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  • 1990-1994  (541)
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  • 1992  (541)
  • Atomic, Molecular and Optical Physics  (441)
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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 147-158 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A review of our own work on ab initio calculation of nonlinear optical properties by the finite oligomer method is presented. Various computational considerations such as geometry determination and choice of basis set are discussed as well as extrapolation to the long-chain limit. For cases where the extrapolation is problematic, two potential solutions are suggested. We also analyze several important areas ripe for investigation including the frequency-dependence, the effect of vibrational distortion, and the role of electron correlation.
    Additional Material: 3 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 573-578 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For the CF4+, PF5+, SF6+, and MoF6+ ions appearing after the F1s photoionization, the possibility of dissociation has been shown by the ab initio MO LCAO method within the Z + 1 core equivalent model. According to the calculations, the decay channel AFn+ → AFn-1+ + F(1s12p6) is energetically open for the ions. So the interpretation of the gas-phase emission FKα spectra, in which the bands are assigned to the discrete transition energies, can be unacceptable for these ions. The conditions and signs of such failure are discussed.
    Additional Material: 1 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 67-80 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The intramolecular (internal) charge sensitivity analysis (ICSA) is presented in the molecular orbital (MO) resolution, with diagonal idempotency constraints being imposed on the MO occupations, n, which can then be considered as independent electron population variables of the system for the assumed fixed shapes of MOS. The standard closed-shell (C.S.) SCF MO framework is adopted with the energy function E(n) approximated by the average energy of a configuration expression to cover fractional MO occupations. The components of the gradient of the auxiliary energy function, including the constraint terms, with respect to n, identified as negative MO internal electron potentials, vanish identically for the ground-state c.s. configuration, whereas their responses to hypothetical intramolecular charge displacements assure the system stability. An application of the ICSA to the iterative SCF procedure is suggested. Illustrative results for the water molecule are reported and general properties of molecular normal orbitals (eigenvectors of the hardness matrix) are discussed. © 1992 John Wiley & Sons, Inc.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 97-111 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 327-336 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio self-consistent field (SCF) molecular orbital calculations were carried out at the MP2/431G* level to determine the effect of substituent groups, —F, —NH2, —OH, —CH3, CH2=CH—, and H—C≡C - on the C≡P triple bond. Although there are considerable differences between the electronegativities of these substituent groups, a characteristic feature found in phospha alkyne derivatives is the weak sensitivity of the C≡P bond length to the substituent at the carbon atom. The molecular electrostatic potential analysis clearly reflects the strength of the electronegativity of these substituent groups and their ability to attract the polarizable charge from the C≡P triple bond. MEP analyzes further indicates that these derivatives of phospha alkynes are most likely to attract incoming electrophiles to the π-electron region of the C≡triple bond. Transition metals are coordinated “sideways” with phospha alkynes, and in this configuration, the lone-pair electrons of the phosphorus atom are unlikely to interact with the metal. © 1992 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 6
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This work examines the possibility of finding an electron-hole and an electron-pair simultaneously in a π-system substituted by an electron-donating (NH2) and/or electron-withdrawing (NO2) group. The contributions of various ionic [(+), ( ↑ ↓ )] structures are calculated from ab initio SCF-CI wave functions, using a recently developed general multielectron population analysis. The molecules studied are two monosubstituted ethylenes, aminoethylene and nitroethylene, and a disubstituted ethylene, the 2-nitroethenamine (push-pull ethylene) in its two configurational forms. The influence of the NH2 and/or NO2 group in delocalization and ionic (vs. covalent) character of the C=C double bond are investigated, along with examining the experimental chemist formalism of electron-pair “displacements” in several resonance structures. Analysis of mutual dependence of an electron-hole and an electron-pair, at short and long distances, leads to the conclusion that a push-pull π-system can stimulate the simultaneous existence of an electronhole and an electron-pair even for nonvicinal positions. The relationship between the electronpair distributions and contributions of the corresponding structures are also examined; the effects of electronic correlation are analyzed as well. © 1992 John Wiley & Sons, Inc.
