract
    • Flory-Huggins (FH) and integral equation theories (IETs) are used to describe the equation of state and the relevant mixing properties of hard sphere binary mixtures in the limit of high size-asymmetry. The results are compared with those obtained from the heuristic equation of state of Mansoori et al. (BMCSL) [J. Chem. Phys. 54, 1523 (1971)] and with the Flory-Huggins scheme of other authors. By choosing a physical recipe for the volume fractions of the two species in the mixture FH theory is shown to be a good approximation to the entropy and the Gibbs free energy of mixing, which shows improvement at high size-asymmetry. In addition, the results of the IETs are found to be in overall quantitative agreement with BMCSL. The implications of our study concerning colloidal systems are discussed.
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10.
  • Björling, Mikael, et al. (författare)
  • Self-consistent-field theory for hard-sphere chains close to hard walls
  • 1992
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 97:9, s. 6890-6900
  • Tidskriftsartikel (refereegranskat)abstract
    • A continuum theory for confined hard-sphere polymers is presented. Starting from fundamental relations and applying defined approximations, a constitutive relation for the conformation probability (analogous to the result in mean-field lattice theories) is developed. The main problem of hard-sphere correlations is attacked by two approximate methods: First, using the Carnahan-Starling equation of state and local volume fractions (CS). Second, by an extension of the lattice theory to spherical components with unequal volumes (LATT). The agreement with Monte Carlo simulations is good for both approximations at low densities, but becomes only qualitative at the higher concentrations. The CS approximation seems to be favored over the LATT approach at the higher concentrations when correlation becomes more important. Both free and grafted chains are treated. The influence of chain length, grafting density, solvent concentration, solvent chain length, and surface curvature on the segment distribution is investigated.
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11.
  • BRINCK, T, et al. (författare)
  • POLARIZABILITY AND VOLUME
  • 1993
  • Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 98:5, s. 4305-4306
  • Tidskriftsartikel (refereegranskat)
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12.
  • Choe, Yong-Kee, et al. (författare)
  • Theoretical study of the electronic ground state of iron(II) porphine. II
  • 1999
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 111:9, s. 3837-3845
  • Tidskriftsartikel (refereegranskat)abstract
    • Ten low-lying electronic states of Fe(II) porphine, (5)A(1g), E-5(g), B-5(2g), (3)A(2g), B-3(2g), E-3(g)(A), E-3(g)(B), (1)A(1g), B-1(2g), and E-1(g) states, are studied with multiconfigurational second-order perturbation (CASPT2) calculations with complete active space self-consistent field (CASSCF) reference functions with larger active space and basis sets. The enlargement of active space and basis sets has no influence on the conclusion of a previous multireference Moller-Plesset perturbation (MRMP) study. The present CASPT2 calculation concludes that the (5)A(1g) state is the ground state. A relativistic correction has been performed by the relativistic scheme of eliminating small components (RESC). For energetics, no significant contribution from the relativistic correction was found. The relative energies and orbital energies are not changed appreciably by the introduction of a relativistic correction. The present result does not agree with all the spectroscopic observations, but is consistent with a magnetic moment study.
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13.
  • Crispin, Xavier, et al. (författare)
  • Chemisorption of acrylonitrile on the Cu(100) surface: A local density functional study
  • 1999
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 111:7, s. 3237-3251
  • Tidskriftsartikel (refereegranskat)abstract
    • The possibility of chemically grafting polyacrylonitrile onto transition metal electrodes via electropolymerization leads to promising applications in the fields of corrosion protection or metal surface functionalization. The initial step of the electrografting mechanism is the adsorption of the acrylonitrile monomer on the metal surface from solution. Here, we investigate theoretically this adsorption process on the copper (100) surface; Density Functional Theory is used in the Local Spin Density approximation to describe the electronic and structural properties of acrylonitrile adsorbed on copper clusters. The chemisorption of acrylonitrile on the copper surface is confirmed experimentally via X-Ray Photoelectron Spectroscopy. The thermodynamic characteristics of the adsorption process are also studied via statistical mechanics. Finally, determining the influence of the copper cluster size on the adsorption of acrylonitrile allows to extrapolate the properties of the acrylonitrile/Cu(100) surface from those of acrylonitrile/copper clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)70231-X].
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14.
  • Dannetun, Per, et al. (författare)
  • Reactions of low work function metals Na, Al, and Ca on α,ω-€diphenyltetradecaheptaene. Implications for metal/polymer interfaces
  • 1994
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 100:9, s. 6765-6771
  • Tidskriftsartikel (refereegranskat)abstract
    • The interactions between different low work function metals aluminium,calcium and sodium, and α,ω‐diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π‐electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic‐like defect for Ca/DP7.
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15.
  • Dannetun, Per, et al. (författare)
  • The chemical and electronic structure of the interface between aluminum and polythiophene semiconductors
  • 1993
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 99:1, s. 664-672
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the chemical nature and the electronic structure of the interface between a low work function metal,aluminum, and a conjugated polymersemiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core‐level x‐ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymerelectronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl‐substituted poly‐3‐octylthiophene and (ii) the α‐sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythiophene with Al atoms is found to modify dramatically the structure of the conjugated backbone, as strong carbon–aluminum bonds are formed in the α positions of the thiophene rings. A large charge transfer takes place from the Al atoms to the polymer chain, and the upper π levels of the polymer are strongly affected. The metallization is contrasted to the doping of conjugated polymers with alkali metals
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16.
