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  • 1995-1999  (974)
  • Computational Chemistry and Molecular Modeling  (973)
  • Nuclear reactions
  • 201
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 341-350 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Wave-operator equations associated with the determination of effective Hamiltonians are generally solved by perturbation methods. However, it is well known that for most actual systems the standard Rayleigh-Schrödinger and Brillouin-Wigner series either converge slowly or diverge. One way how to deal with this problem is to modify or to renormalize the standard wave equations. For that purpose, we derive new derivative and convergence superoperators within the two basic Rayleigh-Schrödinger and Brillouin-Wigner formalisms. Since the direct use of these superoperators would imply difficult operator inversions, we investigate the efficiency of various approximations of these superoperators. It is shown that these approximations can overcome convergence difficulties and also open the way to systematic infinite partial summation schemes. A numerical illustration is presented for a model system of N non-interacting hydrogen molecules. Finally, it is shown that the determination of regular two-component relativistic Hamiltonians is also relevant to this approach. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
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  • 202
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 351-360 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Ab initio valence bond calculations are performed for the three lowest states of the oxygen molecule (3Σ-g, 1Δg, and 1Σ+g). One objective of the present study was to make a contribution to previous valence bond discussions about the oxygen “double” bond. Further, we study the origin of a small barrier in the potential energy surface of the ground state. Two compact models are employed to maintain the clear picture that can be offered by the valence bond method. The first model has only the Rumer structures that are essential for bonding and a proper dissociation. The second model, in addition, has structures which represent excited atoms. These prove to be important for the dissociation energies. For both models, the orbitals are fully optimized. The spectroscopic data obtained are significantly better than are the (few) valence bond results on O2 that have been published and have the quality of multiconfiguration self-consistent field calculations in which the same valence space is used. The “hump” in the potential energy surface of the ground state is shown to arise from a spin recoupling. The free atoms correspond to a spin coupling that is incapable of describing the formation of bonds. Only at short distances, an alternative spin coupling provides bonding and the repulsive curve is converted into an attractive one. Our results on this subject support a valence bond explanation previously given by McWeeny [R. McWeeny, Int. J. Quantum Chem. Symp. 24, 733 (1990)]. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 203
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 373-381 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: On statistical-mechanical grounds, a stochastic optimization technique (generalized simulated annealing) has been recently proposed which contains both classical simulated annealing (Kirkpatrick et al., 1983) and fast simulated annealing (Szu, 1986) as particular cases. This technique can be faster than both in detecting global (and also local) minima. Its utility in quantum chemistry is here illustrated, through the use of a semiempirical quantum method, on molecules of the series CH3-R (C2H6, CH3COH, CH3OH), H2X2 (H2O2, H2S2), X2Y4 (N2H4, P2H4, N2F4), for double bonds (C2H4 and CH2NH), and finally for H2O3. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 204
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 361-372 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Exact quantum molecular overlaplike and Coulomb-like self-similarity measures are studied in a selected series of molecules with the same number of electrons. It is found that quantum molecular overlap self-similarity measures can be used to estimate the concentration of electronic charge in molecules. A good linear relationship between the overlap self-similarity measure and the volume is found for molecules with the same number of electrons when the atoms of the systems being compared belong to the same row of the periodic table. Finally, an upper bound for the quantum molecular overlap self-similarity measure of molecules with a number of electrons up to 54 is given from the atomic quantum self-similarity measures obtained using Slater-type functions. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 205
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 383-388 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We report a fully self-consistent investigation of the electronic structure, relation between charge transfer and electronegativity, bonding characteristics, and alloying of Al, P, S, and Cl with silicon. The present study indicates that the charge transfer from the host Si atoms to the central impurity atom increases with electronegativity of the impurity element. The bonding between host Si atoms and the impurity atoms is ionic in nature. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 206
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 389-398 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The various isomers including stable structures, carbenes, and diradicals on the C3H4 surface have been investigated. The two carbenes propenylidene and cyclopropylidene have been found to have singlet ground states. Vinylmethylene is predicted to have a triplet ground state with a planar diradical type of structure. The syn and anti forms of this state are degenerate. This is in agreement with the observation of two triplet states in the electron spin resonance (ESR) spectra. The π electrons are found to be delocalized over the three carbons. The singlet diradical structures are found to be more stable than the carbene structures, which retain the CH2 (DOUBLE BOND) CH allylic structures. The orbital compositions of the frontier orbitals of all systems have been determined to examine the nature of these orbitals. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 207
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 399-406 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The coupling between the NH hydrogen inversion and the CNC bending modes of dimethyl-amine (DMA) is analyzed from ab initio calculations. For this purpose, the vibrational Hamiltonian is defined from the symmetry properties of the kinetic and potential operators. The two fundamental frequencies are determined with the Möller-Plesset perturbation theory up to four order (MP4). Calculated band positions are compared with data derived from other models in one dimension that reveal the strong effect of the interaction terms on the inversion bands. The frequencies of two components of the wagging fundamental are obtained to be 794.3 and 793.4 cm-1 with the two-dimensional model, in good agreement with experimental data. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 208
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 419-429 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Ab initio calculations of dynamic structure factors are performed with a methodology that requires the crystalline density matrix determined by the periodic Hartree-Fock CRYSTAL code. Amplitudes and orientations of the atomic vibrations are fixed by the mean-square displacements available in the literature. This method, which is here upgraded so as to satisfy normalization conditions, gives rise, for any temperature T, to a “dynamic” density matrix different from the static one. An application to the calculation of the dynamic structure factors of magnesium difluoride (MgF2), which has anisotropic mean-square displacement tensors, is proposed and compared with experiments at 298 K. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 209
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 407-418 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Second-order hyperpolarizabilities (β) and gas-phase transition energies (E) of the para-disubstituted benzene, stilbene and azobenzene derivatives are calculated by the semi-empirical self-consistent-field molecular orbit (SCF MO) method in the Pariser-Parr-Pople (PPP) approximation using a sum-over-state's procedure with singly excited states. The calculated zero-frequency β (β0VEC) values for all the derivatives are directly proportional to their 1/E2 values. The usefulness of mixing ethylenic and azo bridges in long π-conjugated molecules is proposed in order to obtain large β values, by taking the difference between the electronic natures of these bridges into account. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 210
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 211
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 431-439 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The ring-shaped Hartmann potential \documentclass{article}\pagestyle{empty}\begin{document}$$V = \eta \sigma ^2 \varepsilon _0 \left({\frac{{2a_0 }}{r} - \frac{{\eta a_0^2 }}{{r^2 \sin ^2 \theta }}} \right)$$\end{document} was introduced in quantum chemistry to describe ring-shaped molecules like benzene. In this article, fundamental concepts of supersymmetric quantum mechanics (SUSYQM) are discussed. The energy eigenvalues and (radial) eigenfunctions of the Hartmann potential are subsequently rederived using the techniques of SUSYQM. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 212
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 441-451 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We study an example of a generalized Friedrichs model, in which a continuous-continuous coupling produces a pair of resonances as branch cuts of the analytic continuation of the reduced resolvent of the perturbed Hamiltonian to the second sheet of the Riemann surface associated to a transformation of the type w = z + α √z. To define the perturbation, we use the theory of self-adjoint extensions of symmetric operators proposed by the group of St. Petersburg. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 213
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The dependence of the intramolecular charge transfer through the σ-electron systems of substituted hydrocarbons on the spatial arrangement of the X(SINGLE BOND)Cα and Cβ(SINGLE BOND)Cγ bonds has been studied using the perturbation theory for the one-electron density matrix (DM). Analytical expressions for the populations of the orbitals pertinent to the cis- and trans-bonds with respect to the X(SINGLE BOND)Cα bond have been obtained and analyzed. The Hamiltonian matrix elements determining the predominant direction of the above-defined charge transfer (cis or trans) have been revealed. The electron-accepting and electron-donating substituents (X) have been considered separately and the dependence of the resulting charge-transfer direction on the properties of substituent has been established. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 214
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In this work, we evaluated structural and electronic similarities between a new class of acylhydrazones, recently presented as effective inhibitors of a Plasmodium falciparum cysteine protease, and a series of pyrazole arylacylhydrazones with analgesic and antiaggregating (antithrombotic) properties, using AM1. The calculated results suggest that at least one of the pyrazole compounds is similar enough to the active compounds to be considered as a candidate for a future antimalarial series. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 215
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. v 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 216
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 487-496 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Temperature dependence of the experimental photoemission spectra of the high-Tc superconductors have been interpreted and simulated by the recently formulated molecular nonadiabatic theory of the electron-vibrational interaction. The change of the spectral feature below Tc, particularly formation of the narrow, near Fermi surface (FS) edge, peak separated by the dip from a broad spectral band at higher binding energies has been shown to be the consequence of the nonadiabatic electron-phonon coupling and corresponding properties of the nonadiabatic polarons. Simulation yields very good fit to the experimental temperature-dependent spectra, with the calculated value of the gap 30.5 meV. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 217
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 218
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 497-515 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Crystal-orbital calculations on the three-dimensional disodium deoxyguanosine-5′-monophosphate tetrahydrate, disodium uridine-3′-monophosphate tetrahydrate, monosodium monoprotonated deoxyadenosine-5′ monophosphate hexahydrate, and disodium deoxycytidine-5′ -monophosphate heptahydrate crystallohydrates were carried out in a Hartree-Fock all-valence-electron approximation using the CRYSTAL92 routine package. The first, the second, and the last compounds were found to be insulators, whereas the third one should be considered a two-dimensional semiconductor. The structural basis for these findings is discussed, with special reference to the semiconductivity and intermolecular interactions in solid-state samples of polymeric nucleic acids. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 11 Ill.