    Additional Material: 7 Tab.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 319-331 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We explore a hybrid methodology in which the Hartree-Fock energy and density are combined with the nonlocal exchange functional of Becke (1988) and the nonlocal correlation functional of Lee, Yang and Parr (1988) to yield an estimate of the Schrödinger total energy. The resulting mean absolute deviation from a large number of experimental atomization energies, ionization potentials, electron affinities, and proton affinities is 3.86 kcal/mol. Performance comparisons are made with the recently developed G2 theory and nonconventional interpretations of the density functionals are discussed. © 1992 John Wiley & Sons, Inc.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 311-318 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We use the continuous formulation for the matrix Hartree-Fock method, called the generator-coordinate-Hartree-Fock method, to rephrase a theorem of Nakatsuji et al. concerning the improvement of the Hellmann-Feynman forces calculated with nuclei-centered GTO basis functions. We show that we do not need to increase the size of the basis set in order to obtain reliable Hellmann-Feynman forces, but just use a self-consistent set so that, starting with some s-type GTOS, the p-type GTOS are a subset of the derivatives of those s-type GTOS, and so on. We illustrate this feature in calculations on small dyatomic molecules. © 1992 John Wiley & Sons, Inc.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 673-686 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the progress in a study to determine the structure of radiation-induced Mn defect complexes in CaF2. The purpose of this study is to correlate the calculated transition energies with those measured experimentally and to determine what effect the inclusion of the external lattice field has on these transitions. Unrestricted open shell Hartree-Fock Self-Consistent Field (UHF) calculations were performed on the unperturbed F-center [Ca4F6Vac]1+ S = 1/2, the unperturbed Mn2+ center [MnF8]6- S = 5/2, S = 3/2 and the Mn2+ perturbed F-center [Mn2Ca2F6Vac]1+ S = 11/2, S = 9/2 defect clusters. A method by which the effects of the external lattice field may be incorporated in the SCF procedure is developed and used in these calculations. Several transition energies are calculated and Mulliken population results for these clusters are presented and discussed. © 1992 John Wiley & Sons, Inc.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 687-701 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the results of electronic band structure calculations of bulk ZnSe, bulk ZnS, and the (ZnSe)1(ZnS)1 strained-layer superlattice (SLS) using the ab initio factorized linear combination of atomic orbitals method. The bulk calculations were done using the standard primitive nonrectangular 2-atom zincblende unit cell, while the SLS calculation was done using a primitive tetragonal 4-atom unit cell modeled from the CuAu I structure. The analytic fit to the SLS crystalline potential was determined by using the nonlinear coefficients from the bulk fits. The CPU time saved by factorizing the energy matrix integrals and using a rectangular unit cell is discussed. © 1992 John Wiley & Sons, Inc.
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 717-724 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conventional use of pure angular momentum state representations in the algorithms for reactive scattering is discussed and the merits of alternative formulations in a finite element framework are considered with respect to possible numerical effectiveness. Four- and seven-dimensional hyperspheres are discretized as means of describing three- and six-dimensional scattering problems. © 1992 John Wiley & Sons, Inc.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 751-760 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: External field effects are introduced in both charge and orbital control terms of Klopman-Salem formalism for the study of chemical reactivity in condensed phase. An analytical expression is derived for the change of the interaction energy between a nucleophile and an electrophile from gas to solution phase. The resulting simple expression contains the effect of the external field, in terms of the variation of the electrophilic superdelocalizability index associated with the highest occupied molecular orbital (HOMO) of the nucleophile. Two classical reactions are analyzed to illustrate the usefulness and reliability of the proposed formalism. © 1992 John Wiley & Sons, Inc.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 761-771 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this article we generalize the necessary conditions for a topological definition of the concept of molecular structure, showing that it may be independent of the particular criterion chosen, as long as it fulfills some general conditions and may give rise to the concept of a category of stable chemical structures. We proceed to introduce a local metric in the objects of this category by using the properties of metric space bundles, and this allows us to propose a general system of reaction mechanisms within this framework of the theory of categories. We conclude by proposing a single diagram that encompasses the whole theory of stable structures and reaction mechanisms. © 1992 John Wiley & Sons, Inc.
