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1.
  • Karlsson, Hans O, et al. (författare)
  • Density of states of alternant cyclic polyenes (CH)(N) by a direct Lanczos method
  • 1997
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 63:3, s. 719-728
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)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. (författare)
  • Density functional study of chlorine-oxygen compounds related to the ClO self-reaction
  • 1998
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 66:3, s. 203-217
  • Tidskriftsartikel (refereegranskat)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.
  • Himo, Fahmi, et al. (författare)
  • Substituent effects on OH bond strength and hyperfine properties of phenol, as model for modified tyrosyl radicals in proteins
  • 2000
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 76:6, s. 714-723
  • Tidskriftsartikel (refereegranskat)abstract
    • Density functional theory is used to investigate the effects of a variety of substituents (CH3, OH, OCH3, SH, SCH3, NH2, NMe2, NO2, F, Cl, CN, and imidazole) on the phenol O-H bond dissociation energy (BDE) and phenoxyl radical hyperfine properties. Substitutions are made at the ortho position to model modified tyrosine residues found in enzymes. The calculations show that besides the electronic effects of the substituents, intramolecular hydrogen bonds between OH and the substituents will contribute considerably to stabilize the parent species. Substituent effects on anisole O-Me bond strengths can thus not correctly describe the effects on ortho-substituted phenol O-H bond strengths, as previously proposed. This fact is supported by a series of calculations on o-substituted anisoles. The odd-alternant spin pattern of the phenoxyl radical is conserved for most of the substitutions. In particular, it is predicted that the cysteine crosslink to tyrosine, present in the radical enzyme galactose oxidase, and the histidine crosslink, present in cytochrome-c oxidase, will only have minor effects on the BDE and the radical hyperfine coupling constants and spin distribution of the tyrosyl radical.
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4.
  • Konkoli, Zoran, et al. (författare)
  • A new way of analyzing vibrational spectra. II. Comparison of internal mode frequencies
  • 1998
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 67:1, s. 11-27
  • Tidskriftsartikel (refereegranskat)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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5.
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6.
  • Luo, Y., et al. (författare)
  • Some recent developments of high-order response theory
  • 1998
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 70:1, s. 219-239
  • Tidskriftsartikel (refereegranskat)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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7.
  • Sjöqvist, Erik (författare)
  • Degree of electron-nuclear entanglement in molecular states
  • 2000
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 77:2, s. 526-533
  • Tidskriftsartikel (refereegranskat)abstract
    • The degree of electron-nuclear entanglement in molecular states is analyzed. This entanglement has, generally, two sources: delocalization of the electronic and nuclear wave functions and vibronic coupling. For a diatomic molecular ground-state with a single potential energy minimum, it is demonstrated that the entanglement is a function of the product of the vibrational energy and the Born-Huang potential energy correction evaluated at the minimum. In the case of a double-well potential energy; surface, the deviation from maximal entanglement is determined by the overlap of the electronic and nuclear wave functions evaluated at and around the two minima. The adiabatic states of the E x epsilon Jahn-Teller model are shown to be maximally entangled and a relation between the degree of entanglement and Ham's reduction factor for this model is derived. Numerical calculations in the E x epsilon model demonstrate a nontrivial relation between entanglement and vibronic coupling.
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8.
  • Bakker, Albert, et al. (författare)
  • Interaction of aluminum(III) with water. An ab initio study
  • 1999
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 75:4-5, s. 659-669
  • Tidskriftsartikel (refereegranskat)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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9.
  • Carlsen, Henrik, et al. (författare)
  • Quantal trajectories for adiabatic and nonadiabatic regimes of vibronic systems
  • 1999
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 75:4-5, s. 409-416
  • Tidskriftsartikel (refereegranskat)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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10.
  • Ahlstrand, Emma, et al. (författare)
  • Interaction energies between metal ions (Zn2+ and Cd2+) and biologically relevant ligands
  • 2013
  • Ingår i: International Journal of Quantum Chemistry. - Hoboken, NJ : Wiley. - 0020-7608 .- 1097-461X. ; 113:23, s. 2554-2562
  • Tidskriftsartikel (refereegranskat)abstract
    • Interactions between the group XII metals Zn2+ and Cd2+ and amino acid residues play an important role in biology due to the prevalence of the first and the toxicity of the second. Estimates of the interaction energies between the ions and relevant residues in proteins are however difficult to obtain. This study reports on calculated interaction energy curves for small complexes of Zn2+ or Cd2+ and amino acid mimics (acetate, methanethiolate, and imidazole) or water. Given that many applications and models (e.g., force fields, solvation models, etc.) begin with and rely on an accurate description of gas-phase interaction energies, this is where our focus lies in this study. Four density functional theory (DFT)-functionals and MP2 were used to calculate the interaction energies not only at the respective equilibrium distances but also at a relevant range of ion–ligand separation distances. The calculated values were compared with those obtained by CCSD(T). All DFT-methods are found to overestimate the magnitude of the interaction energy compared to the CCSD(T) reference values. The deviation was analyzed in terms of energy components from localized molecular orbital energy decomposition analysis scheme and is mostly attributed to overestimation of the polarization energy. MP2 shows good agreement with CCSD(T) [root mean square error (RMSE) = 1.2 kcal/mol] for the eight studied complexes at equilibrium distance. Dispersion energy differences at longer separation give rise to increased deviations between MP2 and CCSD(T) (RMSE = 6.4 kcal/mol at 3.0 Å). Overall, the results call for caution in applying DFT methods to metalloprotein model complexes even with closed-shell metal ions such as Zn2+ and Cd2+, in particular at ion–ligand separations that are longer than the equilibrium distances.
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11.