  • Engquist, Isak, et al. (författare)
  • Infrared characterization of amorphous and polycrystalline D2O ice on controlled wettability self-assembled alkanethiolate monolayers
  • 1997
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 106:8, s. 3038-3048
  • Tidskriftsartikel (refereegranskat)abstract
    • Infrared reflection-absorption spectroscopy has been used to characterize thin overlayers (1-200 Angstrom) of D2O ice deposited in UHV onto a set of self-assembled alkanethiolate monolayers (SAMs) of controlled wettabilities on gold. The SAMs were prepared from a series of controlled composition, mixed solutions of HS(CH2)(15)CH3 and HS(CH2)(16)OH, making it possible to investigate the whole wettability range from theta approximate to 0 degrees to theta=112 degrees, where theta is the static contact angle with water. Dosing of D2O and infrared measurements were carried out at selected sample temperatures between 82 and 150 K. Experimental spectra of ice overlayers recorded below 100 K on all SAM substrates are in good agreement with simulated reflection-absorption spectra, derived from the optical constants of amorphous ice. This agreement allows accurate film thickness determination. In contrast, lack of correspondence in spectral signature is noted between the spectra of annealed films and simulated polycrystalline (or amorphous) ice spectra. We interpret this discrepancy to suggest that significant substrate-induced differences between thin overlayers and bulk ice persist in the latter case. Spectral indications of ice-substrate interaction are also seen for amorphous ice, and are especially prominent in the case of highly hydrophobic (pure CH3-terminated, theta=112 degrees) substrates. In this case the substrate effect extends up to an average film thickness (150-200 Angstrom) corresponding to similar to 50 ice monolayers, in contrast to highly hydrophilic OH-terminated substrate, where the substrate effects appear to vanish beyond similar to 5 monolayers (15-20 Angstrom average thickness). Annealing of thin ice overlayers (2-3 monolayers) clearly demonstrates a strong correlation between the onset as well as progression of the transition from amorphous to polycrystalline ice and the exact substrate wettability or chemical composition. The data further suggest the existence of metastable intermediate forms, that are neither purely amorphous nor polycrystalline. We discuss these observations in terms of substrate-overlayer interaction. A tentative phase diagram summarizing these results is presented. (C) 1997 American Institute of Physics.
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17.
  • Ferry, Anders (författare)
  • Effects of dynamic spatial disorder on ionic transport properties in polymer electrolytes based on poly(propylene glycol)(4000)
  • 1997
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 107:21, s. 9168-9175
  • Tidskriftsartikel (refereegranskat)abstract
    • The equivalent ionic dc conductivity Λ generally exhibits a dramatic concentration dependence in electrolytic systems based on the host polymer poly(propylene glycol) of molecular weight 4000 (PPG4000). In particular, Λ typically increases rapidly with increasing salt concentration passing through a temperature dependent maximum at high concentration. Prompted by recent reports on a microscopic phase separation occurring in these electrolytes, we here report vibrational spectroscopic, ionic conductivity, and restricted diffusion data for ion-conductors based on PPG4000 complexed with the lithium salts LiCF3SO3 and LiN(CF3SO2)2, in an attempt to resolve seemingly contradictory results concerning ionic transport phenomena in these complexes. We find that the differential salt diffusion coefficient Ds, describing bulk salt motion over long time scales, exhibits a qualitatively similar concentration dependence as Λ. This is contrary to recent 19F pfg-NMR diffusion results for the PPG4000-LiCF3SO3 system which show that the anionic diffusion coefficient decreases monotonically with increasing salt concentration and is inversely proportional to solution shear viscosity. As determined from analyses of characteristic vibrational modes of the [CF3SO3]- and [(CF3SO2)2N]- anions, respectively, the spectroscopic data show very small changes in the distribution of anionic species over the range of electrolyte compositions corresponding to a sharp enhancement of Λ. The results are interpreted in terms of slowly fluctuating salt-rich electrolyte microdomains in equilibrium with salt-depleted polymer regions.
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18.
  • Ferry, Anders, et al. (författare)
  • Ionic interactions and transport in a low-molecular-weight model polymer electrolyte
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 108:17, s. 7426-7433
  • Tidskriftsartikel (refereegranskat)abstract
    • AC impedance, FT-Raman and pulsed field gradient (pfg) NMR measurements have been conducted on solutions of poly(ethylene oxide) dimethyl ether (MW 400) complexed with LiCF3SO3 as a function of temperature and salt concentration. From an analysis of the νS(SO3) and δS(CF3) vibrational band envelopes of the CF3SO3 anion, respectively, the relative concentrations of anions in various chemical environments have been calculated. We find spectroscopic evidence for a redissociation of associated ionic species into spectroscopically "free" anions with increasing salt concentration in dilute solutions. The relative abundance of associated ionic species increases with increasing temperature. Pfg-NMR measurements show that D-(19F) and D+(7Li) are very similar fur all concentrations (i.e., O:Li ≥ 53:1) and temperatures (25-80 oC) investigated. Most notably. the diffusivity of the oligomer solvent, D(1H), is significantly faster than the self-diffusion coefficients of the dissolved ions in all cases, Predicted values for the ionic conductivity were obtained from the NMR diffusivities, using the Nernst-Einstein relation, and compared with those from direct measurement. We find that the calculated values are higher for all concentrations; the discrepancy increases with decreasing salt concentration and increasing temperature. A good correlation is found between the concentration dependence of the ionic redissociation pattern, as determined from the νS(SO3) Raman band envelope, and an increase in equivalent ionic conductivity with increasing salt concentration in dilute solutions (i.e., O:Li ≥ 110:1). We suggest that fluctuating, salt-rich heterogeneities of dissolved ions and polymer segments form at low salt concentrations, and that this may be a general behavior of dilute polymer-salt complexes.
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19.