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 657-665 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An example of an isolated resonance with highly non-Lorentzian line-shape has been found for the gas-surface diffraction system He(21 meV)/Cu(115 ). When the corrugation strength parameter for the surface is very slightly varied, the signature of the narrow resonance structure shifts. © 1992 John Wiley & Sons, Inc.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 667-672 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic and structural properties of the single N and the pair N2 complex substitutional impurity systems in type-IV semiconductors (C, Si, and Ge) are investigated through an ab initio LCAO cluster model. We find that the stable position for N atoms (N and N2) is off center with dislocation in a 〈111〉 direction to form sp2 bonds with the host atoms. Particularly for ionized complex N2+ there is a symmetry break lowering the point group symmetry of the system from D3d to C3v. We present also the results for hyperfine interactions over the impurity and its nearest neighbor which for isotropic terms are in fair agreement with the experiment. © 1992 John Wiley & Sons, Inc.
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 703-716 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the results of calculations of the electronic structure of α-quartz that were made using the first principles, factorized linear combination of atomic orbitals method. Results were obtained for the primitive 9-atom, and orthorhombic 18- and 72-atom unit cells. Application of this method to the calculation of the electronic structure of the neutral oxygen vacancy in α-quartz is discussed and results obtained using a 72-atom unit cell are given. © 1992 John Wiley & Sons, Inc.
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 737-749 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the present study we introduce a time-dependent wave packet method to treat electronically non-adiabatic processes explicitly in the coupled adiabatic state representation. The method is applied to the predissociation of the MgCl (A2II) state and is shown to be easy to implement, with the computational effort being comparable to the corresponding calculation in the diabatic basis. © 1992 John Wiley & Sons, Inc.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 725-736 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Surface-molecule interactions and reactions are important elementary steps of catalytic reactions. Since they involve interactions between infinite and finite systems, modelings are necessary for theoretical investigations of catalytic reactions on a surface. The cluster model (CM) is most frequently used for quantum chemical calculations but neglects the effect of the bulk solid. For including such effect, ( 1 ) embedding the cluster onto a surface (actually into a larger cluster) is a method proposed by Grimley and Pisani, and (2) dipping the adcluster (admolecule + cluster) onto the electron bath of the solid and letting the system be at equilibrium for electron and spin exchanges is another model proposed by Nakatsuji. We show some applications of the embedding cluster model (ECM) and the dipped adcluster model (DAM). We will also report in the lecture on the study of the photochemical decomposition reaction of MnO4- into MnO2- + O2. © 1992 John Wiley & Sons, Inc.
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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 879-894 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ACES II, a new program system for ab initio electronic structure calculations is described. The strengths of ACES II involve the use of many-body perturbation theory (MBPT) and coupled-cluster (CC) theory for calculating the energy, geometry, spectra, and properties of small- to medium-sized molecules. This paper gives a brief overview of the ACES II project, describes many features of the program system, and documents a number of benchmark calculations. © 1992 John Wiley & Sons, Inc.
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 807-815 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Solvation effect on the tunneling rates in the case of coupled motion of two protons along the reaction coordinate was semiempirically, CNDO/2, computed. The following molecular transition-state complexes (TSC) were studied: +AH - BH - A, where A was either NH3 or H2O and BH was HF. Solvation was modeled with water molecules attached to each side of the perpendicular axis, through the bridge HF at different distances F/O. FN and FO (on-line) distances were also varied. Lower and narrower barriers were observed with the close positioning of the solvating molecules. This effect is reflected in higher tunneling probabilities. Thus, solvation favors the tunnel phenomenon in proton-transfer processes. © 1992 John Wiley & Sons, Inc.