    Materialart: Digitale Medien
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  • 219
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 533-533 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: No Abstracts.
    Materialart: Digitale Medien
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  • 220
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 157-166 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The Roothaan equations have been modified to compute molecular interactions between weakly bonded systems at the SCF level of theory without the basis set superposition error (BSSE). The increase in complication with respect to the usual SCF algorithm is negligible. Calculation of the SCF energy on large systems, such as nucleic acid pairs, does not pose any computational problem. At the same time, it is shown that a modest change in basis-set quality from 3-21G to 6-31G changes the binding energy by about 50% when computed according to standard SCF “supermolecule” techniques, while remaining practically constant when computed without introducing BSSE. Bader analysis shows that the amount of charge transferred between the interacting units is of the same order of magnitude when performed on standard SCF wave functions and those computed using the new method. The large difference between the corresponding computed energies is thus ascribed to the BSSE. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 221
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 185-193 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An approach aimed at approximating the extreme (the lowest and/or the highest) eigenvalues of matrices representing many-electron model Hamiltonians from a knowledge of several spectral density distribution moments is proposed. A detailed discussion of the Heisenberg spin Hamiltonian spectrum is presented as an example of an application. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Tab.
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  • 222
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 195-199 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The concept of phase invariance on shifting the origin formulated for the structure factors of crystallography is transferred to momentum space quantum chemistry. The resulting mathematical constraints in the form of inequalities are given for tractable one-electron molecules. © 1996 John Wiley & Sons, Inc.
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  • 223
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 201-211 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The derivation of the reciprocal-space Schrödinger equation is reviewed, as well as Fock's method for solving it for hydrogenlike atoms. It is shown that Fock's solutions (which represent Fourier transformed hydrogenlike orbitals in terms of 4-dimensional hyperspherical harmonics) can be used as basis sets for solving other problems in quantum chemistry. Such basis sets are of the Sturmian type (i.e., all the members of the set correspond to the same energy), and they obey weighted orthonormality relations in both direct and reciprocal space. The kernel of the reciprocal-space Schrödinger equation is expanded in terms of Sturmian basis sets, and this expansion is used to solve the problem of a particle moving in a many-center potential. Both Coulomb and non-Coulomb potentials are treated, and a new method for evaluating the necessary integrals is discussed. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 224
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1291-1301 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We present a computational scheme to study the dynamics of many-electrons in molecular systems by wavepackets method. Several approaches to calculation of nonlinear optical properties for molecules under time-independent or time-dependent external electric fields are presented. Some simple examples of one-dimensional two- or three-electron systems are demonstrated concretely. Implications of these results are discussed in relation to the validity of the many electron wavepackets (MEWP) method. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 225
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1479-1486 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A hybrid quantum/classical model is developed for the photodissociation and recombination dynamics of an I2 molecule in low-temperature rare-gas (Rg) matrices. The simplified model consists of an I2 molecule embedded in a linear chain of Rg atoms. The aggregate is partitioned into a quantum system and a classical bath, which are self-consistently coupled. Two partitioning schemes are used. The first treats the I-I coordinate quantum mechanically and the Rg coordinates classically. The second and more reliable scheme includes in the quantum system both the I-I mode and the symmetric motion of the two nearest Rg atoms. Both models show substantial energy transfer from the dissociating I2 to the solvent, followed by coherent vibrational motion of the recombined I2. It is found that the one-dimensional quantum/classical scheme is consistent with its higher dimensional counterpart, although the latter shows much faster dephasing. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 226
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1457-1468 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The Kohn-Sham eigenvalues were formally introduced into density functional theory as Lagrange multipliers in the implementation of the minimum principle for the total energy of a many-electron system. No general results are available concerning the physical significance of these one-electron eigenvalues (with the exception of the highest occupied level, which equals the Fermi energy). Recent ab initio calculations of dynamical response in metals make explicit use of the Kohn-Sham band structure, and associated wave functions, through the use of spectral representations. This opens up the possibility of examining the significance of the eigenvalues at an “empirical” level, i.e., through direct comparison with the results of spectroscopic measurements. A particularly interesting example is afforded by new inelastic x-ray scattering experiments on A1. For a special wave vector transfer, qo ≈ 1.5kF, the measured spectrum provides a direct mapping of the Kohn-Sham noninteracting spectrum. For a range of wave vectors about qo, the bare Kohn-Sham spectrum still reproduces all the main features of the measurements; this suggests that, in this metal, the Kohn-Sham eigenvalues are good approximations to the quasiparticle energies. We also discuss the interplay between Kohn-Sham bands and the energy of the “anomalous” plasmon in Cs, whose dispersion bears a signature of the excited-state band structure. Finally, and in a more formal framework, we outline the results of a first-principles comparison between quasiparticle amplitudes and Kohn-Sham wave functions at a jellium surface; the latter turn out to be excellent approximations to the former. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 227
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1487-1497 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Minimal basis-set (STO-3G) direct and Fourier space restricted Hartree-Fock (RHF) calculations on the infinite regular (metallic) and alternating chains of lithium molecules, (-Li2-)x, are reported to illustrate two advantages of the Fourier representation method: accurate calculation of all lattice summations and faithful reproduction of the genuine features of the RHF approach for metallic cases. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
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  • 228
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1505-1514 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The dependence of X-ray photoionization spectra on structural characteristics is assessed by one-particle Green's function calculations on the nearly isomeric adamantane, 1,4-dimethyl bicyclo [2,2,2] octane and 1,1,4,4-tetramethyl cyclohexane compounds. These calculations are carried out using the quasi-particle approximation and the diagonal 2ph-TDA renormalization scheme for the self-energy. The simulated spectra fingerprint remarkably the atomic connectivity from their inner valence part, through the interplay of energy degeneracies and the influence of torsional strains on electron binding energies. The boundary region between the inner and outer valence bands provides also specific signatures for molecular characteristics pertaining to conformational questions, namely the enforced conversion from the chair to the boat form of the cyclohexane units generating these compounds. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 229
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1525-1536 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The local self-consistent field (LSCF) method which allows full SCF computations on a fragment of very long molecular systems represented by a classical force field has been adapted to the description of nonmetallic crystals. The periodicity of the network is achieved by modifying self-consistently the basic parameters of the classical subsystem (charges, geometric parameters) along the SCF iterative scheme. The method is tested on α-cristobalite. The parametrization of the quantum classical junction, achieved by localized bond orbitals has been performed with the help of a fragment located in the bulk. The stability of the method with respect to the size of the fragment and the size of the crystalline sample appears to be very good. The properties of fully hydroxylated (010) surface are corrected described. Modeling of water adsorption on the ideal surface as well as on two kinds of surface defects gives rise to very reasonable results with an absorption energies of ca 10 kcal/mol which are close to the upper limit of the experimental data. This preliminary study appears to be quite encouraging regarding the possibilities of using this method, which can be considered as an extension of the embedded cluster approach to covalent solids. Many applications to surface chemical reactivity studies can be imagined. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 230
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 141-155 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The matrix differential calculus is applied for the first time to a quantum chemical problem via new matrix derivations of integral formulas and gradients for Hamiltonian matrix elements in a basis of correlated Gaussian functions. Requisite mathematical background material on Kronecker products, Hadamard products, the vec and vech operators, linear structures, and matrix differential calculus is presented. New matrix forms for the kinetic and potential energy operators are presented. Integrals for overlap, kinetic energy, and potential energy matrix elements are derived in matrix form using matrix calculus. The gradient of the energy functional with respect to the correlated Gaussian exponent matrices is derived. Burdensome summation notation is entirely replaced with a compact matrix notation that is both theoretically and computationally insightful. © 1996 John Wiley & Sons, Inc.
