| 1. |
- Karlsson, Hans O, et al.
(author)
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Density of states of alternant cyclic polyenes (CH)(N) by a direct Lanczos method
- 1997
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In: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 63:3, s. 719-728
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Journal article (other academic/artistic)abstract
- A direct full configuration interaction approach, previously used for studying individual low-lying eigenvalues, is combined with iterative Lanczos calculations, in order to obtain global properties of large Hermitean Hamiltonian matrices. To this effect systematic generation of random start vectors is used to compute statistical approximations to the density of states (DOS). Applications for cyclic polyenes in order to illustrate the viability of the scheme modeled by correlated, tight-binding Pariser-Parr-Pople (PPP) Hamiltonians of increasing complexity are presented. The degree of correlation in the solutions is controlled by the parameter beta. Convergence properties of the DOS for N=10 and N=14 are studied varying the extent of correlation.
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| 2. |
- Fängström, Torbjörn, et al.
(author)
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Density functional study of chlorine-oxygen compounds related to the ClO self-reaction
- 1998
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In: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 66:3, s. 203-217
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Journal article (peer-reviewed)abstract
- Geometrical parameters, vibrational frequencies, relative stabilities, and dissociation energies of the three stable Cl2O2 isomers and the OClO and ClOO radicals were investigated by density functional theory (DFT). The present analysis shows that DFT using hybrid functionals is capable of describing these systems to at least the same degree of accuracy as ab initio methods. The average absolute bond-length deviation of ClClO2,, ClOOCl, and ClO2, from experimental results is 0.024/0.027 Angstrom, with a maximum deviation for the dichlorine peroxide O-O bond equal to 0.072/0.063 Angstrom, for the B3PW91 and B3LYP functionals, respectively. The average absolute bond-angle deviation for the hybrid functionals is 0.8 degrees. Harmonic vibrational frequencies calculated with DFT give for all Cl-O compounds good agreement with experiments. The dissociation energies of ClOOCl, OClO, and ClOO were found to be in good agreement with experiments, the average error being less than 1.2 kcal/mol. The two isomers chloryl chloride (ClClO2,) and dichlorine peroxide (ClOOCl) were found to be approximately 9 kcal/mol more stable than the chlorine chlorite (ClOClO) isomer. The ClOO isomer is predicted to be 3.0 kcal/mol more stable than OClO, in accordance with the experimental value of 4 kcal/mol.
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| 3. |
- Konkoli, Zoran, et al.
(author)
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A new way of analyzing vibrational spectra. II. Comparison of internal mode frequencies
- 1998
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In: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 67:1, s. 11-27
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Journal article (peer-reviewed)abstract
- Adiabatic internal frequencies are compared with c-vector frequencies and intrinsic frequencies. It is shown that c-vector modes are not suitable to characterize molecular fragments φ n since they are not localized in φ n and their definition leads to unreasonable frequency values. Intrinsic frequencies suffer from a strong dependence on the set of internal parameters chosen to describe the geometry of the molecule. Apart from this, they represent averaged frequencies, for which mass effects and electronic effects are not properly separated. Adiabatic frequencies are based on a dynamic principle, separate properly mass effects and electronic effects and do not depend in any way on the set of internal parameters. This is shown for HF/6-31G(d, p) vibrational frequencies of ethene, dichloroethene, benzene, the cyclooctatetraene dication, benzocyclobutadiene, and some of their isotopomers
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| 4. |
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| 5. |
- Luo, Y., et al.
(author)
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Some recent developments of high-order response theory
- 1998
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In: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 70:1, s. 219-239
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Journal article (peer-reviewed)abstract
- We review some recent developments of high-order response theory and illustrate the utility of this theory for a selection of nonlinear properties. © 1998 John Willey & Sons, Inc.
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| 6. |
- Bakker, Albert, et al.
(author)
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Interaction of aluminum(III) with water. An ab initio study
- 1999
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In: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 75:4-5, s. 659-669
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Journal article (peer-reviewed)abstract
- Hydrated Al(3+) ions [Al(H(2)O)(n)](3+), n = 1-6, were examined with ab Initio self-consistent field (SCF) calculations. The relative contributions of two-, three-, and higher-body terms to the total interaction energy for an [Al(H(2)O)(6)](3+) complex were calculated The sum of all three-body contributions amounts to - 30% of the sum of all pair-additive contributions and is opposite in sign. The three-body energy contributions were also derived for two types of [Al(H(2)O)(2)](3+) complexes. in the first type, both water molecules reside in the first hydration shell of Al(3+) and in the second type there is one in the first shell and one in the second. Altogether 15,500 triplets were investigated and analytical two- and three-body potential energy functions were derived via a fitting procedure. (C) 1999 John Wiley & Sons, Inc.