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  • 21
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 793-806 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An ab initio analysis of the reaction of molecular hydrogen with the Zr-imido complex (NH2)2Zr= NH is reported. Several interesting points are noted. The calculated stretching frequency of the Zr=N bond in (NH2)2Zr=NH is 860 cm-1 when properly scaled to account for electron correlation effects. The value supports the assignment of an infrared (IR) band at 865 cm-1 to the Zr=N stretch of a tetrahydrofuran (THF) adduct of the putative reactive intermediate (NHSi′)2Zr=NSi′. Although a weakly bound, H2 complex is found, the interaction is small (1.3 kcal mol-1) compared with experimentally characterized H2 complexes. Variations in bond lengths and intrinsic stretching frequencies demonstrate that π-bonding for amido (NH2) ligands can be substantial in a coordinatively saturated complex. H2 activation by the bis(amido)imido reactive intermediate is calculated to be significantly more favorable by the 1,2-addition of H2 across the Zr=N bond to form the tris(amido)hydride than a sigma-bond metathesis pathway. The transition state (TS) for the addition of H2 across the Zr=N bond of the bis(amido )imido complex is 9.8 kcal mol-1 above the charge transfer (CT) complex, (NH2)2Zr=NH = H2 at the MP2 level; the reverse process, extrusion of H2 from the tris(amido)hydride has a 28.2 kcal mol-1 barrier. The geometry of the four-center TS is of interest, deviating markedly from that of a square and being more “kite” shaped (i.e., one obtuse and three acute angles). Mulliken Bond Overlap Populations suggest that there is some interaction between the Zr and the H atom being transferred (Hl) in the various TSs studied. It is suggested that the interaction plays an important role in the ability of the Zrimido complexes, and indeed other high-valent, multiply bonded complexes, to activate X=H (X=H, C, Si, N, and H) bonds. The design of materials and catalysis precursors which enhance the metalhydrogen interaction in the TS could conceivably lead to lower chemical vapor deposition (CVD) processing temperatures and higher catalytic activities. © 1992 John Wiley & Sons, Inc.
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  • 22
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 837-852 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The implementation of the density-functional scheme requires the knowledge of the exchange and correlation potential, Vxc, as a functional of the electron density. In the local-density approximation (LDA) this potential becomes a function of the local value of the density. This ansatz breaks down qualitatively at a surface because of its neglect of long-range electron-electron correlations in the presence of strong charge inhomogeneity. This breakdown is of relevance in the context of various surface spectroscopies. We outline a scheme for going beyond the LDA without invoking gradient expansions. This scheme is based on establishing an interrelation between density-functional theory and many-body perturbation theory. In this scheme Vxc is obtainable from the knowledge of the electron self-energy Σxc. We solve an exact integral equation relating these two quantities for the electron-gas surface with use of the GW approximation for the self-energy. We establish a “nonlocal” relation between Vxc and the electron density which allows us to carry out nonlocal density-functional calculations with the same ease as LDA-based calculations. We present results of the first application of our method for the case of Al and Pd surfaces. In addition, we report on work in progress devoted to a detailed comparison of the density-functional and quasiparticle pictures of electronic excitations at a metal surface. © 1992 John Wiley & Sons, Inc.
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  • 23
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 377-391 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: After a brief discussion of the deformation of the ground-state electron density asymptotically, in free space homonuclear diatomics by dispersion interactions, dimers in dense homogeneous plasma will be treated. Polarization interaction will be shown to lead to an R-4 interaction energy at large internuclear separation, R, supplementing the well-known long-range oscillatory interaction. The relevance of these interactions, when suitably modified for a heterogeneous environment, to lateral forces between CO molecules chemisorbed on a transition metal surface will then be considered, with specific reference to experiments on thermal desorption. Finally, the theory of the inhomogeneous electron liquid in intense magnetic fields will be briefly discussed, for both localized and delocalized electrons. © 1992 John Wiley & Sons, Inc.
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  • 24
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    International Journal of Quantum Chemistry 44 (1992), S. 443-453 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The He I and He II photoelectron (PE) spectra of methyl pseudohalides (MeNCO, MeNCS, MeNCSe, MeSCN, and MeSeCN) have been recorded. Assignments are based on ab initio quantum chemical calculations, and their comparison with high-resolution He I spectra and He I/He II band intensity ratios. Contradictions found in previous works are largely resolved. The strikingly different spectra of the isomeric forms are interpreted using a consistent model for a methyl-distorted NCX substructure. The effects of the heavy atoms are also discussed. © 1992 John Wiley & Sons, Inc.