  • Ai, Yuejie, 1982-, et al. (författare)
  • Exploring concerted effects of base pairing and stacking on the excited-state nature of DNA oligonucleotides by DFT and TD-DFT studies
  • 2011
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:10, s. 2366-2377
  • Tidskriftsartikel (refereegranskat)abstract
    • We have taken (dA)5, (dT)5, and (dA)5•(dT)5 as model systems to study concerted effects of base pairing and stacking on excited-state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFTmethods. The spectroscopic states are determined to be of a partial A →A charge transfernature in the A•T oligonucleotides. The T → T charge-transfer transitionsproduce dark states, which are hidden in the energy region of the steady-stateabsorption spectra. This is different from the previous assignment that the T → Tcharge-transfer transition is responsible for a shoulder at the red side of the first strongabsorption band. The A →T charge-transfer states were predicted to have relativelyhigh energies in the A•T oligonucleotides. The present calculations predict that the T→A charge-transfer states are not involved in the spectra and excited-state dynamics ofthe A•T oligonucleotides. In addition, the influence of base pairing and stacking on thenature of the 1nΠ* and 1ΠΠ* states are discussed in detail.
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12.
  • Arteca, Gustavo A., et al. (författare)
  • Generalized electronic diabatic approach to structural similarity and the Hammond postulate
  • 2007
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 107:2, s. 382-395
  • Tidskriftsartikel (refereegranskat)abstract
    • We revisit the notion of structural similarity along a reaction path within the context of a generalized electronic diabatic (GED) molecular model. In this approach, a reaction involving two closed-shell stable species is described as the evolution of a quantum state that superimposes at least three diabatic electronic species (reactant, product, and an open-shell transition state) coupled by an external electromagnetic field. Reactant and product amplitudes in this general state are also modulated by changing the geometry of a system of classical positive charges interacting with the electrons. By mapping these amplitudes over nuclear configurational space, we can follow the total quantum state along a reaction coordinate and establish its similarity to each of the diabatic species. As a result, chemical processes, and useful notions such as those of energy barriers and the Hammond postulate, emerge as consequence of Franck-Condon-like transitions between quantum states.
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13.
  • Arteca, Gustavo A., et al. (författare)
  • Generalized electronic diabatic approach to structural similarity in two-dimensional potential energy surfaces of various topologies
  • 2008
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 108:4, s. 651-666
  • Tidskriftsartikel (refereegranskat)abstract
    • Active site properties in some proteins can be affected by conformational fluctuations of neighbor residues, even when the latter are not involved directly in the binding process. A local environment thus appears to alter the relevant potential energy surface and its reaction paths. Here, some aspects of this phenomenon are simulated within a generalized electronic diabatic (GED) scheme to study the geometry and structural similarity for a class of two-dimensional (2D) energy surfaces. The electronic quantum state is a linear superposition of diabatic basis functions, each of which is taken to represent a single (pure) electronic state for the isolated material system. Here, we describe a model reaction of isomerization by shifts in amplitudes for three diabatic species (reactant, product, and an open-shell transition state) coupled in an external field. The "effective" 2D energy surface in the field is characterized in terms of critical points, and the amplitudes along the main reaction paths. A new feature is the introduction of a phase diagram where all possible potential-energy-surface topologies (consistent with three-state systems in two linear coordinates) are matched with actual model parameters. By varying the coupling strengths between diabatic states, we classify regions of this phase diagram in terms of electronic and structural similarities; some regions comprise models whose reaction paths have geometries that belong to the catchment region of the reactant, yet are electronically akin to the diabatic transition state or product.
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14.
  • Arteca, Gustavo, et al. (författare)
  • Simulating trends in reaction path geometry as a function of external fields : A generalized electronic diabatic model for two-dimensional energy surfaces
  • 2008
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 108:10, s. 1810-1820
  • Tidskriftsartikel (refereegranskat)abstract
    • We introduce a protocol to represent quantum states as a linear superposition of model electronic diabatic basis states coupled in an external (static) elec. field. By considering an entire family of these models, we uncover trends in reaction-path geometry and the topol. of potential-energy surfaces, including all those that can be realized in a two-dimensional configurational space. Our approach can be used as a tool to model the key parameters (e.g., diabatic basis states, external field intensity) that yield desired geometrical characteristics in an actual potential energy surface. In this work, external agents such as laser fields, or a group of neighboring charges, are regarded as essential requirements to prompt, or trigger, the occurrence of a chem. process. In these cases, reaction path geometry can be modulated externally so as to yield processes that would appear to occur far from gas-phase geometries. This phenomenol. is intrinsically nonadiabatic. Our present approach accounts for the possibility of such features, i.e., the occurrence of quantum states whose electronic structures resemble products, while at geometries that are more similar to those of reactants.
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15.
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16.
  • Bengtson, Charlotta, et al. (författare)
  • Quantum nonlocality in the excitation energy transfer in the Fenna-Matthews-Olson complex
  • 2016
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 116, s. 1763-1771
  • Tidskriftsartikel (refereegranskat)abstract
    • The Fenna-Matthews-Olson (FMO) complex - a pigment protein complex involved in photosynthesis in green sulfur bacteria - is remarkably efficient in transferring excitation energy from light harvesting antenna molecules to a reaction center. Recent experimental and theoretical studies suggest that quantum coherence and entanglement may play a role in this excitation energy transfer (EET). We examine whether bipartite quantum nonlocality, a property that expresses a stronger-than-entanglement form of correlation, exists between different pairs of chromophores in the FMO complex when modeling the EET by the hierarchically coupled equations of motion method. We compare the results for nonlocality with the amount of bipartite entanglement in the system. In particular, we analyze in what way these correlation properties are affected by different initial conditions. It is found that bipartite nonlocality only exists when the initial conditions are chosen in an unphysiological manner and probably is absent when considering the EET in the FMO complex in its natural habitat. It is also seen that nonlocality and entanglement behave quite differently in this system. In particular, for localized initial states, nonlocality only exists on a very short time scale and then drops to zero in an abrupt manner. As already known from previous studies, quantum entanglement between chromophore pairs on the other hand is oscillating and exponentially decaying and follow thereby a pattern more similar to the chromophore population dynamics. The abrupt disappearance of nonlocality in the presence of nonvanishing entanglement is a phenomenon we call nonlocality sudden death; a striking manifestation of the difference between these two types of correlations in quantum systems. 