  • Ferry, Anders, et al. (författare)
  • Spectroscopic studies of luminescent and ionically conducting Eu[N(CF3SO2)2]3-PPG4000 complexes
  • 1998
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 109:7, s. 2921-2928
  • Tidskriftsartikel (refereegranskat)abstract
    • Alternating current impedance, Fourier transform Raman/infrared (IR), and luminescence excitation (continuous and time-resolved) measurements have been conducted on solutions of poly(propylene glycol) (MW 4000) complexed with Eu[N(CF3SO2)2]3 salt, EuTFSI3, along with differential scanning calorimetry (DSC) studies. From observed frequency shifts of characteristic internal anionic vibrational modes (Raman and IR), we conclude that the salt is solvated by the polymer host. The TFSI anions, however, interact extensively with Eu3+ cations at all concentrations investigated. Ion-polymer interactions are manifested as changes in characteristic vibrational modes of the polymer. Continuous and time-resolved site-selective luminescence data give, respectively, evidence for two different types of chemical environments for solvated Eu3+ cations. In particular, the strongly forbidden non-degenerate 5D0 - 7F0 transition exhibits a structured two-component profile in the spectra. DSC data show that the glass transition temperature, Tg, is only marginally affected by the introduction of a relatively high concentration of salt into the host matrix, whereas the resulting polymer-salt complex is of rubbery character, distinctly different from the pure host polymer, which is a viscous liquid at room temperature. The present findings are interpreted in terms of a phase-segregated microstructure. This conjecture is supported by previous studies on PPG4000-based electrolytes indicating microscopic phase anomalies.
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20.
  • Gel'mukhanov, F, et al. (författare)
  • Soft and hard x-ray Raman scattering by oriented symmetrical molecules : Selection rules, interference, and dephasing mechanisms
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 109:12, s. 5060-5069
  • Tidskriftsartikel (refereegranskat)abstract
    • Resonant x-ray Raman scattering in chaotical, partial, and fixed oriented symmetric molecules is analyzed for x-ray photon excitation frequencies in both the soft and the hard x-ray regions. Different dephasing mechanisms and their connection with channel interference and the observation of selection rules are investigated. It is predicted that for harder x-ray energies the scattering cross sections become strongly anisotropic and oscillatory due to channel interference. The orientational dephasing is predicted to be an important coherence-blocking mechanism and can be introduced even by zero-point vibrational or librational motions. The connections between selection rules, symmetry and phase factors of the photon wave function, Bragg conditions and the channel interference show that the selection rules may operate for oriented, surface adsorbed, molecules even in the hard x-ray region. The possibility of using the interference effect for structure determination of adsorbates is discussed.
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21.
  • Guo, J.-H., et al. (författare)
  • Resonant and nonresonant x-ray scattering spectra of some poly(phenylenevinylene)s
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 108:14, s. 5990-5996
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structure of some poly(phenylenevinylene)s have been investigated by resonant and nonresonant x-ray inelastic scattering spectroscopies. The nonresonant as well as all resonant spectra for each polymer demonstrate benzene-like features, indicating a local character of the x-ray emission in which the phenyl ring acts as a building block. Theoretical simulations of x-ray energies and intensities taking the repeat unit as a model molecule of the polymer agree with the experimental spectra fairly well. The edges of the occupied bands have been identified in the nonresonant spectra of each polymer. By subtracting the emission energy of the highest occupied molecular orbital in the nonresonant spectrum from the core excitation energy in the resonant spectrum an alternative way to determine the optical band gap is obtained. As for free benzene the outer π band in the polymer spectra show a depletion of the emission going from the nonresonant to the resonant x-ray emission spectra. It is demonstrated that this transition, which is strictly symmetry forbidden for free benzene, becomes effectively forbidden in the polymer case as a result of strong interference effects, and it is argued that this is the general case for resonant x-ray emission of conjugated polymers as far as the frozen orbital approximation holds.
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22.
  • Halle, B., et al. (författare)
  • Microemulsions as macroelectrolytes
  • 1995
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 103:4, s. 1655-1668
  • Tidskriftsartikel (refereegranskat)abstract
    • Water-in-oil microemulsions, composed of discrete aqueous droplets dispersed in a continuous oil medium, constitute a special class of electrolyte solutions. Such macroelectrolytes are analogous to conventional electrolyte solutions in most respects, with the notable difference that, in a microemulsion, the ionic (droplet) charge is not fixed but depends on the droplet interactions. Describing the microemulsion as a primitive-model electrolyte mixture with ions of variable charge and evaluating the statistical mechanics within the mean-spherical approximation (MSA), we construct a self-consistent theory of charge fluctuations and droplet interactions in ionic microemulsions. The droplet charge distribution is calculated as a function of the size, shape, polydispersity, and volume fraction of the droplets. We argue that the net droplet charges can have a decisive influence on microemulsion structure, especially at the higher volume fractions where clustering and spinodal decomposition are observed. At lower volume fractions, where the MSA treatment should be quantitatively accurate, the Coulomb interaction between charged droplets has no effect on the structure factor deduced from scattering data.
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23.
  • Hellsing, B, et al. (författare)
  • Photoinduced desorption of potassium atoms from a two dimensional overlayer on graphite
  • 1997
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 106:3, s. 982-1002
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an experimental and theoretical investigation of K atom desorption from the basal plane of graphite at 83 K induced by low energy photons (3-6 eV). The 2D potassium overlayer is characterized by low energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and work function measurements. At monolayer coverage (5.2 x 10(14) atoms cm(-2)), the dependence of the cross section on photon energy has a threshold at (h) over bar omega approximate to 3.0 eV and rises up to a maximum of 1.8 +/- 0.4 x 10(-20) cm(2) at 4.8 eV. The coverage dependence of the photoyield reflects the existence of two phases of adsorbed K, dilute ionized photo-active and close-packed photo-neutral, respectively. The observed photodesorption is a single-photon, nonthermal event, consistent with a substrate-mediated mechanism. The desorption results from attachment of optically excited hot electrons to the empty 4s state of ionized potassium. The theory predicts in this case a Gaussian line shape of the photoyield vs photon energy. Fitting the model parameters to the experimental data, we determine (i) the energy and slope of the excited state potential energy curve, and (ii) the position and width of the potassium-induced 4s resonance. The present findings combined with other available data for potassium on graphite are used to construct 1D potential energy curves along the surface normal for K+ and K-0. The calculated cross sections for s- and p-polarized Light are in qualitative agreement with the measurements. (C) 1997 American Institute of Physics.