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  • 231
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 157-172 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The size-extensive quadratic CI method with single(s), double(D), and triple (T) excitations, QCISDT, is compared with QCISD, QCISD(T), CCSDT-n, and CCSDT. It is shown that QCISDT results are more accurate than are either QCISD or QCISD(T) results. In particular, QCISDT turns out to be more stable than are QCISD and QCISD(T) in cases with considerable multireference character. QCISDT and CCSDT results are of similar accuracy with slight advantages for the former method. Since QCISDT is much easier to implement on a computer than is CCSDT, it is an attractive alternative to CCSDT. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 232
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 173-182 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: This article presents remarks on the meaning of stopping power and of its magnitude. More precisely, the first set of remarks concerns the connection of stopping power with elements of particle-transport theory, which describes particle transport and its consequences in full detail, including its stochastic aspects. The second set of remarks concerns the magnitude of the stopping power of a material and its relation with the material's electronic structure and other properties. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 233
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 183-198 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The virial field V(r) is defined by the local statement of the quantum mechanical virial theorem, as the trace of the Schrödinger stress tensor. This field defines the electronic potential energy density of an electron at r and integrates to minus twice the electronic kinetic energy. It is the most short-ranged description possible of the local electronic potential energy and it exhibits the same transferable behavior over bounded regions of real space (corresponding to the functional groups of chemistry) as does ρ(r). This article establishes a structural homeomorphism between -V(r) and ρ(r), showing that the two fields are homeomorphic over all of the nuclear configuration space. The stable or unstable structure defined by the gradient vector field Δρ(r; X) for any configuration X of the nuclei can be placed in a one-to-one correspondence with a structure defined by the field -ΔV(r; X′). In particular, a molecular graph for ρ(r) defining a molecular structure is mirrored by a corresponding virial graph for V(r) and the lines of maximum density linking bonded nuclei in the former field are matched by a set of lines of maximally negative potential energy density in the latter. The homeomorphism is also geometrically faithful, an equilibrium geometry in general, exhibiting equivalent structures in the two fields. The demonstration that the virial field, whose integrated value equals twice the total energy, is essentially just a locally scaled version of the electron density is suggestive of possible new approaches in density functional theory. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 234
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 229-233 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Restricted open-shell Hartree-Fock and unrestricted Hartree-Fock calculations of the electric-field gradient in atoms B, N, O, Al, S, and Cl were performed by relieving the spherical symmetric constraint. The Sternheimer's core polarization effect is then automatically taken into account. The orbitals produced by the axial symmetric self-consistent field are found to have axial symmetry of s-d and p-f mixing types. However, the nonequivalence of the three p orbitals also gives rise to ambiguity. © 1996 John Wiley & Sons, Inc.
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  • 235
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 235-243 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The diagonalization of a class of lattice spin models of a particular structure is first reviewed and secular polynomials for these models are calculated explicitly from the corresponding secular matrices. The structure of the eigenvectors of the given secular matrices is investigated and used to determine the eigenvalues theoretically, and proofs which have not appeared are presented. These results can be compared to the results obtained from the full secular polynomials. © 1996 John Wiley & Sons, Inc.
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  • 236
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 237
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 259-259 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 238
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The influence of a hydroxyl group simulating Ser-48 in the hydride-transfer step characteristic of liver alcohol dehydrogenase is studied on the hydride-transfer reaction as modeled by a methanolate anion interacting with a cyclo propenyl cation. It is shown first that this is an adequate model by comparing it to the methanolate-pyrydinium cation model transition structure, (TS). The side-chain effect is modeled first by adding water and then with methanol located at the position that Ser-48 occupies in the enzyme; a supermolecule approach is used. It is found that (i) the normalized advance coordinate (NAC) for the exchanged hydrogen has an invariant value at the TS and the reactant, while for the product, the NAC depends upon the external perturbation introduced by the ancillary molecule (the TS is reactant-like); (ii) the products are strongly destabilized, so the (activation) barrier with respect to the TS diminishes; (iii) the energy gap between reactants and products is sensibly diminished by the presence of methanol; (iv) the alcoholate moiety in the hydride transfer complex is not spontaneously protonated; and (v) there is a negligible charge transfer between the hydride-transfer system and models of Ser-48. In the present simplified model, methanol appears to have a catalytic effect via hydrogen bonding. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 239
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 281-293 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: It is shown how the regularized two-component relativistic Hamiltonians of Heully et al. and Chang, Pelissier, and Durand can be viewed as arising from a perturbation expansion that unlike the Pauli expansion remains regular even for singular attractive Coulomb potentials. The performance of these approximate Hamiltonians is tested in the framework of the local density approximation and the relation of their spectrum to that of the Dirac Hamiltonian is discussed. The circumstances under which the current approximations are superior to the Pauli Hamiltonian are analyzed. Finally, it shown how the Hamiltonians could be used within the context of conventional Hartree-Fock theory. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 240
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 295-308 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Chief features of dimensional scaling methods are exemplified by briefly reviewing prototypical applications and recent developments. The pseudoclassical large-D limit usually can be evaluated exactly regardless of the magnitude, nature, and number of strong, nonseparable dynamical interactions. Often, this limit can be accurately linked to D = 3 by perturbation or interpolation methods. This is because the dimension dependence of many-body effects tends to be smooth and mild when calibrated by appropriate one- or few-body problems. A simple renormalization procedure applied to atoms with up N ∼ 100 electrons yields a major part of the correlation energy. From Hartree-Fock input, a renormalized nuclear charge is determined which renders the dimensionally scaled energy at D → ∞ a good approximation to that for D = 3 with the actual Z. Prospects are discussed for other means to exploit dimensional scaling, including an analogous renormalization procedure for molecules. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 9 Ill.
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  • 241
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 265-280 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Sequence transformations, which transform a slowly convergent or divergent sequence {sn}∞n&=0 into a new sequence {sln}∞n=0 with hopefully better numerical properties, are useful computational tools to overcome convergence and divergence problems. In this article, sequence transformations are discussed that use explicit remainder estimates [E.J. Weniger, Comput. Phys. Rep. 10, 189 (1989)]. Because of the explicit incorporation of the information contained in the remainder estimates, these transformations are potentially very powerful and well suited for the summation of strongly divergent series. The Rayleigh-Schrödinger perturbation series for the ground-state energy of the quartic, sextic, and octic anharmonic oscillator is a typical example of a perturbation series that diverges quite strongly for every nonzero coupling constant. It can be summed efficiently even in the very challenging strong coupling regime, if sequence transformations are combined with a suitable renormalization technique described by F. Vinette and J. C˘íz˘ek [J. Math. Phys. 32, 3392 (1991)]. Moreover, a renormalized strong coupling expansion for the ground-state energy of an anharmonic oscillator can be constructed, which apparently converges for all coupling constants β ε [0,∞) and which makes the computation the ground-state energy almost trivial. Other applications of sequence transformations in quantum mechanical and quantum chemical calculations are also reviewed. © 1996 John Wiley & Sons, Inc.