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17.
  • Bischof, Gerhard, et al. (författare)
  • Quantum chemical study of the molecular dynamics of hydrated Li+ and Be2+ cations
  • 1997
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 65:5, s. 803-816
  • Tidskriftsartikel (refereegranskat)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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18.
  • Björkman, Torbjörn, et al. (författare)
  • Adaptive Smearing for Brillouin Zone Integration
  • 2011
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:5, s. 1025-1030
  • Tidskriftsartikel (refereegranskat)abstract
    • We suggest a simple scheme for automatically determining the width parameter of smearing methods of the Brillouin zone integration in electronic structure calculation. The scheme retains one free parameter that at any time can be eliminated by choosing a denser k-space mesh until the desired accuracy is obtained. The tests are carried out in the context of Methfessel-Paxton smearing. This adaptive Gaussian smearing (AGS) is easily implemented, variational with respect to partial occupancies and free from spurious occupancies that are negative or larger than one. Its convergence properties are similar to those obtained with the modified tetrahedron method for energy resolution of >= 0.1 meV.
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19.
  • Blomberg, Margareta R. A. (författare)
  • How Quantum Chemistry Can Solve Fundamental Problems in Bioenergetics
  • 2015
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 115:18, s. 1197-1201
  • Tidskriftsartikel (refereegranskat)abstract
    • Three different enzymes are discussed, cytochrome c oxidase, involved in aerobic respiration, cytochrome c dependent nitric oxide reductase, involved in denitrification (anaerobic respiration), and photosystem II, involved in photosynthesis. For all three systems, free energy profiles for the entire catalytic cycle are obtained from quantum mechanical calculations on large cluster models of the active sites, using hybrid density functional theory with the B3LYP* functional. The free energy pro-files are used to solve different fundamental problems concerning energy conservation, enzymatic reaction mechanisms and structure, and also to explain experimental results that seem to be in conflict with each other. Possible future applications to related problems using similar methodology are suggested.
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20.
  • Bondarchuk, S. V., et al. (författare)
  • Theoretical study of the triplet state aryl cations recombination : A possible route to unusually stable doubly charged biphenyl cations
  • 2013
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 113:24, s. 2580-2588
  • Tidskriftsartikel (refereegranskat)abstract
    • The self-recombination reactions of 4-aminophenyl cations and parent phenyl cations, each in ground triplet states, are studied within the framework of density functional theory. Only the total zero spin (singlet state) is chosen, as the quintet and triplet counterparts are nonreactive in these systems. The recombination products are the benzidine and biphenyl doubly charged cations. These species are unexpectedly stable. The transition state of the 4-aminophenyl cations reaction is located at the distance of about 4.0 Å between the ipso-carbon atoms. The activation barrier is predominantly formed by electrostatic repulsion between two cations and is estimated to be 27.6 kcal mol−1 [B3LYP/6–311+G(d,p)]. Similar results are obtained for the phenyl cations recombination. The general importance of the participation of other aryl cations in analogous organic reactions is discussed.
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21.
  • Borgh, Magnus, et al. (författare)
  • Correlation and spin polarization in quantum dots: Local spin density functional theory revisited
  • 2005
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 105:6, s. 817-825
  • Tidskriftsartikel (refereegranskat)abstract
    • Using quantum dot artificial atoms as a simple toy model, we reflect on the question of whether spin density functional theory (SDFT) can accurately describe correlation effects in low-dimensional fermion systems. Different expressions for the local density approximation of the exchange-correlation energy for the two-dimensional electron gas, such as the much-used functional of Tanatar and Ceperley, and the recent suggestion by Attaccalite et al., are compared with the results of a numerical diagonalization of the many-body Hamiltonian matrix in the limit of small electron numbers. For systems with degeneracies, as shown in the present work for the example of a spin triplet with S = 1, the direct comparison with configuration interaction (Cl) methods demonstrates that the spin representation of SDFT may, under certain circumstances, produce artificial energy splittings between states that belong to the same spin multiplet. For a singlet ground state with S = S = 0, however, the correlation functions of the Cl solutions confirm the spin-density wave states found earlier within the SDFT method.
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22.
  • Boström, Jonas, et al. (författare)
  • Analytical gradients of the second-order Møller–Plesset energy using Cholesky decompositions
  • 2014
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 114:5, s. 321-327
  • Tidskriftsartikel (refereegranskat)abstract
    • An algorithm for computing analytical gradients of the second-order Møller–Plesset (MP2) energy using density fitting (DF) is presented. The algorithm assumes that the underlying canonical Hartree–Fock reference is obtained with the same auxiliary basis set, which we obtain by Cholesky decomposition (CD) of atomic electron repulsion integrals. CD is also used for the negative semidefinite MP2 amplitude matrix. Test calculations on the weakly interacting dimers of the S22 test set (Jurečka et al., Phys. Chem. Chem. Phys. 2006, 8, 1985) show that the geometry errors due to the auxiliary basis set are negligible. With double-zeta basis sets, the error due to the DF approximation in intermolecular bond lengths is better than 0.1 pm. The computational time is typically reduced by a factor of 6–7.
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23.