  •  
24.
  • HERMANSSON, KERSTI (författare)
  • ABINITIO CALCULATIONS OF THE FUNDAMENTAL OH FREQUENCY OF BOUND OH- IONS
  • 1991
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 95:5, s. 3578-3588
  • Tidskriftsartikel (refereegranskat)abstract
    • In contrast to the OH stretching frequencies of bound H2O molecules, which are always found at lower wave numbers compared to the free molecule, the experimentally determined frequency of the OH- ion can be either lower or higher than the free-ion value. Optimized geometries and fundamental stretching frequency of OH- have been calculated here by ab initio methods at the Hartree-Fock and second-order Moller-Plesset levels for a number of cation-OH-, HOH...OH-, cation-OH-.q-, and cation-OH-.OH2 complexes for Li+, Mg2+, and Al3+. The importance of electrostatic effects on the OH- frequency has been assessed by comparison with calculations of different point-charge and homogenous-field OH- systems. As long as the interaction is not dominated by electronic overlap, the frequency shift is found to be largely determined by electrostatic forces: with increasing field strength the OH- frequency rises to a maximum and then decreases. The OH- dipole moment and Mulliken charges vary monotonically with the field strength, whereas the equilibrium OH distance goes through a minimum and the bond electron density through a maximum. In strongly polarizing fields, such as in the optimized Al3+.OH- and Mg2+.OH-...OH2 systems, the OH- frequency falls below the free-ion value. Ar experimentally observed frequency downshift for an OH- ion in the condensed phase cannot be used as a criterion for the existence of H bonding. The OH- ion acts as an H-bond donor only when strongly polarized by a neighbor on its oxygen side.
  •  
25.
  • HERMANSSON, KERSTI (författare)
  • ELECTRIC-FIELD EFFECTS ON THE OH VIBRATIONAL FREQUENCY AND INFRARED-ABSORPTION INTENSITY FOR WATER
  • 1993
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 99:2, s. 861-868
  • Tidskriftsartikel (refereegranskat)abstract
    • The variations of the anharmonic OH frequency and the infrared absorption intensity with field strength have been calculated for the uncoupled OH stretching vibration of a water molecule in a static, homogeneous electric field using ab initio methods at the MP4 level with a nearly saturated basis set. The OH frequency is found to be virtually independent of the field components perpendicular to the vibrating OH bond. For the parallel component, the frequency vs field curve is close to quadratic, with a maximum for a slightly negative (directed from H to 0) field strength. The external field perturbation, defined as V(ext)(E(parallel-to), r(OH)) = V(tot)(E(parallel-to), r(OH)) - V(free)(r(OH)), is found to be closely linear in r(OH), except when the field strength Ell is both large and negative. The linear external force constant is almost perfectly accounted for by the sum of two terms, -E(parallel-to).dmu(parallel-to)free/dr(OH) and -1/2.E(parallel-to).partial derivative mu(parallel-to)induced/partial derivative r(OH). These derivatives are quite insensitive to the choice of basis-set. The partial derivative mu(parallel-to)induced/partial derivative r(OH) derivative is approximately proportional to E(parallel-to), and gives rise to the arclike shape of the frequency vs field curve. The frequency maximum occurs where partial derivative mu(parallel-to)tot/partial derivative r(OH) almost-equal-to 0. It is the sign of dmu(parallel-to)free/dr(OH) which determines that the frequency maximum occurs at a negative field strength for water (but at a positive field strength for OH-, for example), i.e., that a frequency red-shift (blue-shift for OH-) occurs when the molecule is bound. The linear relationship between the infrared absorption intensity and frequency of the water OH vibration is derived.
  •  
26.
  • HERMANSSON, KERSTI, et al. (författare)
  • THE OH VIBRATIONAL-SPECTRUM OF LIQUID WATER FROM COMBINED ABINITIO AND MONTE-CARLO CALCULATIONS
  • 1991
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 95:10, s. 7486-7496
  • Tidskriftsartikel (refereegranskat)abstract
    • The infrared vibrational OH stretching spectrum of isotopically isolated HDO molecules in liquid water has been calculated by ab initio methods at the MP2 level for a number of geometrical configurations taken from a Monte Carlo simulation. Each vibrating water molecule with its environment was described by a pentamer supermolecule, surrounded by a large number of point charges representing polarized water molecules. The anharmonic stretching potentials (MP2 force constants up to fifth order) for 40 uncoupled OH water vibrators were calculated. The average computed r(e) distance found for liquid water is 0.01 angstrom longer than the free-water value. The frequencies were obtained by solving the one-dimensional Schrodinger equation variationally for each OH potential curve. Using the squared dipole moment derivatives, which vary by a factor of 7 over the frequency band, the density-of-states histograms were converted to intensities. The resulting computed average frequency downshift is approximately 260 cm-1, compared to approximately 310 cm-1 (experimental), with a bandwidth in good agreement with experiment. The remaining discrepancy between theoretical and experimental frequency shifts is to a large part due to the charge transfer within the water clusters. This charge transfer gives rise to an electrostatic field which, at the site of the vibrating H atom, counteracts the downshift induced by the other environmental effects. The agreement between experiment and theory is very satisfactory when this charge transfer effect is corrected for or when point-charge embedded heptamer clusters are considered.
  •  
27.
