| 1. |
- Karlsson, Hans O, et al.
(författare)
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Density of states of alternant cyclic polyenes (CH)(N) by a direct Lanczos method
- 1997
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 63:3, s. 719-728
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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)
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Density functional study of chlorine-oxygen compounds related to the ClO self-reaction
- 1998
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 66:3, s. 203-217
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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)
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Substituent effects on OH bond strength and hyperfine properties of phenol, as model for modified tyrosyl radicals in proteins
- 2000
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 76:6, s. 714-723
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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)
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A new way of analyzing vibrational spectra. II. Comparison of internal mode frequencies
- 1998
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 67:1, s. 11-27
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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)
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Some recent developments of high-order response theory
- 1998
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 70:1, s. 219-239
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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)
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Degree of electron-nuclear entanglement in molecular states
- 2000
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 77:2, s. 526-533
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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)
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Interaction of aluminum(III) with water. An ab initio study
- 1999
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 75:4-5, s. 659-669
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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)
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Quantal trajectories for adiabatic and nonadiabatic regimes of vibronic systems
- 1999
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Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 75:4-5, s. 409-416
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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)
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Interaction energies between metal ions (Zn2+ and Cd2+) and biologically relevant ligands
- 2013
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Ingår i: International Journal of Quantum Chemistry. - Hoboken, NJ : Wiley. - 0020-7608 .- 1097-461X. ; 113:23, s. 2554-2562
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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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