  • Bredenberg, Johan, et al. (författare)
  • Modeling zinc sulfhydryl bonds in zinc fingers
  • 2001
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 83:3-4, s. 230-244
  • Tidskriftsartikel (refereegranskat)abstract
    • Molecular dynamics simulations have been carried out employing three different model descriptions of the zinc sulfhydryl interactions in class II fingers. One bonded and two nonbonded models were studied. Two variant structures of the glucocorticoid receptor DNA-binding; domain and a NMR structure from a fragment of methionyl-tRNA synthetase were subjected to long-time MD simulations with these models. Our analysis is focused on comparison with experimental and quantum mechanical data, concerning the local Zn-finger and overall structural and dynamic properties for these models. All models performed M ell, but the nonbonded models appeared to reproduce the protein dynamics in better agreement with experimental data than does the bonded description.
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24.
  • BRINCK, Tore, et al. (författare)
  • MOLECULAR-SURFACE ELECTROSTATIC POTENTIALS AND LOCAL IONIZATION ENERGIES OF GROUP-V-VII HYDRIDES AND THEIR ANIONS - RELATIONSHIPS FOR AQUEOUS AND GAS-PHASE ACIDITIES
  • 1993
  • Ingår i: 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
  • Tidskriftsartikel (refereegranskat)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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25.
  • BRINCK, Tore, et al. (författare)
  • SURFACE ELECTROSTATIC POTENTIALS OF HALOGENATED METHANES AS INDICATORS OF DIRECTIONAL INTERMOLECULAR INTERACTIONS
  • 1992
  • Ingår i: 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
  • Tidskriftsartikel (refereegranskat)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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26.
  • Brändas, Erkki, et al. (författare)
  • Darboux functions and power series expansions with examples from isoelectronic sequences
  • 1972
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 6:S6, s. 59-72
  • Tidskriftsartikel (refereegranskat)abstract
    • A procedure for summing a power series based on a Darboux function ansatz is developed. It involves computation of poles and residues of logarithmic derivatives using rational functions. Applications to the helium isoelectronic sequence condense the power series expansion (˜ 40 coefficients) to a few-parameter formula, which exhibits the singularity determining the radius of convergence. Accurate extrapolations of electron affinities and shielding factors are obtained.
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27.
  • Brändas, Erkki, Professor Emeritus, 1940- (författare)
  • It from bit : AI and the laws of nature
  • 2023
  • Ingår i: International Journal of Quantum Chemistry. - : John Wiley & Sons. - 0020-7608 .- 1097-461X. ; 124:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The present rate of growth of powerful AI systems motivates an accurate comparisonbetween the notion of computers and the workings of natural sciences. Statements such as “intelligence doesn't require flesh, blood or carbon atoms” or “it's computation all the way down” incite a substrate-independent view, providing shortcuts for Darwinian evolution and the possible appearance of sentient machines. This view is discussed and contrasted from a quantum chemical perspective. The qualitative difference between the developed AI and the evolved HI is recognized and the importance of a material constituent, formulated in terms of energy-temperature, conjugate to an immaterial ingredient, in the context of time-entropy, is pointed out as a necessary feature. The popular dictum “it from bit” does not appear valid unless amended with its obverse “bit from it.”
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28.
  • Brändas, Erkki J. (författare)
  • A theorem for complex symmetric matrices revisited.
  • 2009
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 109:13, s. 2860-2865
  • Tidskriftsartikel (refereegranskat)abstract
    • In this contribution we will revisit the celebrated theorem that every   square matrix is similar to a (complex) symmetric matrix and that every   symmetric matrix is orthogonally similar to a given normal canonical   form. Specifically we will re-examine the proof as well as the   derivation of an explicit n-dimensional complex symmetric form. We will   extend the formula to incorporate the various powers of the original   normal form, a derivation not previously provided. Some examples of   these complex symmetric forms in chemical and physical applications are   indicated.
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29.
  • Brändas, Erkki J. (författare)
  • Arrows of time and fundamental symmetries in chemical physics
  • 2013
  • Ingår i: International Journal of Quantum Chemistry. - New York : Wiley-Blackwell. - 0020-7608 .- 1097-461X. ; 113:3, s. 173-184
  • Tidskriftsartikel (refereegranskat)abstract
    • The field of chemical and physical processes, using principal microscopic techniques, should today allegedly be identified as a fundamental branch of physics. The discipline, by tradition known as chemical physics, is undergoing rapid progress. This assessment, illustrated by the advances presented at this Congress, is characterized by modern developments and novel trends with a concrete bearing on original theoretical understanding, with the possibility to go beyond traditional interpretations and explanations. In this vein, we do not only consider nonrelativistic treatments of various types of molecules including interactions between these and polymers, theoretical examinations of chemical reactions, surface states and interface states, disordered phenomena etc., but also recent progress together with non-Hermitean extensions to quantum mechanics and statistical mechanics. The latter leads to a united edifice of theoretical constructions including the law of self-reference, which emerges in analogy with the illustrious Godel theorem(s) of mathematical logic, that is, the assertion of the inherent limitations of all nontrivial axiomatic systems. The current development begets the foundation of temporal processes and associated invariance principles including the valuation of the various arrows of time. The present conjugate operator array formulation supports the possible gravitational origin of molecular chirality and other principal symmetry violations.
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30.
  • Brändas, Erkki J. (författare)
  • Godelian Structures and Self-Organization in Biological Systems
  • 2011
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:7-8, s. 1321-1332
  • Tidskriftsartikel (refereegranskat)abstract
    • In 1986, Seel and Ladik asked, which role Godel's incompleteness theorem should have in a basic theory of biology. Recently, the author has tried to collect the conditions, which such a meta-theory must fulfill. A further argument concerned the deeper connection between classical canonical forms of so-called (triangular) Jordan blocks in the description of open quantal systems far from equilibrium and those of self-referential contradictions and paradoxes in philosophy and mathematical logic. Related examples were quoted from the emergence of self-organization in so-called dissipative structures with applications to both fundamental-and of higher order levels of organization. To bring this analogy closer together, we have developed a quantum logical formalism, describing such a Godelian situation, via the characterization of a well-defined "truth matrix." In this setting, the modus operandi of exploiting self-referential traits and paradoxical inconsistencies emphasize the possibility of a meta-code in complicated enough (biological) systems. In conclusion, we will revisit situations were the aforementioned self-referential property, together with the laws of physics and chemistry will guide our understanding of biology. We will finally consider subsequent implications on the various positions on artificial intelligence.