  • Inaba, Akira, et al. (författare)
  • Lattice vibrations and thermodynamic stability of polymerized C60 deduced from heat capacities
  • 1999
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics. - 0021-9606 .- 1089-7690. ; 110:24, s. 12226-12232
  • Tidskriftsartikel (refereegranskat)abstract
    • Heat capacities of 1D and 2D pressure polymerized C60 as well as the thermally depolymerized C60 have been measured at temperatures between 4 and 350 K and the results analyzed for the lattice vibrations and the thermodynamic stability. It was found from the low-temperature heat capacity (T < 100 K) that on polymerization (1) the lattice vibrations, both translational and rotational, stiffen substantially, (2) an anisotropic nature emerges in response to the lower dimensionality, and (3) an anharmonicity still remains as in pristine C60. The normal C60 has an excess entropy of 67.8 and 99.4 J K – 1 mol – 1 at 300 K relative to the 1D and 2D polymerized C60, respectively. The thermodynamic stability is considered with two possible phase diagrams.
  •  
28.
  • Irbäck, A. (författare)
  • Hybrid Monte Carlo simulation of polymer chains
  • 1994
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 101:2, s. 1661-1667
  • Tidskriftsartikel (refereegranskat)abstract
    • We develop the hybrid Monte Carlo method for simulations of single off-lattice polymer chains. We discuss implementation and choice of simulation parameters in some detail. The performance of the algorithm is tested on models for homopolymers with short- or long-range self-repulsion, using chains with 16≤N≤512 monomers. Without excessive fine tuning, we find that the computational cost grows as N2+z′ with 0.641N∼Nν(ln N)-α.
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29.
  • Irbäck, Anders, et al. (författare)
  • Local interactions and protein folding : A model study on the square and triangular lattices
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 108:5, s. 2245-2250
  • Tidskriftsartikel (refereegranskat)abstract
    • We study a simple heteropolymer model containing sequence-independent local interactions on both square and triangular lattices. Sticking to a two-letter code, we investigate the model for varying strength κ of the local interactions; κ=0 corresponds to the well-known HP model [K. F. Lau and K. A. Dill, Macromolecules 22, 3986 (1989)]. By exhaustive enumerations for short chains, we obtain all structures which act as a unique and pronounced energy minimum for at least one sequence. We find that the number of such designable structures depends strongly on κ. Also, we find that the number of designable structures can differ widely for the two lattices at a given κ. This is the case, for example, at κ=0, which implies that the HP model exhibits different behavior on the two lattices. Our findings clearly show that sequence-independent local properties of the chains can play an important rote in the formation of unique minimum energy structures.
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30.
  • Irbäck, Anders, et al. (författare)
  • Local interactions and protein folding : A three-dimensional off-lattice approach
  • 1997
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 107:1, s. 273-282
  • Tidskriftsartikel (refereegranskat)abstract
    • The thermodynamic behavior of a three-dimensional off-lattice model for protein folding is probed. The model has only two types of residues, hydrophobia and hydrophilic. In absence of local interactions, native structure formation does not occur for the temperatures considered. By including sequence independent local interactions, which qualitatively reproduce local properties of functional proteins, the dominance of a native state for many sequences is observed. As in lattice model approaches, folding takes place by gradual compactification, followed by a sequence dependent folding transition. Our results differ from lattice approaches in that bimodal energy distributions are not observed and that high folding temperatures are accompanied by relatively low temperatures for the peak of the specific heat. Also, in contrast to earlier studies using lattice models, our results convincingly demonstrate that one does not need more than two types of residues to generate sequences with good thermodynamic folding properties in three dimensions.
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31.
  • Irbäck, Anders, et al. (författare)
  • Monte Carlo study of the phase structure of compact polymer chains
  • 1999
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 110:24, s. 12256-12262
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the phase behavior of single homopolymers in a simple hydrophobic/hydrophilic off-lattice model with sequence independent local interactions. The specific heat is, not unexpectedly, found to exhibit a pronounced peak well below the collapse temperature, signalling a possible low-temperature phase transition. The system size dependence at this maximum is investigated both with and without the local interactions, using chains with up to 50 monomers. The size dependence is found to be weak. The specific heat itself seems not to diverge. The homopolymer results are compared with those for two nonuniform sequences. Our calculations are performed using the methods of simulated and parallel tempering. The performances of these algorithms are discussed, based on careful tests for a small system.
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32.
  • Johansson, N., et al. (författare)
  • Electronic structure of tris(8-hydroxyquinoline) aluminum thin films in the pristine and reduced states
  • 1999
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 111:5, s. 2157-2163
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structure of tris(8-hydroxyquinoline) aluminum (Alq(3)) has been studied in the pristine molecular solid state as well as upon interaction (doping) with potassium and lithium. We discuss the results of a joint theoretical and experimental investigation, based on a combination of x-ray and ultraviolet photoelectron spectroscopies with quantum-chemical calculations at the density functional theory level. Upon doping, each electron transferred from an alkali metal atom is stored on one of the three ligands of the Alq(3) molecule, resulting in a new spectral feature (peak) in the valence band that evolves uniformly when going from a doping level of one to three metal atoms per Alq(3) molecule. (C) 1999 American Institute of Physics. [S0021-9606(99)50628-4].
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33.
  • Jonsson, D., et al. (författare)
  • Cubic response functions in the muiticonfiguration self-consistent field approximation
  • 1996
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 105:15, s. 6401-6419
  • Tidskriftsartikel (refereegranskat)abstract
    • Cubic response functions and various related residues are derived for multiconfigurational self-consistent field reference states. Compact computable expressions are given which render the possibility to simultaneously consider several types of electric and magnetic response properties referring to ground and excited states and to consider multiphoton spectra between ground and excited states or between different excited states. The perturbational expression for the excited-ground state difference in a frequency dependent property can be identified from a double residue of the cubic response function, and from one single reference wave function the property of interest can in principle be given for the whole manifold of excited states. With the present theory calculations for excited state properties can thus be conceived in two ways; by cubic response calculations with the ground state as the reference state and by linear response calculations using the particular excited state as reference state. The two approaches are identical at the full configuration interaction limit, while at the Hartree-Fock level only the former is possible. The applications include calculations of frequency dependent hyperpolarizabilities and excited state polarizabilities of lithium hydride and carbon monoxide and correlated Cotton-Mouton constants for hydrogen fluoride. The results for LiH and CO indicate that the cubic response function approach is quite rewarding already at the self-consistent field level, giving excited state, polarizabilities comparable to those obtained from excited state multiconfiguration self-consistent field calculations. At this level the cubic response function calculation scales as ordinary self-consistent field calculations with applicability to large systems. The role of the presently proposed method for quantum chemistry of excited states is briefly discussed. © 1996 American Institute of Physics.