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  • 242
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 309-319 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Gradient corrections to the local spin density (LSD) approximation for the exchange-correlation energy are making density functional theory as useful in quantum chemistry as it is in solid-state physics. But which of the many gradient-corrected density functionals should be preferred a priori? We make a graphical comparison of the gradient dependencies of some popular approximations, discussing the exact formal conditions which each obeys and identifying which conditions seem most important. For the exchange energy, there is little formal or practical reason to choose among the Perdew-Wang 86, Becke 88, or Perdew-Wang 91 functionals. But, for the correlation energy, the best formal properties are displayed by the nonempirical PW91 correlation functional. Furthermore, the real-space foundation of PW91 yields an insight into the character of the gradient expansion which suggests that PW91 should work especially well for solids. Indeed, while improving dissociation energies over LSD, PW91 remains the most “local” of the gradient-corrected exchange-correlation functionals and, thus, the least likely to overcorrect the subtle errors of LSD for solids. To show that our analysis of spin-unpolarized functionals is sufficient, we also compute spin-polarization energies for atoms, finding PW91 values only slightly more negative than LSD values. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
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  • 243
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Ab initio calculations at the MP2/6-31G*//HF/3-21G* level have been carried out to study Diels-Alder reactions of 2-substituted-1,3-dienes with sulfur dioxide. The CH ··· O electrostatic interaction detected in some of the transition structures located could be decisive in the control of the exo/endo stereoselectivity of this type of reaction. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 244
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 245
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 501-518 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A recently proposed pair population scheme is investigated for ab initio SCF and spin-coupled wave functions. All the required expressions are rederived, in very compact form, and efficient computational schemes are presented for their evaluation. As representative examples of different types of bonding, pair populations and related quantities are examined for H2, LiH, BeH, CH4, H2O, and B2H6. Various “effective valencies” are also considered. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 246
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 519-532 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: On the grounds of results from quantum chemical calculations, we introduce spin-catalysis as a new concept, and classify different types of chemical reactions according to this concept. We propose a general definition of spin-catalysis as phenomena in which chemical reactions are promoted by substances which assist in overcoming spin-prohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst. A number of known phenomena fall into this definition and can be classified according to two main categories: (1) spin-orbit coupling induced and (2) paramagnetic-exchange induced spin-catalysis. Other types of spin-catalysis are also discussed: (3) processes with participation of substances which assist in (a) photochemical and (b) thermal or electrochemical generation of active particles by energy and - by electron transfer [radicals, diradicals, O2(1Δg), etc.], reacting further without spin-prohibition: (4) processes induced by an external magnetic field. Processes (3) and (4) are quite general and well known; their inclusion in the spin-catalysis classification does not introduce any new findings for the chemical kinetics, but the general features, which unite them with the important catalytic processes of the first two types, serve as a useful guide in catalysis theory. Few models of spin-catalytic processes have been simulated for the purpose of illuminating the principles of spin-catalysis; the cis-trans isomerization of ethylene catalyzed by molecular oxygen, the external heavy atom effect in ethylene photochemistry, and some others. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 247
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 533-542 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The performance of numerical basis sets in relation to Gaussian basis sets is examined, by studying 20 small sulfur-containing molecules. The results of geometry optimization calculations are reported for each molecule using both density functional and Hartee-Fock methods. In comparison with experimental data, it is shown that the use of numerical bases tend to overestimate structural parameters, particularly bond lengths, and, in most cases, more than Gaussian basis sets. It is also shown that the use of a larger Gaussian basis set in DFT calculations has the effect of reducing bond lengths. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 248
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 543-550 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Vibrational population factors for the nascent Penning ions HD+ (v′)(… He) and energy of the corresponding Penning electrons are calculated for the ionization event He(23S)(SINGLEBOND)HD(v′ = 0) → [He … HD+(v′)] + e- taking place at a range of the He*(SINGLEBOND)HD separations and orientations accessible by the system during thermal energy collisions. The vibrational population factors are obtained from the local widths of the He(23S)(SINGLEBOND)HD(v′ = 0, N) state with respect to autoionization to HD+(… He) in its v′th vibrational level. The initial overall picture of the autoionization event is consistent with the He(23S)(SINGLEBOND)H2(v′ = 0) one. On the other hand, the vibrational population factors are different from the approximate average populations used in initial model theoretical considerations about the Penning processes in the system. Variation of the calculated considerations about the Penning processes in the system. Variation of the calculated quantities with changes in the He*(SINGLEBOND)HD separations and orientations is found to be smooth enough to guarantee that the present data might form a sound basis for construction of analytical representations of the corresponding 2D surfaces and for future study of the dynamics of the collision system. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 249
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 235-248 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Extremal pair functions for an n-electron wave function of a closed-shell state are defined as linear combinations of spin-orbital-product pair functions that make some functionals (e.g., r212 or r-112) extremal. They are related to the natural spin geminals in the uncorrelated limit and are useful both for an analysis of wave functions in view of an understanding of the chemical bond and for the treatment of electron correlation. Numerical examples are shown and discussed for He2 as well as the 10-electron systems Ne, HF, H2O, NH3, and CH4. © 1996 John Wiley & Sons, Inc.
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  • 250
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 273-285 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We explain that the symmetrized Rayleigh-Schrödinger (SRS) perturbation theory formula for intermolecular interaction energies cannot be derived under assumptions which are generally true, and that the formula must generally become indeterminate when the perturbation expansion is carried to infinite order. Although one can use a limiting procedure to calculate the interaction energy to infinite order, there is no unique value for this limit when the perturbation expansion is divergent, which is generally the case. The exact ground-state interaction energy is among the possible limits, but there is no known prescription for reaching that limit. For LiH we present numerical results for one of the possible limits, showing that the SRS energy formula can give very inaccurate results over a range of nuclear separations, its accuracy decreasing rapidly with increasing separation. We review reasons for believing that the SRS theory can, however, give results of useful accuracy when truncated to second order in the energy. Because this study completes our demonstration that the simplest intermolecular perturbation theories are generally invalid when carried to infinite order, we review the alternative theories that promise to be valid. © 1996 John Wiley & Sons, Inc.
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  • 251
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 321-329 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Extended Fokker-Planck (FP) equations are generalized stochastic equations which describe the evolution of a set of coordinates, loosely referred to as solute, coupled with a relevant set of solvent variables. They are useful for the analysis of molecular dynamics in liquids, when a time-scale separation between probe motions and relaxation times of interactions with surroundings particles cannot be performed, because of persistence of slowly fluctuating components. In this article, we focus attention on a model system, made up of an angular coordinate and its conjugate momentum, submitted to a bistable potential, and coupled to a dissipative harmonic mode, to investigate the influence of polar solvents on reactive dynamics. The results are appropriate to describe dielectric effects, solvent-controlled conformational changes involving charge transfer which occur in photophysical processes, and the dynamic Stokes shifts observed in time-resolved fluorescence experiments. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 252
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 347-358 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We developed in this article a spin-adapted formulation of the coupled-cluster based linear response theory (CC-LRT) for computing double-ionization potentials (DIPs), which may be experimentally observed by Auger spectroscopy. CC-LRT is a multireference generalization of the CC theory where the energy differences have no disconnected vacuum (core) diagrams, signifying core-extensivity. For the spin-adaptation of the CC-LRT equations for the singlet and triplet manifolds, we used the Young-Yamanouchi orthogonal spin-eigenfunctions. The orbital version of the CC-LRT equations are then automatically generated by the conjugate projection operators of Young-Yamanouchi spin functions. We illustrated the working of our spin-adaptation procedure by confining our CC-LRT equations to the space of 2h and 1p-3h ionized determinants. As numerical application of our formalism, we computed the Auger kinetic energies of HF and H2O. We also analyzed the nature of size-extensivity of the DIPs generated by CC-LRT and showed explicitly that when the molecule is composed of two noninteracting fragments the computed DIPs are either DIPs of fragment A or B or a composite DIP depending on both A and B, which are just not sum of ionization potentials (IPs) of A and B. This analysis is done to underscore the fact that DIPs from CC-LRT is only core-extensive and not fully extensive. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 253
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 421-431 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The chemical bond in the hydrogen molecule is examined using the electron density and the generalized overlap amplitudes. Logarithmic derivatives of the electron density provide a clear picture of its behavior in the bonding region as well as in the outer region. The GOA expansion of the density is used to examine the dependence of the rate of decay of the density on the GOA ionization potentials. The increase in the electron density at the nuclei and in the bonding region coincides with the higher ionization potential of H2 over the H atom. The density in the bonding region along the internuclear axis does not decay exponentially, but its shape is very nearly an inverted Gaussian. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 11 Ill.