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  • 25
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 487-495 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Elements of the frequency-dependent polarizability tensor are obtained from the quadratic response function (QRF). This is accomplished from equations of motion (EOM) which relate QRFS and the linear response function (LRF). The response functions are evaluated within the random-phase approximation (RPA), where the EOM are exact in a complete basis. Hence, the agreement between LRF and QRF polarizability provides a criterion for basis set selection when calculating second- as well as third-order properties. Numerical examples are provided for the static and dynamic polarizability of H2O. © 1992 John Wiley & Sons, Inc.
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  • 26
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    International Journal of Quantum Chemistry 44 (1992), S. 505-515 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potential energy surface of glycolic acid was investigated using ab-initio 4-31G SCF calculations. Geometries, energies, and wavenumbers for all seven symmetry-unique local minima are reported together with all reaction paths and their energetical barriers. © 1992 John Wiley & Sons, Inc.
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  • 27
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    International Journal of Quantum Chemistry 44 (1992), S. 543-550 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calculations of electronic structure and bonding via the SCMEH-MO method with limited Cu 3d, 4s, and 4p configuration interaction, have been carried out on the tetragonally coordinated (D2h symmetry) CuCl6(NH4)4 structural cluster unit in crystalline (NH4)2CuCl4. The results are compared with those of other calculations. The calculated electronic spectrum, electric field gradient (EFG), and magnetic data are compared with experiment. © 1992 John Wiley & Sons, Inc.
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  • 28
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    International Journal of Quantum Chemistry 44 (1992), S. 563-573 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Gas phase acrylonitrile electron affinity has recently been estimated to be in the range 0.01 to 0.02 eV. In spite of a strong theoretical prediction on the existence of stable anions for molecules having dipole moments larger than the critical 1.625 D, it has not been possible to obtain a positive electron affinity for acrylonitrile (dipole moment = 3.9 D) by ab initio SCF and MP2 calculations using 6-31G, 6-3IG*, 6-31+G(d), 6-31++G(d, p), and 6-311++G(2d,2p) bases. © 1992 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
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  • 29
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    International Journal of Quantum Chemistry 44 (1992), S. 575-585 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stability of different conformational isomers of partially deuterated radical cations of ethane, butane, and cyclopropane is studied at the HF/6-31G** and MP2/6-31G** levels. It is shown that the superposition patterns of spectra corresponding to different isomers, observed in ESR spectroscopy, are accurately reproduced by Boltzmann statistics based on differences in vibrational zero-point energies (ZPE), provided that the temperature is high enough to overcome existing barriers toward internal pseu-dorotation in the cations. For the ethane and butane cations, the most stable conformations are, as expected, those which are deuterated in the short CH bonds, while this is found not to be the case for the cyclopropane cation. The latter result is explained by shifts in the low-frequency bending modes, which counteract the anticipated isotope effect on the C—H stretching modes. © 1992 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
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  • 30
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    International Journal of Quantum Chemistry 44 (1992), S. 153-160 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work use is made of algebraic techniques developed for the evaluation of Vibration-Translation energy transfer in atom-diatom collisions to obtain the energy eigenvalues of the one-dimensional quartic oscillator. We have found that even our zero order approximation results are very close to the exact ones and when second-order perturbation theory is used, they improve even for high values of the anharmonicity parameter. © 1992 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
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  • 31
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: TheMoenergy level patterns of molecules deduced directly from the pictures of molecules (VIF method) are shown to implicitly include the overlap integrals between highly nonorthogonal AO's and to be invariant under crucial nonunitary transformations. Further, machine-computed MO levels are found to readily yield chemically interpretable information such as the nonbonding MO levels and the nature of the HOMO and LUMO once the proper invariant quantity found here is subtracted out from the numerical energies. The resulting EHT, Gaussian STO-3G, … type computed MO energy level patterns then show correspondence with the VIF-pictorially deduced level patterns obtained without point group symmetries. © 1992 John Wiley & Sons, Inc.