  •  
31.
  • Brändas, Erkki J. (författare)
  • Microscopic self-organization and self-referential systems : a progress report.
  • 2009
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 109:14, s. 3500-3504
  • Tidskriftsartikel (refereegranskat)abstract
    • It is contended that (the classical canonical form of) Jordan blocks, play a role analogous to those of paradoxes and self-referential contradictions in philosophy and mathematical logic. As examples we will briefly discuss the occurrence of such triangular units in appropriately generalized quantum theory of microscopic as well as open dissipative systems with structures appearing on both the fundamental as well as in higher order levels of organization. The mathematical structure centers on specific transformations within coherent-dissipative ensembles that exhibit certain factorization properties allowing prime number algorithms, cf. the Godel encoding system used to derive the celebrated incompleteness theorem. This prompts the suggestion that an additional meta-code, cf. the genetic code, might be a-scripted for the mapping between the genotype and phenotype spaces.
  •  
32.
  • Brändas, Erkki J. (författare)
  • Non-hermitian quantum mechanics
  • 2012
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 112:15, s. 2764-2765
  • Recension (övrigt vetenskapligt/konstnärligt)
  •  
33.
  • Brändas, Erkki J. (författare)
  • Some Comments on the Problem of Decoherence
  • 2011
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:2, s. 215-224
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider a universally valid superposition principle derived from a general complex symmetric ansatz. The approach concerns open, dissipative systems and its significance for the examination and analysis of system-environment correlations. In particular, we discuss the concept of decoherence in the ensuing fundamental operator equations, incorporating simultaneously holistic and classical regimes. Despite its reductionist character, the theory applies to both micro-and macroscopic systems. Examples are drawn from general relativity, for example, the law of light deflection, the gravitational redshift, the time delay and the perihelion motion of Mercury, and from the macroscopic domain, for example, the emergence of selforganization in complicated enough systems. The preceding portrayal commends autonomicity and provides legitimate concepts of central importance in biological organization and artificial intelligence.
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34.
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35.
  • Brändas, Erkki, Professor Emeritus, 1940-, et al. (författare)
  • Preface
  • 2018
  • Ingår i: International Journal of Quantum Chemistry. - New York : John Wiley & Sons. - 0020-7608 .- 1097-461X. ; 118:1, s. 1-2
  • Tidskriftsartikel (refereegranskat)abstract
    • This volume collects 11 selected papers from the scientific contributions presented at the Ninth Congress of the International Society for Theoretical Chemical Physics (ISTCP-IX), organized by the team led by Professor Mark Hoffmann at the University of North Dakota, Grand Forks, North Dakota, U.S.A., from July 17 to 22, 2016. The ISTCP-IX Congress in Grand Forks followed the format established at the eight previous meetings: ISTCP-I:               Professor Ramon Carbo-Dorca, Girona (Spain), June 28 - July 3, 1993ISTCP-II:             Professor Sean P. McGlynn, New Orleans (LA, USA), April 9 - 13, 1996ISTCP-III:            Professor Miguel Castro, Mexico City (DF, Mexico), November 8 - 13, 1999ISTCP-IV:            Professor Jean Maruani, Marly-le-Roi (Paris, France), July 9 - 16, 2002ISTCP-V:             Professor Peter Politzer, New Orleans (LA, USA), July 20 - 26, 2005ISTCP-VI :           Professor Yan Alexander Wang, Vancouver (BC, Canada), July 19 - 24, 2008ISTCP-VI I:          Professor Hiromi Nakai, Waseda (Tokyo, Japan), September 2 - 8, 2011ISTCP-VIII:         Professor Péter Surján, Eötvös (Budapest, Hungary), August 25 – 31, 2013.  The 2016 venue offered the possibility for the approximately 300 participants from 26 countries to join the Congress. Speakers from each of the countries were joined by students from 9 countries.  Despite being nearly 2000 km from any sea coast (1200 km, if one counts the Arctic Ocean outlet Hudson Bay),  this Congress continued to reflect the strong international characteristics of ISTCP. Countries sending 4 or more delegates include China, Denmark, France, Germany, Hungary, Israel, Japan, Mexico, Netherlands, Norway, Switzerland, and the United Kingdom, besides the U.S.A. The International Society for Theoretical Chemical Physics, ISTCP, was founded in 1990 by Professor János Ladik at the University of Erlangen, Germany. ISTCP has the objectives to promote theoretical developments at the frontier between physics and chemistry. Additionally the goal is to allow younger researchers to interact with leading contributors of the field at regularly organized International Congresses. The Society involves an Honorary Board, a Board of Directors gathering together about 60 scientists (including 5 Nobel Laureates and 2 Wolf Prize laureates) in the fields of Theoretical Chemistry and Physics, and a Board of National Representatives covering about 35 countries/regions. The current President, since July 2000, is Professor Erkki J. Brändas, from Uppsala University, Sweden. ISTCP Congress Proceedings have been published regularly in the special issues of the International Journal of Quantum Chemistry (IJQC) and partly (2002, 2008) co-published in special volumes of Progress in Theoretical Chemistry and Physics (PTCP). Following this tradition, a small and highly targeted set of articles were solicited from researchers in several forefront fields represented at ISTCP IX.  These 11 articles are divided into 4 reviews, 3 tutorial reviews, 2 perspectives and 2 papers. ISTCP-IX was organized into 9 thematic Symposia, plus a special symposium honouring Per-Olov Löwdin.  The co-organizers of each of the symposia had significant latitude in inviting leading scientists in their areas, with attention paid to overall geographical, career stage and gender diversity.  Moreover, in an effort to stimulate conversation and cross-disciplinary inquiries, each speaker was limited to only 1 talk, and there were never more than 3 parallel sessions.  