    Materialart: Digitale Medien
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  • 254
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 467-473 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The adiabatic energy surfaces of the lowest three electronic states [2(2A′ and 2A′)] and 2Σ+[2A′] of the C2F radical were investigated by the Hartree-Fock multiconfiguration self-consistent field (HF - MCSCF) ab initio method using a large set of atomic natural orbitals (ANO) and an extended configuration space, and the results were shown to be in agreement with the predictions of valence theory for this radical. The electronic ground state was found to have a bent equilibrium structure, hence contradicting the Walsh rule which predicts for the isoelectronic molecules a 2 linear state. The three states were found to be nearly degenerate and the potential energy surfaces of the two lowest electronic states exhibit an avoided crossing at an energy ∼2000 cm-1 above the ground-state minimum, lower than the highest vibrational fundamental. The strong adiabatic interaction which is responsible for the ordering of the electronic states and their equilibrium geometry involves not only the bending coordinate as normally found for Renner-Teller pairs of states, but also the C - C stretching coordinate, due to the near degeneracy of the 2Σ+ and the 2 lowest electronic states at linear geometries. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 255
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 493-504 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In this article, we present a study of the localization and properties of the molecular orbitals (MOs) of polyatomic systems by using a comprehensive version of the G1 model. In this version, the wave function is written as a DODS product of univocally determined spin orbitals (MOs), “projected” on the singlet ground state. A procedure for determining the MOs is given and applied to the BeH2 ground state. Equivalent split shell and localized MOs are found. The Be orbitals are seen to exhibit sp hybridization and the localized valence MOs are found to produce - 13.7 kcal/mol localization energy. Multistructural calculations are carried out and show that the present approach is able to describe localized and well-oriented bonds whenever the molecule under study presents only a single well-defined nonresonant chemical structure. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 256
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 529-534 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The 13C shielding of the isotopomers CH3Cl, CH2 DCl, CHD2Cl, and CD3Cl has been calculated for a range of temperatures from an self-consistent field (SCF) shielding surface computed by Buckingham and Olegario. It is found that each successive deuterium substitution increases the shielding by about 0.19 ppm and that a very slight nonadditivity occurs. The principal factor which governs the nuclear motion correction for each isotopomer is the stretching of the bonds with both first- and second-order terms being significant. Angle bending contributions are very small at first order but quite substantial at second order. Not only should the 13C-isotope shifts in this experimentally uninvestigated series be easily measured but the temperature dependence of the shielding in any one isotopomer should be observable provided that careful measurements are made. The 13C-shielding difference between CH3 35Cl and CH3 37Cl has also been calculated and is found to agree well with experiment. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
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  • 257
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 567-578 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Complete geometry optimizations using ab initio SCF/3-21G* calculations were performed on silatrane and azasilatrane, on their fluoro derivatives, and on methylsilatrane. By comparison with optimized geometries obtained for a series of model systems, the predicted transannular internuclear distances were interpreted in terms of the combined effects of electronegative substituents on Si and the anomeric interaction in the silatranes. The predicted geometries indicate that there is a weak transannular interaction in silatrane and a more significant one in azasilatrane, the predicted equilibrium distances being 2.66 and 2.15 Å, respectively. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 258
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 259
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 609-616 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Coupled Hartree-Fock theory is used to compute and map the current density induced in planar hydrocarbons by an external magnetic field. Results of useful accuracy can be obtained with modest (6-31G**) basis sets by employing a continuous gauge transformation. Maps are presented and discussed for benzene, naphthalene, anthracene, tetracene, pentacene, heptacene, and biphenylene. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 11 Ill.
    Materialart: Digitale Medien
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  • 260
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 641-648 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Trigonometric basis sets are used in a Rayleigh-Ritz variational method for computing two-sided eigenvalue bounds of the Schrödinger equation in one dimension. The method is based on truncating the infinite interval and solving an eigenvalue problem which obeys the von Neumann and the Dirichlet boundary conditions, respectively. Highly accurate numerical results are presented for the asymmetrical two-well oscillators. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 261
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 667-680 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In the previous article we introduced a method to obtain an ensemble density describing a molecule in a crystal from diffraction experiment structure factors. Here the method is applied to the CoCl42- molecular ion in a Cs3CoCl5 crystal for which accurate magnetic structure factors are known. First, the approximations involved in the interpretation of polarized neutron experiment are reviewed with special emphasis on the collinearity approximation which has been avoided in this work. Second, the derivation of magnetic structure factors corresponding to theoretical ensemble densities is explained (the spin and the exact orbital contributions have been included). Third, the fitting procedure is presented and results at different levels of approximation are discussed. The main conclusions are: (1) A density built by using several molecular wave functions can give a very good agreement with the experimental data. (2) The ensemble representability constraint is necessary to retrieve physical information from the optimized parameters. (3) Taking into account the proper paramagnetic contribution to the magnetization improves significantly the agreement between theory and experiment. (4) Neglecting the diamagnetic contribution and the fact that the magnetization may be locally noncollinear to the applied external field is fully justified for the system under study. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 262
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 699-708 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Electrostatic potential maps of certain biologically important molecules (guanine, adenine, cytosine, thymine, and two conformers of glycine) were studied employing a “hybridization displacement charge (HDC)” correction to Löwdin charge distributions and using the conventional Mulliken charge distributions of the molecules. In these calculations, the magnitude of each of the point charges was distributed in 3-dimension in a spherically symmetric manner. The method of computing HDC using MNDO wave functions was optimized by introducing two adjustable parameters. It has been clearly demonstrated that HDC-corrected Löwdin charge distributions of the molecules mentioned above are able to reproduce most of the important features of their ab initio MEP maps, while the conventional Mulliken charge distributions fail in many respects. Thus, HDC-corrected Löwdin charge distributions are shown to be quite reliable for the study of molecular electrostatic properties, particularly for molecules with more than one competing electrophilic site. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 263
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 715-715 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 264
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 717-729 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An alternative approach to secular problems for Hamiltonian matrices H of regular quasi-one-dimensional systems is suggested. The essence of this approach consists of the inverted order of operations against that of the traditional solid-state theory, viz., taking into account the local structure of the system is followed by regarding the translational symmetry of the whole chain. The first step is performed by reducing the initial system of secular equations into an effective N × N-dimensional secular problem, wherein a single equation corresponds to each of N elementary fragments of the initial chain. An implicit form of the dispersion relation and the level density function follow directly from the reduced problem without passing into the delocalized description of the system. The resulting eigenfunctions of the matrix H prove to be expressed as the Bloch sums of N nonorthogonal eigenvalue-dependent local-structure-determined orbitals of algebraic form, each of them corresponding to a definite elementary fragment of the chain. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 265
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 657-665 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The ground-state electronic structure of heliumlike systems can be described by several simple wave functions expressed in terms of the three distances between particles (one nucleus and two electrons). The dependence of (e, 2e) ionization cross sections on the description of radial and angular correlations between target electrons is analyzed. The cases of plane wave and Coulomb wave representations of ejected electron trajectories are examined together with the feasibility of analytic calculations. The relative sensitivity of (e, 2e) cross sections to the description of correlations between K-shell electrons is also investigated as a function of nuclear charge. Existing experimental data on helium are used to support a comparison with calculated (e, 2e) cross sections. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 9 Ill.