    Additional Material: 3 Tab.
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  • 32
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    International Journal of Quantum Chemistry 43 (1992), S. 119-134 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new formalism and algorithm is developed for solving the general-order time-dependent Hartree-Fock (TDHF) problem. It is shown that for any order a generalization of the TDHF equations can be derived where all lower-order solutions constitute a constant term. This makes it very easy to obtain high-order solutions. As the space required for the mapping of density matrices to Fock matrices in a problem of a giver order is largely reduced, we can perform the most time-consuming steps within the core memory of the machine and easily manipulate vector products via optimum routines. The second hyperpolarizability γ is obtained from the secondorder TDHF solution via a 2n rule. The formalism also allows for expressing all terms in the equation diagrammatically, which provides additional physical insight and a more systematic evaluation of terms. To illustrate the method, TDHF results are presented for trans-butadiene and carbon monoxide for several optical processes, including correlation corrections to their static hyperpolarizabilities obtained via coupled cluster (CCSD) and many-body perturbation theory. The hybrid TDHF/CCSD method provides excellent agreement with the DC-SHG experiments (χ||(2) = 11.4 x 10-32 esu/mol compared to 12.9 ± 11.4 × 10-32 esu/mol and χ||(3) = 149 compared to 144 ± 4 × 10-39 esu/mol).
    Additional Material: 6 Tab.
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  • 33
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    International Journal of Quantum Chemistry 43 (1992), S. 135-146 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio and model calculations on linear clusters of acetylene molecules have been carried out to identify the effect of weak intermolecular interaction on the intrinsic dipole polarizability (-) and the second hyperpolarizability (γ) and, in particular, to determine the three-body and higher-order effects. Ab initio and model calculations on a linear dimer show clearly that the evolution of properties from the separated limit is largely a consequence of the mutual polarization as the monomers begin to interact. This reveals that intermolecular quantum features are not significant. Furthermore, higher-order response properties play a negligible role in the evolution of lower-order cluster properties. For instance, the hyperpolarizability γ of the monomer is relatively unimportant in the evolution of α of the dimer; the change in α due to dimer formation is dictated largely by the mutual polarization via the intrinsic α′s of the monomer. With this determination, an electrical interaction model can be used to explore α and γ of long acetylene chains, and this serves as a guide to the vibronic coupling effects on hyperpolarizabilities of longchain species. We find, in particular, that the hyperpolarizability of the subunits of long acetylene chains are dramatically enhanced by intermolecular electrical interaction.
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  • 34
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    International Journal of Quantum Chemistry 43 (1992), S. 171-173 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 35
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    International Journal of Quantum Chemistry 43 (1992) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 36
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    International Journal of Quantum Chemistry 43 (1992), S. 159-169 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Components of the crystal quadratic susceptibility tensor χ(2) for second-harmonic generation are calculated for the title compound, 4-(N, N-dimethylamino)-2-acetamido-nitrobenzene. Input data are the crystal structure and refractive indices and CNDO hyperpolarizabilities; two different sets of refractive indices give similar results. The calculations yield effective polarizabilities as well as both rigorous and anisotropic Lorentz local electric fields. Susceptibility components are close to 10 pm V-1, rather smaller than those deduced experimentally and showing a weaker dependence on direction; Kleinman symmetry is mostly observed within 10%. Screened dipoledipole interactions using CNDO dipole moments amount to -80 kJ mol-1, implying permanent fields of 2 GV m-1.
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  • 37
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    International Journal of Quantum Chemistry 43 (1992), S. 175-195 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The correlation between topological features of molecular structure and physicochemical as well as biological properties of molecules were examined for some sets of compounds, using graph-theoretical desciptions. It was found that the topological similarity plays a role in the determination of molecular properties for almost all compound sets studied. As a conclusion, one can presume that the less elaborated and complicated similarity measure based on the graphtheoretical description is sufficient in the case of distinct structure-property dependence.