It is the careful thought and hard work of the Symposium Organizers that contributed to the success of the Congress.  The Symposia and their Organizers are:  1. Accurate Thermochemistry (Angela Wilson, Branko Ruscic) 2. Chemical Insights (Paul Ayers, Pedro Salvador) 3. Complex Systems (Jiali Gao, Nandini Ananth) 4. Dynamics (George Schatz, Keli Han) 5. Electronic Structure (Piotr Piecuch, Jiri Pittner) 6. Subsystems in Density Functional Theory (Tomasz Wesolowski, Christoph Jacob) 7. Emerging Methods for Quantum N-body Problem (Seiichiro Ten-no, Edward Valeev)  8. Molecular Properties (Trygve Helgaker)  9. Per-Olov Löwdin Symposium (Erkki Brändas)10. Relativistic Methods (Wenjian Liu, Jochen Autschbach) In addition to symposia, there were 9 plenary talks for which all participants were gathered.  The early and enthusiastic support of the plenary speakers were critical to providing high visibility for the conference, and we are grateful to them.1. Kim Baldridge, Structure-Property Relationships of Curved Aromatic Materials from First Principles2. Ria Broer, Theoretical and Computational Studies for the Design of Organic Photovoltaic Materials3. Benedetta Mennucci, Ab Initio Simulation of the Optical Spectroscopy of Multichromophoric Systems4. William Miller, Symmetrical Quasi-Classical Model for Classical Molecular Dynamics Simulations of Electronically Non-adiabatic Processes5. Debashis Mukherjee, A Survey of the Unitary Group Adapted MRCC and MRPT Theories: SU vs SS Approaches6. Martin Quack, The Quantum Dynamics of Chiral and Achiral Molecules including Electroweak Parity Violation: Theory and Experiment7. Andreas Savin, Multireference Density Functional Theory8. Henry F. Schaefer III9. Tamar Seideman, Coherent Alignment in Complex Systems This Preface does not allow a comprehensive account of all the excellent contributions to the conference or to the articles submitted to these proceedings. The 4 Reviews consider relativistic treatment of molecular properties, charge transfer in molecular crystals and in organic polymeric materials, and advances in subsystem embedding.  These are complemented by Tutorial Reviews on molecular motors, the inverse approach to exchange-correlation potentials, and the random phase approach in the context of reduced density matrices.  One full paper considers the chemistry of new super heavy elements and the other on data considerations in petascale computations of chemical and biological systems.  There are Perspectives on non-collinear electronic structure calculations and calculations of atoms and molecules in strong magnetic fields.  The articles in the proceedings can be grouped roughly into extension of theory and calculations into much larger systems than could be considered just a few years ago and extension of precision in theory and calculations. We are grateful to all organizers for their exceptional work. In particular we want to thank Professor Janos Ladik, Founder of the Society and Honorary Chair. We were sorry to learn that he could not participate in person but his kind interest and strong support in the various matters of the venue were indeed a positive factor. We are indebted to our excellent organizing committee that guided us in producing a well-balanced, global perspective on cutting-edge chemical physics: Gustavo Aucar, T. Daniel Crawford, Peter Gill, Anna Krylov, Hiromi Nakai, Katarzyna Pernal, Péter Surján and Ágnes Szabados. We are also grateful to all session chairs, speakers, poster presenters, as well as all student volunteers, contributing significantly to the great success of the meeting. For more details regarding the Congress we refer to our web site http://istcp-2016.org/. The ISTCP-IX Congress took place at the Alerus Center, near the University of North Dakota campus. The assistance of the staff at the Alerus Center and at the Greater Grand Forks Convention and Visitors Bureau were critical in facilitating an event of this complexity in this city of only 60,000 people.  But, most of all, it was the unwavering support of former UND President Robert Kelley, Vice President of Academic Affairs and Provost Tom DiLorenzo, Vice President for Research and Economic Development Grant McGimpsey, and Division of Research Staff Cathy Lerud and Carla Kellner that made this happen. We are pleased to express our sincere thanks to our sponsors.  In addition to generous support from the University of North Dakota and the Greater Grand Forks Convention and Visitors Bureau, which allowed low-cost registration and accommodations for students, we are pleased to be able to acknowledge additional support from Physical Chemistry Chemical Physics and Department of Energy. These contributions from our sponsors have enabled us to maintain the high-quality standard of the Congress. The guest editors of this Special Issue, finally, want to thank the authors, who accepted our invitation to contribute to these proceedings, and in so doing provide a perspective of some cutting edge areas of inquiry in chemical physics.  The IXth Congress of ISTCP included both these areas and many more. We hope that all researchers with a great interest in theory and methods related to fundamental scientific problems and future progress of our field will appreciate this volume. Mark HoffmannErkki Brändas  
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36.
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37.
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38.
  • Bunta, Juraj, et al. (författare)
  • Solvating, manipulating, damaging, and repairing DNA in a computer
  • 2007
  • Ingår i: International Journal of Quantum Chemistry. - New York : Wiley. - 0020-7608 .- 1097-461X. ; 107:2, s. 279-291
  • Tidskriftsartikel (refereegranskat)abstract
    • This work highlights four different topics in modeling of DNA: (i) the importance of water and ions together with the structure and function of DNA; the hydration structure around the ions appears to be the determining factor in the ion coordination to DNA, as demonstrated in the results of our MD simulations; (ii) how MD simulations can be used to simulate single molecule manipulation experiments as a complement to reveal the structural dynamics of the studied biomolecules; (iii) how damaged DNA can be studied in computer simulations; and (iv) how repair of damaged DNA can be studied theoretically.