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  • 266
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 825-832 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The G and F matrices in the molecular vibration problem, the secular matrix in Hückel calculation including some nonneighbor interactions, and the Fock matrices at any stage of iteration in the Pariser-Parr-Pople (PPP) calculations on cis- and trans-butadiene, benzene, and s-triaminobenzene systems have been shown to be factorizable by representing them graphically and then applying the generalized technique of splitting of graphs with n-fold symmetry. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 267
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 959-969 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The Wiener index is a graphical invariant that has found extensive application in chemistry. We define a generating function, which we call the Wiener polynomial, whose derivative is a q-analog of the Wiener index. We study some of the elementary properties of this polynomial and compute it for some common graphs. We then find a formula for the Wiener polynomial of a dendrimer, a certain highly regular tree of interest to chemists, and show that it is unimodal. Finally, we point out a connection with the Poincaré polynomial of a finite Coxeter group. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 268
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 1-2 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: No Abstracts.
    Materialart: Digitale Medien
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  • 269
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 109-119 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Accurate ab initio CASSCF, CASPT2, and DFT computations have been performed on three different model systems which emulate the oxygenated active site of hemocyanin (a Cu+ - Cu+ dimer that binds oxygen as peroxide to form oxyhemocyanin). The three models differ in the number of the ammonia molecules (0, 4, and 6 molecules, respectively) which emulate the real histidine metal ligands of the protein matrix. While the CASSCF computations indicate that the ground state wave function of the oxyhemocianin active site is in all cases a singlet, the CASPT2 and the DFT approaches provide a significantly different description and suggest that the greater stability of the singlet versus the triplet state (experimentally observed) is not an intrinsic property of the oxygenated form of the hemocyanin active site but depends on the presence of ligands on copper atoms. These results indicate that the dynamic correlation contributions (included in the CASPT2 and DFT methods) are essential to obtain a proper description of these systems that cannot be correctly emutated using models where metal ligands are not included. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 270
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 123-132 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The theoretical underpinnings of the linear combination of Gaussian-type orbitals (LCGTO) calculations of the density functional (DF) energy of molecules and clusters are described. The generating function for three-center integrals of arbitrary angular momenta is given in the solid-spherical-harmonic basis. Variational fitting is described and its accuracy tested. The LCGTO-DF method is used to address questions related to the problem of how it is that the methods of cluster science, i.e., high-energy beams or currents, can be used to make C60 in bulk quantities. In particular, it is shown that C60 is neither especially stable nor is it the only large, stable, perfectly round, approximately sp2 carbon molecule. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 271
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 133-146 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: When conventional matrix algebra is used to solve the semiempirical self-consistent field equations for large systems, the time required rises as the third power of the size of the system. A consequence of this is that self-consistent calculations of large systems such as enzymes are impractical. By using localized molecular orbitals instead of matrix methods, the time required for these systems can be made almost proportional to the size of the system. In partial geometry optimizations, the time required depends only upon the size of the fragment being optimized and is almost independent of the size of the whole system. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 272
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 147-152 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The performance of effective core potentials adjusted at the Hartree-Fock level but applied in density functional calculations has been tested in a set of calculations using various basis sets and/or core potentials. Test molecules have been the first-row transition-metal carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4 and the second-row carbonyls Mo(CO)6, Ru(CO)5, and Pd(CO)4. Only “small-core” potentials have been used, and these are able to reproduce molecular structures and bond energies from all-electron calculations. Relativistic effects have been estimated for the second-row carbonyls by using quasi-relativistic core potentials. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 9 Tab.
    Materialart: Digitale Medien
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  • 273
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 153-159 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A coherent computational scheme on a very large molecule in which the subsystem that undergoes the most important electronic changes is treated by a semiempirical quantum chemical method, though the rest of the molecule is described by a classical force field, has been proposed recently. The continuity between the two subsystems is obtained by a strictly localized bond orbital, which is assumed to have transferable properties determined on model molecules. The computation of the forces acting on the atoms is now operating, giving rise to a hybrid classical quantum force field (CQFF) which allows full energy minimization and modeling chemical changes in large biomolecules. As an illustrative example, we study the short hydrogen bonds and the proton-exchange process in the histidine-aspartic acid system of the catalytic triad of human neutrophil elastase. The CQFF approach reproduces the crystallographic data quite well, in opposition to a classical force field. The method also offers the possibility of switching off the electrostatic interaction between the quantum and the classical subsystems, allowing us to analyze the various components of the perturbation exerted by the macromolecule in the reactive part. Molecular dynamics confirm a fast proton exchange between the three possible energy wells. The method appears to be quite powerful and applicable to other cases of chemical interest such as surface reactivity of nonmetallic solids. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 274
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 161-173 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Magnetic and optical properties of transition-metal complexes are governed by the ground state and the low-energy excitation spectrum of the d-shell of the central transition metal ion. These spectra are successfully fit to the crystal field theory. We present here an account of the effective Hamiltonian method recently developed to calculate the ground state and the excitations of the d-shells of transition-metal complexes and report the results of its application to some complexes of particular interest. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Tab.
    Materialart: Digitale Medien
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  • 275
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1257-1260 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In carrying out our plan for doing multicenter molecular integrals over Slater-type orbitals, it is necessary to evaluate the Löwdin α-function over a grid from the origin of the coordinate system to the displacement distance of the center of the orbital. A previous article obtained excellent results by expanding the exponentials in the α-function, for both interior and exterior regions. However, if the displacement distance multiplied by the screening constant, i.e., the ζa parameter, is larger than 16, we suggest that it may be more efficient in time and storage if we use the closed formula for the α-function for values of the radial distance r greater than 8. This remarkable rule of thumb was tested for a variety of orbitals up to ζa=64 and one to ζa=128. Also, in the exterior region, the formula may always be used if ζa≥16. This strategy necessitates using the formula in quadruple precision arithmetic.   © 1996 John Wiley & Sons, Inc.
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  • 276
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1261-1270 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We have already developed the many-electron wavepackets (MEWP) method in order to study the dynamics and electronic structure of molecular systems. We extended the MEWP method to study the nonadiabatic effects and formulated a nonadiabatic molecular theory, where both electron and nucleus are treated equivalently. Then we applied our method to the isotope series of hydrogen molecule i.e., H2, HD, and D2, and calculated the total energy and the average distance between nucleus-nucleus, electron-electron, and nucleus-electron in order to analyze numerically the nonadiabatic effect in the molecule. Finally we calculated the real-time evolution of the polarization by means of Chebyshev scheme; and by Fourier transforming this, we found out the excitation spectrum of the system, which corresponds to the electronic excitation and the nuclear vibrational frequency. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 277
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1311-1318 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Dirac bra-ket notation is introduced for the Whittaker cardinal (Sinc) functions and a previously unreported completeness relation for these quantities is presented and derived. With the use of this completeness relation it becomes simple to transform to a Sinc-basis the eigenvalue equations arising from a light-cone quantization of field theory or the similar equations occurring in nonrelativistic quantum mechanics. The simplicity and power of Sinc-function expansions is illustrated by computation of the eigenvalues and eigenfunctions of the position-space planar Coulomb equation, a problem for which convergence has not been achieved by a variety of other computational methods. © 1996 John Wiley & Sons, Inc.