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  • 38
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    International Journal of Quantum Chemistry 43 (1992), S. 209-214 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We generalize the usual complex virial theorem using a procedure of complex scaling for only some of the coordinates of the system. This result can also be considered as an extension of the virial theorem obtained in a previous work using real scaling for a subset of coordinates.
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  • 39
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    International Journal of Quantum Chemistry 43 (1992), S. 197-207 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Attempts to calculate the metal-ring distance in the ferrocene molecule using a well-tried minimal basis reveal serious deficiencies in the minimal-basis SCF model of the electronic structure of organometallics. It is found that the lowest state of the molecule in this approximation is a triplet and that there is a whole manifold of “states” in the ground-state region that have broken spatial symmetry and high-spin multiplicity.
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  • 40
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    International Journal of Quantum Chemistry 43 (1992), S. 215-219 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio MP2/4-31G** calculations indicate that the most stable form of C3NH is bent and singlet and that the linear structure corresponds to a maximum. The effect of changing the CNH angle on the total energy is slight, but it is quite pronounced on the molecular polarity. The wider angle tends to increase the polarity of C3NH. MP2/4-31G** calculations predict a difference of polarity between linear and bent structures of 0.8 D.
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  • 41
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    International Journal of Quantum Chemistry 43 (1992), S. 221-250 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that in the framework of coupled cluster theory both the correlation energy per unit cell and quasi-particle band structures of polymers can be computed directly from matrix elements of the excitation operator and the two-electron integrals calculated in localized orbital basis. Further, it is described how to thake advantage of the localized nature of the orbitals applied. Ab initio test calculations on a finite model system similar to the Pariser-Parr-Pople Hamiltonian are presented.
    Additional Material: 3 Ill.
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  • 42
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    International Journal of Quantum Chemistry 43 (1992), S. 251-258 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The variation-iteration method of Svartholm has been applied to the momentum-space Schrödinger equation for one-dimensional two-electron systems. The first and second iteration momentum-space wave functions have been evaluated in analytical forms. The momentum representation of the exact Hartree-Fock ground-state wave function is chosen as the initial function. The influence of electron correlation on the distribution of momentum-space probability density has been studied. It is shown for the one-dimensional McWeeny-Coulson problem that the numerical value of the ground-state energy of the one-dimensional two-electron atom is between the minimum energy values ε1/2 and ε1.
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  • 43
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    International Journal of Quantum Chemistry 43 (1992), S. 273-280 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new way of computing two-electron integrals over Slater functions by means of optimized Gaussian expansions is proposed. Expansions up to five terms have been optimized for 1s, 2s, and 2p STOs and applied to the computation of many-center two-electron integrals. All these integrals can be quickly evaluated and the SCF energy computed from them is very accurate.
    Additional Material: 5 Tab.
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  • 44
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    International Journal of Quantum Chemistry 43 (1992), S. 259-272 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The single-particle Green's function is used to generate a new zero-order Hamiltonian. The idea to generate a new zero order from the previous zero order by incorporating perturbative corrections up to certain order is attractive since it allows an iterative procedure to repeatedly improve the results by decreasing the perturbation. In particular, in those cases where the Hartree-Fock Hamiltonian is not a good approximation to the full Hamiltonian and where perturbation theory usually does not produce sufficiently accurate results, one might hope that such a repetitive procedure ultimately yields an improved zero order and accurate perturbative corrections from this newly generated zero order. Two such approaches are investigated: first, one in which the ω-independent part of the self-energy is fully incorporated in the zero order and, second, one in which the correlation energy is incorporated in a one-electron potential in an average way. Numerical calculations are reported.
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  • 45
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    International Journal of Quantum Chemistry 43 (1992), S. 311-312 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 46
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    International Journal of Quantum Chemistry 43 (1992), S. 281-300 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A simple valence-bond approach is considered for treating extended systems of equivalent atoms. In this approach, a limited nonorthogonal basis set of atomic orbitals is utilized and only nearest-neighbor interactions are supposed. By exploiting then the translational invariance of the system, expressions for the matrix elements between antisymmetrized products of such nonorthogonal orbitals are derived in an algebraic form suitable for the numerical implementation on a computer. The one-band linear case is explicitly considered. Some possible applications of the proposed approach and its connections with model Hamiltonians such as the Hubbard and the Heisemberg ones are also discussed.