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39.
  • Bustad, Johnny, et al. (författare)
  • Semi-empirical configurational interaction calculation of shake-up satellites in Ni(CO)4
  • 1998
  • Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 69:5, s. 649-657
  • Tidskriftsartikel (refereegranskat)abstract
    • INDO/CI calculations were used to analyze the C1s and O1s shake-up spectra of nickel tetracarbonyl, Ni(CO)4. The satellite structure in both cases is dominated by excitations from metal–ligand bonding (2Πb) to metal–ligand antibonding (2Πa) orbitals and by excitations within the core-ionized CO molecule, ΠCO—Π*CO.
  •  
40.
  • Chen, Shu-Feng, et al. (författare)
  • Multireference theoretical studies on the solvent effect of firefly multicolor bioluminescence
  • 2011
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:13, s. 3371-3377
  • Tidskriftsartikel (refereegranskat)abstract
    • In concert with the recent spectroscopic studies of the light-color modulation mechanism of firefly (Hirano et al., J Am Chem Soc 2009, 131, 2385), quantum chemical calculations using complete active space SCF (CASSCF), multistate complete active space second order perturbation (MS-CASPT2) theory as well as a time-dependent density functional theory (TD-DFT) approach with the Coulomb attenuated hybrid exchange-correlation functional (CAM-B3LYP) were performed on the excited state (S1) of the keto-form oxyluciferin (keto-OxyLH2). Benzene, DMSO, CH3CN, and H2O were chosen as polar solvents. The polarization effect of less polar solvent was considered by a simple model, complex of keto-OxyLH2, and NH3 with different covalent character. The calculated results supported the experimental conclusion: (1) the light emitter of bioluminescence is the S1 state of keto-OxyLH2 anion [(keto-1)*], and (2) light emission from (keto-1)* is modulated by the polarity of surrounding environment and the degree of covalent character of hydrogen bond between (keto-1)* and a protonated basic moiety. The mechanism of the multicolor bioluminescence was discussed from the theoretical viewpoint.
  •  
41.
  • Cicero, Giancarlo, et al. (författare)
  • Study of the oxidative half-reaction catalyzed by a non-heme ferrous catalytic center by means of structural and computational methodologies
  • 2007
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 107:6, s. 1514-1522
  • Tidskriftsartikel (refereegranskat)abstract
    • Deacetoxycephalosporin C synthase (DAOCS) is a mononuclear ferrous enzyme that catalyzes the expansion of the five-membered thiazolidine ring of the penicillin nucleus into the six-membered dihydrothiazine ring of the cephalosporins. In the first half-reaction with dioxygen and 2-oxoglutarate, a reactive iron-oxygen species is produced that can subsequently react with the penicillin substrate to yield the cephalosporin. We describe quantum mechanical calculations of the first part of the reaction based on the high-resolution structures of the active site of DAOCS and its complexes with ligands. These studies are aimed at understanding how the reactive species can be produced and contained in the active site of the enzyme. The results demonstrate the priming of the active site by the co-substrate for oxygen binding and hint to the presence of a stable iron-peroxo intermediate in equilibrium with a more reactive ferryl species and the formation of CO2 as a leaving group by decarboxylation of 2-oxoglutarate. A conclusion from these studies is that substitution of CO2 by the penicillin substrate triggers the oxidation reaction in a booby-trap-like mechanism.
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42.
  • Crespo, R., et al. (författare)
  • A Theoretical Study of Semiclassic Models : Toward a Quantum Mechanical Representation of Chemical Processes
  • 2011
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:2, s. 263-271
  • Tidskriftsartikel (refereegranskat)abstract
    • CASPT2 level of theory and aug-cc-pV6Z atomic basis set for hydrogen and 6 base states is sufficient to accurately reproduce the historical calculations of Kolos-Wolniewicz for H-2(1) Sigma(+)(g) potential energy curve. We found that for H + H process experimental and adiabatic theory results are contradictory. By analyzing different levels of semiclassic theory, we identify the nature of the failure. We propose an approximate scheme that may bridge semiclassic to QM model.
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43.
  • Dahlen, Nils-Erik, et al. (författare)
  • Variational energy functionals of the Green function tested on molecules
  • 2005
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 101:5, s. 512-519
  • Tidskriftsartikel (refereegranskat)abstract
    • It was recently proposed to use variational functionals based on manybody perturbation theory for the calculation of the total energies of many-electron systems. The accuracy of such functionals depends on the degree of sophistication of the underlying perturbation expansions. The energy functionals are variational in the sense that they can be evaluated at rather crude approximations to their independent variables which are the one-electron Green function, or the one-electron Green function and the dynamically screened electron interaction. The functionals were previously applied to the electron gas and shown to be extraordinarily accurate already at the level of the so-called GW approximation (GWA). In the current work we have tested the functional due to Luttinger and Ward, which is a functional of the Green function. Using density functional theory (DFT) and Hartree-Fock Green functions as input variables, we have calculated total energies of diatomic molecules at the level of the GWA as well as with second-order exchange effects included. We will also discuss various other variational energy functionals, including DFT orbital functionals based on many-body perturbation theory.
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44.
  • de Almeida, K. J., et al. (författare)
  • Theoretical insights into the visible near-infrared absorption spectra of Bis(hexafluoroacetylacetonate) copper(II) in pyridine
  • 2012
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 112:13, s. 2571-2577
  • Tidskriftsartikel (refereegranskat)abstract
    • The density functional theory calculations were performed to investigate the specific solvent effects on the optical absorption spectrum of copper(II) hexafluoroacetylacetonate complex in pyridine. The effects of single and double coordination of pyridine molecules at axial position of bis(hexafluoroacetylacetonate) copper(II) indicate that both positions and intensities of 3d3d electronic transitions are strongly dependent on the coordination environment around the copper(II) complex. The results indicate that the nature of the electron-acceptor atoms in the equatorial ligands plays an important role in the number of solvent molecules in the first solvation shell of copper(II) acac systems.