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  • 278
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 911-930 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The mivazerol and three of its methylated derivatives are studied at the ab initio SCF level within the MINI-1 and 6-31G basis sets. This study aimed at determining some probable conformations, either neutral or protonated, that could interact with the α2 adrenoceptors. The solvent effect was also studied within the Onsager's solvent model at the two dielectric constant (ε) values of 4 and 80. The interaction with the environment (either solvent or receptor) is schematically studied by positioning either two water molecules or one formamide or/and one formic acid near the amide and the imidazole for few isomers. The medium polarization, through ε, and the solvation effect, through the interaction with the solvent molecules or the receptor side chains, stabilize the same isomers. Post-SCF calculations are performed at the CISD level, the first excited singlet and triplet states energies are determined, and the question of the wave-function stability is addressed. The results indicate the probability of a spin-orbit coupling with the first excited triplet state, thus opening the question of such a possibility within an enzyme active site. © 1996 John Wiley & Sons, Inc.
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  • 279
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 941-941 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 280
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1001-1005 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A simple method of calculation of the integrals of the Hylleraas-Ore wave function, used in the description of the four-particle system analogous to the H2 molecule, is presented. The integrals corresponding to the kinetic energies of particles, to the Coulomb energy terms, and to the interparticle distances are given by the appropriate partial derivatives of a functional depending on the wave-function parameters. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 281
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1037-1044 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The geometries and the energies of conformers of PnHn + 2 (n = 2-9) have been studied with PM3 method. To test the quality of the semiempirical results, ab initio calculations have been carried out on P3H5. All results of P2H4 and P3H5 with PM3 are consistent with the experimental and ab initio data. According to the analysis of P3H5 and P4H6 results, it is concluded that gauche interaction between adjacent lone electron pairs and gauche interaction between polar P-H bond with adjacent polar P-P bond are important for predicting the stable conformer of open-chain phosphoanes. The calculations of PnHn + 2 (n 〉 4) give further support to this conclusion. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
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  • 282
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1015-1026 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The effective core potential (ECP), using a basis set of different qualities, and ab initio full-electron (FE) calculations were carried out for MoS-24, MoO-24, and MoOCl4 molecules. The topology of - ▿2p(rcp) (the negative Laplacian of the charge density at its critical points) in the atomic valence shell was studied. Results clearly indicate that semicore (ECP2) approaches are able to reproduce, in a qualitative way, the topology of the Laplacian distribution with respect to those obtained by the FE method. Modifications of basis sets, such as introduction of polarization functions on the ligands, affect the electronic charge distribution (number of critical points in MoOCl4) for FE as well as for ECP2 approaches. The ECP2 scheme predicts correctly the order of - ▿2px(rcp) (X = O, S, Cl, Mo) in the valence shell; nevertheless, it fails in the relative magnitudes of - ▿2pMo(rcp) between Mo compounds in respect to FE calculations. A scaling factor consistently improves the values of - ▿2p(rcp) and p(rcp), which are larger than those obtained with FE, particularly the - ▿2p(rc) values. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
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  • 283
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1045-1055 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Numerical experiments were carried out to determine the timewise self-consistency of different physical processes involved in the energy transfer in green plant photosynthetic units. A 6 × 6 × 6 array of chlorophyll-a with cubic lattice constants a = b = c = 20 Å was chosen as a model of the thylakoid disc. Another model aggregate was obtained by substituting chlorophyll-b molecules for some of the chlorophyll-a molecules. In both models, a reaction center occupied a central site in the last xy plane. Two extreme arrangements were considered for the orientation of molecules. In one, the transition moments of all molecules were directed along the y-axis. The other had chlorophyll molecules randomly oriented. The four resulting model systems were used in our investigation on exciton generation, transport, decay by fluorescence, and trapping. All excitons were assumed to be generated by a 20 ms exposure to sunlight at high altitudes. The general trends noticed from these computations are as follows: The number of excitons generated is influenced by lattice disorders. Disorders also increase the time for the establishment of an equilibrium distribution. The decay of excitons by fluorescence is always a monotonic function of time. The energy transfer is adversely affected by a lower degree of orientation in the crystal: The trapping time increases with disorder. The number of trappings decreases with the onset of fluorescence of the host molecules and the trap. From these investigations, we also made three specific observations: (1) The efficiency of exciton utilization varies from 12% for a completely random arrangement of transition dipoles to 46% for a perfectly ordered arrangement. This agrees with the experimental efficiency, about 20%. (2) The number of excitons trapped varies from one to six. This tallies with the time scale of electron transfer along the Z-scheme that requires at least two excitons trapped in about 20 ms. Thus, the photon density and the exciton transfer rate are consistent with the rates of electron transfers. (3) The trapping rate also indicates that the thylakoid disc must resemble a considerably ordered system. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 284
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1393-1401 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Starting from a formally exact density-functional representation of the frequency-dependent linear density response and exploiting the fact that the latter has poles at the true excitation energies, we develop a density-functional method for the calculation of excitation energies. Simple additive corrections to the Kohn-Sham single-particle transition energies are derived whose actual computation only requires the ordinary static Kohn-Sham orbitals and the corresponding eigenvalues. Numerical results are presented for spin-singlet and triplet energies. © 1996 John Wiley & Sons, Inc.
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  • 285
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Experimental observations of the photoinduced excited-state lifetime of cis-stilbene have shown a distinct dependence on solvation processes. The rate of decay, dominated by a cis-trans isomerization, is more rapid in polar solvents than in nonpolar solvents. Linear solvation energy relationship (LSER) techniques show that this can be explained in terms of polarity and polarizability parameters for the solvent. Theoretical linear solvation energy relationship (TLSER) analysis shows that this can be explained in terms of solvent polarizability and electrostatic basicity. New TLSER descriptors based on calculated solvent bond diplole parameters are also successful in describing this solvent dependence. Solvent-dependent dipole moments are calculated for an approximate stilbene transition-state geometry using the polarizable continuum model (PCM), which suggests the usefulness of a more detailed study of this photoisomerization process using current solvation theory and computational techniques. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 286
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1633-1642 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Ab initio electron-correlated calculations of the equilibrium geometries, dipole moments, and static dipole polarizabilities are reported for benzene and 12 heteroaromatic six-membered rings obtained from it by aza-substitution. Our geometries and dipole moments agree well with available experimental microwave determinations. The polarizabilities are in reasonable agreement with the fragmentary experimental data available. Uncoupled Hartree-Fock calculations indicate that as much as half the polarizability comes from the σ-electrons. Simple empirical formulas based on atom- and bond-additive models correlate the calculated polarizabilities of 33 five- and six-membered heteroaromatic rings (10 azoles, 10 oxazoles, 13 azines) quite well. The correlation improves significantly if systematic data of uniform quality are used. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 287
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1673-1677 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The SCMEH-MO method with average relativistic and spin-orbit effects has recently been applied to study the electronic structure and bonding in samarium pentamethylcyclopentadienyls. In this report the same approach has been utilized in studying the electronic structures of Nd and Sm hexacarbonyls. In contrast to the stable transition metal d-block carbonyls, these lanthanide carbonyls are found to be quite unstable. These findings are based on calculated electronic structures and bond energies. © 1996 John Wiley & Sons, Inc.