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34.
  • Jonsson, D., et al. (författare)
  • Electric and magnetic properties of fullerenes
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 109:2, s. 572-577
  • Tidskriftsartikel (refereegranskat)abstract
    • We present fully analytical ab initio calculations of the electric polarizability, the second hyperpolarizability, and the magnetizability of the fullerenes C 70 and C 84 at the self-consistent field level of theory using large basis sets and - in the case of the magnetizability - London atomic orbitals in order to obtain gauge-origin independent results. These calculations are the first ab initio studies of such properties for C 70 and C 84, and all results are expected to be of near Hartree-Fock limit quality. By comparison with similar results reported earlier for C 60, valuable insight into the electronic structure of the fullerenes is obtained. © 1998 American Institute of Physics.
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35.
  • Jonsson, D., et al. (författare)
  • Excited state polarizabilities in solution obtained by cubic response theory : Calculations on para-, ortho-, and meta-nitroaniline
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 109:15, s. 6351-6357
  • Tidskriftsartikel (refereegranskat)abstract
    • We show that response theory implemented with a self-consistent reaction field theory model is a viable approach to simulate excited state polarizabilities of molecules in solution. The excited state polarizabilities are in this approach given by the double residue of the ground state cubic response functions, accounting for both equilibrium and nonequilibrium contributions to the interaction with the outer medium. The effects of the inertial polarization of the solvent on the polarizabilities of the solutes are shown to strongly depend on the solvent configuration, whether the solvent is in equilibrium or in nonequilibrium with the charge distribution of the investigated compound. The inertial polarization vector in the nonequilibrium solvent configuration represents the equilibrated solvent configuration when solvating the ground state of the solute. This inertial polarization vector is not in equilibrium with any of the excited states and therefore one observes a rather different behavior between nonequilibrium and equilibrium solvent descriptions of the solute. Illustrative calculations are presented for para-, meta-, and ortho-nitroanilines in gas and solution phases. Results have been compared with experimental data where available. © 1998 American Institute of Physics.
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36.
  • Jonsson, D., et al. (författare)
  • Response theory for static and dynamic polarizabilities of excited states
  • 1996
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 105:2, s. 581-587
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate a fully analytical method for calculations of excited state static and dynamic polarizabilities. From the double residues of a single reference state cubic response function all excited state polarizabilities can be determined. The method forms also a basis for calculations of excited state hyperpolarizabilities through the finite-field approach. A demonstration is given for the first excited state polarizabilities and hyperpolarizabilities of long polyene chains. © 1996 American Institute of Physics.
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37.
  • Jonsson, D., et al. (författare)
  • The hypermagnetizability of molecular oxygen
  • 1997
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 106:20, s. 8552-8563
  • Tidskriftsartikel (refereegranskat)abstract
    • The hypermagnetizability and the hypermagnetizability anisotropy of the oxygen molecule are computed using cubic response theory applied to multi-configurational self-consistent field wave functions. The effects of basis set, electron correlation, frequency dispersion, zero point vibrational averaging and pure vibrational contributions are discussed. The result for the anisotropy (Δη = + 2.65 a.u. at λ = 632.8 nm), even taking into account possible limitations in the treatment of electron correlation and in the incompleteness of the basis set, maintains a different sign and is more than two orders of magnitude smaller than the experimental values published in the literature. Possible reasons for this large discrepancy are discussed. © 1997 American Institute of Physics.
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38.
  • Karlsson, Hans O (författare)
  • Accurate resonances and effective absorption of flux using smooth exterior scaling
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 109:21, s. 9366-9371
  • Tidskriftsartikel (refereegranskat)abstract
    • A general coordinate transformation is used to derive smooth exterior scaling (SES). Different complex paths are discussed and it is also shown how to derive a complex absorbing potential (CAP) from the SES. Accurate resonance values are computed both for short range and long range potentials. It is shown that the SES absorbs outgoing flux very effectively. The approximation of not scaling the potential and its relation to CAPs is discussed. It is emphasized that the SES can be implemented as easy as CAPs for grid methods.
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39.
  • Karlsson, Hans O (författare)
  • Exterior complex dilation for grid methods : Application to the cumulative reaction probability
  • 1998
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 108:10, s. 3849-3853
  • Tidskriftsartikel (refereegranskat)abstract
    • Exterior complex dilation (ECD) is introduced for the discrete variable representation (DVR) via a general coordinate mapping. The procedure leads to a computationally efficient and easily implemented approach for imposing outgoing boundary conditions, comparable with absorbing potentials in terms of minimizing the grid and parameters used. ECD relies on a rigorous mathematical framework in contrast to absorbing potentials. The DVR-ECD approach is illustrated by computation of the cumulative reaction probability for the H+H-2 reaction in one and two dimensions.
  •  
40.