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| 7. |
- Carlsen, Henrik, et al.
(author)
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Quantal trajectories for adiabatic and nonadiabatic regimes of vibronic systems
- 1999
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In: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 75:4-5, s. 409-416
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Journal article (peer-reviewed)abstract
- Exact and averaged nuclear pseudorotational quantal trajectories are compared for Various adiabatic and vibronic states of the Longuet-Higgins E x epsilon Jahn-Teller model. It is argued that the usual averaging over the electronic motion could be understood as being a consequence of ergodicity. The failure of the Born-Oppenheimer factorization to obey the ergodic hypothesis was examined. A quantitative separation of the electronic and nuclear time-scales is, nevertheless, achieved for all regimes. It is shown that the short-time deviations from the global "drift" of the electronic and nuclear motions are perfectly correlated.
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| 8. |
- Bischof, Gerhard, et al.
(author)
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Quantum chemical study of the molecular dynamics of hydrated Li+ and Be2+ cations
- 1997
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In: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 65:5, s. 803-816
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Journal article (peer-reviewed)abstract
- Molecular dynamics simulations of clusters of Li+ and Be2(+) cations with up to 12 water molecules were performed calculating the particle trajectories using Hartree-Fock-derived forces. It was found that independent of the starting configuration tetrahedral clusters are obtained in less than 1 ps. Only for Li+ clusters, transition states with five and three water molecules are found. We discuss the structures of the clusters and the vibrational dynamics of the water molecules and compare them with other ab initio simulations, with simulations using analytical potential functions, and with static calculations. Water-dissociation (hydrogen transfer) reactions which take place at elevated temperatures were investigated. (C) 1997 John Wiley & Sons, Inc.
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| 9. |
- BRINCK, Tore, et al.
(author)
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MOLECULAR-SURFACE ELECTROSTATIC POTENTIALS AND LOCAL IONIZATION ENERGIES OF GROUP-V-VII HYDRIDES AND THEIR ANIONS - RELATIONSHIPS FOR AQUEOUS AND GAS-PHASE ACIDITIES
- 1993
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In: International Journal of Quantum Chemistry. - UNIV NEW ORLEANS,DEPT CHEM,NEW ORLEANS,LA 70148. : JOHN WILEY & SONS INC. - 0020-7608 .- 1097-461X. ; 48:2, s. 73-88
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Journal article (peer-reviewed)abstract
- We have computed ab initio HF/6-31 + G* electrostatic potentials and average local ionization energies on the molecular surfaces of the Group V-VII hydrides and corresponding anions of the first three rows of the periodic table. The surfaces were defined to be specified contours (0.002 or 0.001 au) of the molecular electronic density. The most negative potentials, V(S,min), and lowest ionization energies I(S,min)BAR, were located and determined. Their magnitudes separately satisfy limited correlations with gas-phase protonation enthalpies and aqueous pK(a) values. Our results indicate that V(S,min) and I(S,min)BAR are complementary, the former reflecting electrostatic factors and the latter being related to charge transfer/polarization. More general relationships for protonation enthalpies are obtained when both V(S,min) and I(S,min)BAR are explicitly included. Solution-phase and gas-phase acidities are shown to correlate very well if electrostatic effects are explicitly taken into account. (C) 1993 John Wiley & Sons, Inc.
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| 10. |
- BRINCK, Tore, et al.
(author)
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SURFACE ELECTROSTATIC POTENTIALS OF HALOGENATED METHANES AS INDICATORS OF DIRECTIONAL INTERMOLECULAR INTERACTIONS
- 1992
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In: International Journal of Quantum Chemistry. - UNIV NEW ORLEANS, DEPT CHEM, NEW ORLEANS, LA 70148 USA. : WILEY. - 0020-7608 .- 1097-461X. ; 44:19, s. 57-64
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Journal article (peer-reviewed)abstract
- Using an ab initio self-consistent-field molecular orbital approach, we have computed 6-31G*//STO-3G* electrostatic potentials for CH3F, CF4, CH3Cl, CCl4, CH3Br, and CBr4. It is demonstrated that the potentials along the carbon-halogen bonds of these systems can explain the observed directional preferences of the halogens' intermolecular interactions. Our surface potentials for the chlorinated and brominated molecules favor the observed ''side-on'' and ''head-on'' interactions with electrophiles and nucleophiles, respectively, of C-Cl and C-Br in crystals, whereas the potentials of CH3F and CF4 are indicative of fluorine interacting only with electrophiles, as is found experimentally. The strongly positive potentials at the ends of the chlorines and bromines in CCl4 and CBr4 are consistent with complexes that these form with electron donors, e.g., the pi regions of benzene and p-xylene and the lone pairs of pyridine and tetrahydrofuran.
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