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  • 288
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1701-1707 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We investigate the fission process Ag2+23 → Ag+12 + Ag+11 in order to compare total energies that calculated by the shell correction method and jellium models. A cavity potential and a Woods-Saxon-type potential are used as the shell potential for the shell correction method, with which the single-particle energy levels are calculated. Shell corrections are obtained by using the Balian-Bloch formula and by smoothing the discrete energy levels in the shell potentials. The jellium model calculations are performed in the framework of the local spin density functional approximation. The conventional jellium model and stabilized jellium model are used. Although the qualitative agreement between the shell correction method calculations and the stabilized jellium calculations is very good, an improvement of the liquid drop energy will be necessary for the satisfactory quantitative agreement. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 289
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1559-1566 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Self-consistent electronic structure calculations of δ-doped quantum wells (QW) in the presence of in-plane magnetic fields B up to 20 Tesla are carried out within the frameworks of the effective mass and the local density approximations. QWs composed of two layers of Ga1-xA1xAs, separated by a layer of GaAs with a donor δ-doped sheet in the center, are considered. The width of the GaAs layer was varied from 100 to 400 Å. It is shown that the diamagnetic shift increases with the increasing of the GaAs QW width. The magnetic field induces remarkable changes in the energy dispersions of electrons and holes, along an in-plane direction perpendicular to B. The most striking effect occurs in the nature of the band gap of these systems. We found that the valence band displays a double-maximum character instead of a single maximum at the center of the Brillouin zone. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 290
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1577-1583 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The interaction on and the rotation of C60 in akali-doped C60 solids, AXA′3-XC60 (X = 1, 2, 3; A, A′ = alkali), have been calculated with Buckingham potential model. The results show that the total interaction on C60 changes dramatically when the pure C60 solid is alkali-doped into K3C60. The interaction on C60 in K3C60 is about 20 times greater than that in pure C60. And the main component in the former, occupying 〉 90% is electrostatic, while in the latter, the main components, occupying 〉 90%, are dispersive and repulsive. The results also show that in contrast to the whole-region free rapid rotation of C60 molecule in its pure solid, the rotation of C60 in K3C60 is mostly forbidden due to a 10 times increase (reaching about 300 kJ/mol) in potential barrier, except for the region from 0° to 50° where a broad, smooth, and shallow potential well exists. Calculations for alkali-doped complexes other than K3C60, i.e., AXA′3-XC60 (X = 1, 2, 3; A, A′ = K, Rb, Cs), come to the same conclusion. Finally, an interesting and meaningful result is that the superconducting transition temperatures of AXA′3-XC60 (X = 1, 2, 3; A, A′ = K, Rb, Cs) change inversely with the total interactions on C60. © 1995 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 291
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1623-1631 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Multireference configuration interaction wave functions with single and double excitations were calculated for the 1Σ+g ground state of the C2 molecule and the excited states of C+2 with symmetries 2Σ+g, 2Σ-u, 2Πu, and 2Πg. The corresponding σg, σu, πu, and πg valence Dyson orbitals were calculated. Most of the density due to the valence electrons is accounted for by three σg, one σu, and one degenerate pair of πu Dyson orbitals. Electron correlation plays an important role in the bond strength of C2 by increasing the occupation of the σg valence orbitals and decreasing the occupation of the σu and πu valence orbitals. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
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  • 292
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1661-1671 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: For formaldehyde, the C — O stretch potential of 1(π, π*) crosses all 1A1 Rydberg potentials, such as n, 3py, n, 3dyz, etc., thereby transferring the intensity of the unassigned 1(π, π*) ← X??? system to these Rydberg states. For thioformaldehyde, the situation is similar but a shift in the potentials allows for direct observation of 1(π, π*). In its 1(π, π*) state, H2CO is planar, having a low barrier of about 0.2 eV toward the nonplanar 1(σ, π*) state. For H2CS, the planar conformation of 1(π, π*) is a saddle point, with 1(π, π*) being the global minimum on the 21A′ surface. The triplet π, π* states of H2CO and H2CS are nonplanar, having inversion barriers of 0.1 and 0.05 eV, respectively. For both H2CO and H2CS, the π, π* configuration also crosses the ground-state configuration, which explains predissociation and radiationless transitions of some Rydberg states. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 11 Ill.
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  • 293
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1789-1795 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A theoretical analysis based upon large-scale self-consistent Hartree-Fock calculations at a semiempirical quantum theory level (CNDO/S) is performed to investigate long-range electron transfer in a donor-DNA-acceptor molecule, where the donor and acceptor moieties are tethered to the DNA. The π-stacked base pairs are found to dominate the long-range electronic coupling. Despite the π-electron mediated coupling, the exponential distance decay constant of the electron transfer rate is ∼ 1.2-1.6 Å-1, values typical of electron transfer proteins. The calculated long-range electron transfer rate of the order of 106 s-1 for a metal-to-metal distance of 21 Å is found to be in agreement with kinetic measurements by Meade and Kayyem. Based on the current analysis, the π-electrons dominate the long-range electronic coupling interactions in DNA, but they do not lead to one-dimensional molecular wire-like properties. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 294
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1775-1787 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Estimation of brain uptake of solutes and drugs would be useful in designing novel therapeutic agents. Given the problems associated with fragment-based models, including the need for strict additivity, we used AM1-derived molecular parameters in attempting to generate an equation for predicting the log of the brain-to-blood concentration ratios (log B:B) for a collection of compounds. Sequential fitting of the data using 10 AM1-derived variables indicated that dipole generated the most predictive one-term equation, dipole and log P, the best two-parameter system, dipole, log P, and molecular weight produced the optimal three-term model, and dipole, log P, molecular weight, and HOMO energy resulted in the best four-parameter paradigm. These systems were statistically similar to those generated using general solvation equations, but with the use of fewer parameters. An expanded parameter set may generated models of even better predictability. © 1996 John Wiley & Sons, Inc.
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  • 295
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 461-469 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A possibility of stabilization in an external homogeneous electric field on the HeH2(21 A′) metastable state relative to its nonadiabatic decay into the 1 1A′ state is investigated theoretically. If the lowest vibrational levels of the HeH2 (2 1A′) state might be stabilized, one would try to detect the radiative transition 1 1A′ ← 2 1A′ from these levels and thus to verify experimentally the existence of the HeH2 (2 1A′) state. It is shown that the separation between the potential energy surfaces (PESs) of the states in question and the energy of their avoided crossing point do increase in the field. Moreover, it is shown that in the fields with strength up to 107 V/cm the nonadiabatic decay of the 2 1A′ state along one of the vibrational modes may be closed. However, along two other modes such decay still may occur. It is noted nevertheless that the question of such stabilization possibility in inhomogeneous fields (i.e., in ionic crystal or on its surface) is still open. The effect of the field was taken into account within an analytic two-level model. The unperturbed PESs were calculated by the CASSCF method. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 10 Ill.
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  • 296
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 517-532 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Alkali-doped polyacetylene is considered as an electron-transfer system. To estimate the reorganization energy due to bond-length changes when electrons are added or subtracted, we applied the (U)MP2 and CASSCF methods to small systems of the type H(CH)NH. The simple tight-binding (Hückel) model with bond-length-dependent resonance integrals has been applied to the same and larger polyenes. The bond lengths are obtained via the bond orders for the various oxidation states. The results agree very well with the ab initio results and experiments for small polyenes. Odd-N and even-N systems behave differently. In odd-N systems, a structural “soliton” exists in the neutral molecule. An electron can be added or removed without bond-length change. In even-N chains, with perfect bond alternation in the neutral molecule, the bond length changes when an electron is accepted occur over about 20 carbon atoms. The reorganization energy tends to a constant value (0.22 eV) as the chain length is increased. Soliton structure is studied as a function of out-of-plane torsional defects and it is found that an additional electron is localized primarily on a segment with an odd number of carbon atoms. In the presence of positively charged ions, electrons are attracted toward this charge and positive solitons formed at some distance from the perturbing ion. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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  • 297
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 298
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 59 (1996), S. 259-260 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 299
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 59 (1996), S. 271-279 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The diffusion constant of dynamics simulation data evaluated by the time-dependent displacement or the velocity autocorrelation function provides equivalent results. The diffusion constant (D) increases with the cutoff distance for electrostatic energy. An extended version of the TIP3P water model provides a proper value of D at small cutoff distance (8.5 Å); the SPC/E water model requires a larger cutoff distance (11.0 Å). Considering Langevin dynamics with a total friction γ, the Einstein relation (D ∼ 1/γ) is valid for large enough friction (γ 〉 5 ps-1) only. Heating of a protein-water system by stochastic dynamics at the boundary is studied in detail. The calculated thermal conductivity of water agrees with experiment. The thermal conductivity of a protein molecule is about a factor of two smaller. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 300
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 83-90 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A computational scheme is proposed for generating effective orbital sets for electron groups within the framework of the multiconfiguration self-consistent field-configuration interaction (MCSCF-CI) approach in the ab initio model potential method. © 1996 John Wiley & Sons, Inc.
    Materialart: Digitale Medien
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