  • Karlsson, Hans O (författare)
  • The quasi‐minimal residual algorithm applied to complex symmetric linear systems in quantum reactive scattering
  • 1995
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 103:12, s. 4914-4919
  • Tidskriftsartikel (refereegranskat)abstract
    • The solution of systems of linear equations Ax=b with complex symmetric coefficient matrix A of size N, typically appearing in quantum-reactive scattering problems, is discussed. The quasiminimal residual (QMR) method is introduced to solve the complex symmetric linear system and is compared to the generalized minimal residual (GMRES) method. The methods are applied to two different chemical problems: the initial state-selected reaction probability for the H-2+OH-->H +H2O reaction, and the cumulative reaction probability for the isomerization of ketene, both with N>10(4). It is shown that the QMR method behaves more favorably, i.e., converges faster, than the GMRES for large N, especially when high accuracy is needed
  •  
41.
  • Karlsson, Hans O, et al. (författare)
  • Theoretical calculation of photodetachment intensities for H3O-
  • 1996
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 105:13, s. 5387-5396
  • Tidskriftsartikel (refereegranskat)abstract
    • We have calculated total and arrangement-selected photodetachment intensities for the H3O- anion (and its deuterated form, D3O-) using a Green's function in a discrete variable representation with absorbing boundary conditions. A multiply-shifted quasiminimal residual method is used to obtain the Green's function for many energies at once. We present spectra obtained by explicitly treating two and four degrees of freedom. Comparison with experiment indicates that the bending angles in the anion and neutral are more similar than in the current potential energy surfaces. The calculated spectra are also consistent with the suggestion that the barrier should be ''earlier.'
  •  
42.
  • KARLSSON, HO (författare)
  • THE QUASI-MINIMAL RESIDUAL ALGORITHM APPLIED TO COMPLEX SYMMETRICAL LINEAR-SYSTEMS IN QUANTUM REACTIVE SCATTERING
  • 1995
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 103:12, s. 4914-4919
  • Tidskriftsartikel (refereegranskat)abstract
    • The solution of systems of linear equations Ax=b with complex symmetric coefficient matrix A of size N, typically appearing in quantum-reactive scattering problems, is discussed. The quasiminimal residual (QMR) method is introduced to solve the complex symmetric linear system and is compared to the generalized minimal residual (GMRES) method. The methods are applied to two different chemical problems: the initial state-selected reaction probability for the H-2+OH-->H +H2O reaction, and the cumulative reaction probability for the isomerization of ketene, both with N>10(4). It is shown that the QMR method behaves more favorably, i.e., converges faster, than the GMRES for large N, especially when high accuracy is needed. (C) 1995 American Institute of Physics.
  •  
43.
  • KNUTS, SÖREN, et al. (författare)
  • AN AB-INITIO STUDY OF THE OH STRETCHING FREQUENCIES IN ICE-II, ICE-VIII, AND ICE-IX
  • 1993
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 99:4, s. 2917-2928
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio studies of the uncoupled, anharmonic OH and OD stretching frequency shifts in the three proton-ordered ice phases known, ice II, ice VIII, and ice IX, are presented. The ice structures are simulated by (H2O)5 supermolecules surrounded by point charges representing the correct crystal potentials. The calculations include electron correlation at the MP2 (DZP) level. For the eight different OH (OD) vibrators studied, the crystal environment leads to a downshift of the anharmonic OD frequency in the range 195-265 cm-1, in good agreement with experimental values (222-281 cm-1) when corrections are made for the limited supermolecular size (approximately - 45 cm-1), and, for ice VIII, also for the effects of the nonhydrogen bonded network (approximately + 75 cm- 1). Also the agreement between absolute experimental and theoretical OD frequencies is good when errors due to basis set limitation (approximately - 75 cm-1 ) are taken into account. The calculations suggest a reassignment of two of the experimental OD bands in ice II and all three experimental OD bands in ice IX. Calculations for charge-embedded (H2O)9 and (H2O)13 ice clusters show that at least a nonamer is needed to avoid boundary effects from the size of the supermolecule. Theoretical correlation curves between H-bond parameters-R (O ... 0), nu(OH), r(e)(OH), and infrared absorption intensity-are presented for the three ice phases and are compared to liquid water computations.
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44.
  •  
45.
  •  
46.
  • Lindh, Roland, 1958-, et al. (författare)
  • Accurate abinitio calculations of the quadrupole-moment of acetylene : a combined study of basis set, correlation, and vibrational effects
  • 1991
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 94:6, s. 4356-4368
  • Tidskriftsartikel (refereegranskat)abstract
    • The quadrupole moment of acetylene has been studied at the multiconfiguration self-consistent field (MCSCF) and multireference single and double configuration interaction (MRSDCI) level of theory. At the MCSCF level the pi-CI complete active space SCF (CASSCF) and the valence-CI CASSCF were employed. The subsequent MRSDCI calculations were continued until the reference space included all configuration state functions (CSFs) of the MCSCF wave function with a coefficient larger than 0.01 [MRSDCI(0.01)]. The higher level basis sets in this study were all based on van Duijneveldt’s C(13s 8p) and H(6s) and extensions of that basis set. The study shows in a consistent way that both the one- and n-particle spaces are saturated at the highest level of theory. The study has revealed that in addition to the well known increase of the quadrupole moment due to the inclusion of polarizing functions in the basis (typically 0.20 a.u.), the inclusion of electronic correlation in the model wave function as well as vibrational corrections will decrease the quadrupole moment significantly more, -0.66, -0.49, and -0.36 a.u., for the correlation correction and zero-point correction for HCCH and DCCD, respectively. The most accurate computations predict the quadrupole moment of HCCH, including zero-point correction, to be 4.29 +/- 0.12 a.u., which discriminates the experimental estimates of 4.03 +/- 0.30, 4.28 +/- 0.30, and 4.57 +/- 0.30 a.u. (the first being the favored value). The quadrupole moment of DCCD is computed to 4.42 +/- 0.10 a.u. In the study it was observed that in contradiction to previous experiences the use of the model equilibrium geometries rather than the experimental geometry gives a smoother convergence as the level of theory is increased. The effects of basis set quality and electron correlation on the quadrupole moment are studied in detail. These effects are analyzed with reference to the redistribution of the electronic charge.