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45.
  • de Marothy, Sven (författare)
  • Autocatalytic decomposition of carbonic acid
  • 2013
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 113:20, s. 2306-2311
  • Tidskriftsartikel (refereegranskat)abstract
    • Long held to be unstable or metastable in the gas phase, carbonic acid has successfully been produced and identified in its gaseous form in recent decades. Theoretical studies have indicated that isolated carbonic acid in the gas phase may in fact be quite stable, its decomposition attributable to the catalytic effect of water molecules, either present or produced in a chain reaction by an initially slow decomposition. In this study, a previously unreported autocatalytic decomposition route is studied using high-accuracy ab initio quantum chemical methods. Results indicate that a carbonic acid dimer may react and decompose in a single-step, highly concerted reaction. The transition state of this reaction was characterized, and the reaction pathway was found to have significantly lower activation energy than the uncatalyzed decomposition, and comparable or lower energy than the water-catalyzed reaction. The results indicate that gaseous carbonic acid should be unstable even in the absence of water.
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46.
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47.
  • Devarajan, Ajitha, et al. (författare)
  • Role of Electronic Curve Crossing of Benzene S-1 State in the Photodissociation of Aryl Halides, Effect of Fluorination : RASSI-SO MS-CASPT2 Study
  • 2009
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 109:9, s. 1962-1974
  • Tidskriftsartikel (refereegranskat)abstract
    • An ab initio study of the role of electronic curve crossing of benzene S-1 state in the photo dissociation dynamics of the iodobenzene and effect of fluorination is presented. Two dissociative life times observed in iodobenzene is attributed to the coupled repulsive potential energy curves of the low-lying n-sigma*, pi-sigma*, pi-pi* states. The direct channel is attributed to the alkyl like transition and the indirect channel is attributed to the mixing of the alkyl like transitions with the low lying benzene pi-pi* transitions. Fluorination of iodobenzene results in a substantial increase in the direct channel product. To analyze the possible role of electronic curve crossing of these transitions, potential energy curves of low-lying n-sigma*, pi-sigma*, pi-pi* states were studied including spin-orbit and relativistic effects using the Restricted Active Space state interaction multistate complete active space perturbation theory (RASSI-MS-CASPT2) method. Crossing behavior of spin-free and spin-orbit potential energy curves was analyzed for the role of the benzene S-1 state. Our results indicate the curve crossing region to be around 2.00-2.35 angstrom for both C6H5I and C6F5I. Analysis of effect of fluorination on the energies of states corresponding to benzene pi-pi* and n-sigma* transitions suggests an increase in the energy of benzene pi-pi* states and a decrease in the energy of the states corresponding to n-sigma* transitions. Increased spin-orbit gap, increased separation of the benzene S-1(pi-pi*) state and n-sigma* states in the region of curve crossing, lesser mixing of the pi-pi* and n-sigma* states, an order of magnitude decrease in the transition strength to the benzene singlet transition all contributed to the observed Substantial increase in the quantum yield of the direct channel product on fluorination of aryl halides.
  •  
48.
  • Dungsrikaew, V., et al. (författare)
  • Comparison of methods for point-charge representation of electrostatic fields
  • 2004
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 96, s. 17-22
  • Tidskriftsartikel (refereegranskat)abstract
    • The calculation of the electrostatic potential resulting from an infinite or extended array of charges in the interior of a region of interest is a frequent task in computational chemistry. In case of a periodic potential this can, for example, be done by Ewald summation or by multipole methods. An important alternative are those methods where arrays of auxiliary point charges are optimized with respect to charge and/or position to reproduce the original electrostatic potential. In the literature different variations are reported. We compare the performance of some of these with respect to their ability to reproduce the original potential and the computational effort required. Between (1) surface charges determined by the conductor-boundary condition, (2) optimized surface charges, and (3) surface charges floating on the surface we find that (2) offers good quality with small computational costs involved.
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49.
  • Dunne, Lawrence J., et al. (författare)
  • Superconductivity from repulsive electronic correlations on alternant cuprate and iron-based lattices
  • 2013
  • Ingår i: International Journal of Quantum Chemistry. - New York : Wiley. - 0020-7608 .- 1097-461X. ; 113:17, s. 2053-2059
  • Tidskriftsartikel (refereegranskat)abstract
    • A key question in the theory of high-temperature superconductivity is whether off-diagonal long-range order can be induced wholly or in large part by repulsive electronic correlations. Electron pairs on cuprate and the iron-based pnictide and chalcogenide alternant lattices may interact with a strong short-range Coulomb repulsion and much weaker longer range attractive tail. Here, we show that such interacting electrons can cooperate to produce a superconducting state in which time-reversed electron pairs effectively avoid the repulsive part but reside predominantly in the attractive region of the potential. The alternant lattice structure is a key feature of such a stabilization mechanism leading to the occurrence of high-temperature superconductivity with d(x2-y2) or sign alternating s-wave or s +/- condensate symmetries. 
  •  
50.
  • Elander, Nils, et al. (författare)
  • Complex scaling and self adjoint dilations
  • 1993
  • Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 46:3, s. 415-418
  • Tidskriftsartikel (refereegranskat)abstract
    • Complex scaling of the Schrodinger equation on the halfaxis with a nontrivial boundary condition at the origin is investigated. A self-adjoint dilation of the corresponding dissipative operator is constructed. The relations between the scattering problems for the operator and it's dilation are clarified
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