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47.
  • Lindh, Roland, 1958-, et al. (författare)
  • The energy separation between the classical and nonclassical isomers of protonated acetylene : An extensive study in one‐ and n‐particle space
  • 1991
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 94:12, s. 8008-8014
  • Tidskriftsartikel (refereegranskat)abstract
    • The energy separation between the classical and nonclassical forms of protonated acetylene has been reinvestigated in light of the recent experimentally deduced lower bound to this value of 6.0 kcal/mol. The objective of the present study is to use state-of-the-art ab initio quantum mechanical methods to establish this energy difference to within chemical accuracy (i.e., about 1 kcal/mol). The one-particle basis sets include up to g-type functions and the electron correlation methods include single and double excitation coupled-cluster (CCSD), the CCSD(T) extension, multireference configuration interaction, and the averaged coupled-pair functional methods. A correction for zero-point vibrational energies has also been included, yielding a best estimate for the energy difference between the classical and nonclassical forms of 3.7 +/- 1.3 kcal/mol.
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48.
  • Lindh, Roland, 1958-, et al. (författare)
  • The fraternal twins of quartet O+4
  • 1994
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 100:1, s. 224-237
  • Tidskriftsartikel (refereegranskat)abstract
    • Eleven stationary geometries of quartet O4+ have been studied by ab initio methods. The geometries were optimized at the complete active space self-consistent field (CASSCF) level of theory and the energies were calculated by the multiconfigurational second order pertubation method (CASPT2), using double-zeta plus polarization (DZP), triple-zeta plus double polarization (TZ2P), average atomic natural orbital (ANO) [5s4p2d] and average ANO [6s5p3d2f] basis sets. The rectangular and trans-planar structures are found to be the most stable, with an energy barrier to conversion between the two at the threshold of dissociation. Both have a delocalized hole and are stable relative to separated O2 and O2+ by 11.0 and 11. 5 kcal/mol for the rectangular and the trans-planar structure, respectively, compared with the experimentally deduced energy in the range of 9.2 to 10.8 kcal/mol. The adiabatic ionization potentials of O4 and O2 are computed to be 11.67 and 12.21 eV, while experimental values are 11.66 and 12.07 eV, respectively. The vibrational frequencies have been computed for all degrees of freedom at the CASSCF level of theory. Symmetry breaking is found to be a particular problem in the computation of the antisymmetric stretch frequency for the delocalized structures at the CASSCF level of theory. Attempts to rectify these problems using the restricted active space self-consistent field (RASSCF) method leads to additional difficulties, but further analysis yields insight into the symmetry breaking and problems with earlier calculations. Finally, a nonorthogonal configuration interaction (CI) calculation based on the interaction of localized CASSCF wave functions using the complete active space state interation (CASSI) method leads to a balanced treatment of the antisymmetric stretch which is free from symmetry breaking. The study explains the four most prominent absorption frequencies observed in the partially unassigned IR spectrum of 04+ isolated in solid neon as the antisymmetric OO stretch, and the combination band of the symmetric and antisymmetric OO stretch of both the rectangular and trans-planar structures.
  •  
49.
  • Lindh, Roland, 1958-, et al. (författare)
  • The reduced multiplication scheme of the Rys quadrature and new recurrence relations for auxiliary function based two‐electron integral evaluation
  • 1991
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 95:8, s. 5889-5897
  • Tidskriftsartikel (refereegranskat)abstract
    • The reduced multiplication scheme of the Rys quadrature is presented. The method is based on new ways in which the Rys quadrature can be developed if it is implemented together with the transfer equation applied to the contracted integrals. In parallel to the new scheme of the Rys quadrature improvements are suggested to the auxiliary function based algorithms. The two new methods have very favorable theoretical floating point operation (FLOP) counts as compared to other methods. It is noted that the only significant difference in performance of the two new methods is due to the vectorizability of the presented algorithms. In order to exhibit this, both methods were implemented in the integral program SEWARD. Timings are presented for comparisons with other implementations. Finally, it is demonstrated how the transfer equation in connection with the use of spherical harmonic Gaussians offers a very attractive path to compute the two-electron integrals of such basis functions. It is demonstrated both theoretically and with actual performance that the use of spherical harmonic Gaussians offers a clear advantage over the traditional evaluation of the two-electron integrals in the Cartesian Gaussian basis.
  •  
50.
  • Lugez, C.L., et al. (författare)
  • Infrared spectra of (NO)(2)(+), (NO)(2)(-), and (NO)(3)(-) trapped in solid neon
  • 1999
  • Ingår i: Journal of Chemical Physics. - 1089-7690 .- 0021-9606. ; 110:21, s. 10345-10358
  • Tidskriftsartikel (refereegranskat)abstract
    • New studies of the infrared spectra of the products which result on codeposition at approximately 5 K of a Ne:NO sample with Ne atoms that have been excited in a microwave discharge have led to new and revised assignments for several ionic species. The appearance of the ν1 absorption of ONNO+ for several new species with asymmetric isotopic substitution, but for no symmetrically substituted species, confirms the trans ground-state configuration for ONNO+. The behavior of a neon-matrix product absorption at 1227.5 cm−1 parallels that of an argon-matrix absorption at 1221.0 cm−1 which has recently been assigned to trans-ONNO−. The identity of the carrier of a product absorption at 1424.1 cm−1, contributed by a vibration of two symmetrically equivalent NO groups, has not been definitively established. This absorption exhibits complex photodestruction behavior. Three absorptions are assigned to cis,cis-(NO)3−, which has C2v symmetry, with the aid of density functional calculations of the isotopic substitution pattern for the vibrational fundamentals of this species. Similar calculations of the isotopic substitution patterns for other structures result in poor agreement with the experiments. Photodestruction of cis,cis-(NO)3− trapped in solid neon yields the N2O⋯NO2− complex.
  •  
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