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  • 47
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    International Journal of Quantum Chemistry 43 (1992) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 48
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    International Journal of Quantum Chemistry 43 (1992), S. 301-309 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground states of atoms and molecules Li-, Be, LiH, LiH2-, and Li2 have been calculated using the n-electron wave functions built up with two kinds of geminals. As a comparison, the above systems have been calculated with the Hartree-Fock self-consistent field and the multi-configuration self-consistent field method as well. The results show that the wave functions in this work are capable of describing the electron correlations, and both kinds of geminals can be taken as a starting point in building up n-electron ground states.
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  • 49
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    International Journal of Quantum Chemistry 43 (1992), S. 313-320 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We propose a novel method for calculation of the electronic correlation energy of molecular systems in the path integral formalism. The procedure consists of saddle-point expansion of the path integral around the Hartree-Fock energy. By one loop-expansion of the molecular path integral into the generalized space phase of the molecular orbitals, the shift energy is obtained.
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  • 50
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    International Journal of Quantum Chemistry 43 (1992), S. 321-326 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio Hartree-Fock crystal orbital results were used as input for the calculation of exciton energies in the Tamm-Dancoff and random-phase approximation for polymers with helical symmetry. The calculations were applied to a hydrogen fluoride chain. We show that the Tamm-Dancoff method is a good approximation to the random-phase theory. Furthermore, the influence of the band-band interaction in the exciton calculation has been investigated.
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  • 51
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    International Journal of Quantum Chemistry 43 (1992), S. 343-352 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the present paper, the role of (n - 1)ƒ orbitals in metal-metal quadruple bonds was studied. It was shown by the calculations that the probabilities for finding the σ-, π-, and δ-electrons between two metal atoms, under the influence of the ƒ orbitals on the metal-metal quadruple bonds, increased while their mean kinetic energy components along the metal bond axis decreased. In addition, the effects of the ƒ orbitals upon the σ, π, and δ metal-metal bonds were different. In general, σ 〈 π 〈 δ.
    Additional Material: 6 Ill.
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  • 52
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    International Journal of Quantum Chemistry 43 (1992), S. 327-342 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The local origin/local orbital (LORG) method of Hansen and Bouman has been implemented with the intermediate neglect of differential overlap Hamiltonian for spectroscopy (INDO/S). The method is shown capable of demonstrating the inductive effects associated with electron-withdrawing substituents through the diamagnetic shielding term. In addition, the method is capable of differentiating chemical shift in differing bond environments. The calculated paramagnetic contribution, however, is deficient for substituents that saturate the minimal basis such as oxygen and fluorine, which severely limits the general utility of the procedure. Through the utilization of reduced linear equations for the paramagnetic term, the method is amenable to any molecule for which a self-consistent field can be performed and therefore can potentially be used to study very large systems. At present, however, the LORG method when used with the rapid INDO/S model Hamiltonian does not reliably reproduce the paramagnetic contribution to the shielding.
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  • 53
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    International Journal of Quantum Chemistry 43 (1992), S. 353-374 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Following the recent developments of the charge sensitivity analysis (CSA) in the atoms-in-molecules (AIM) resolution, the corresponding CSA quantities in the orbital (or shell) resolution (OR) are defined. The OR electron population variables, in the ordinary closed-shell SCF problem, are the elements of the bond-order matrix P, and their conjugates, “chemical potentials,” FT = ∂E/∂P, are the respective Fock matrix elements, appropriate for the representation in question; here E is the SCF energy. The second derivatives ∂2E/∂P∂P define the OR hardness tensor from which all related OR CSs, e.g., the hardness, softnesses, Fukui function (FF) indices, etc., can be determined. The rigid potentials and hardness tensor, corresponding to the “frozen” orbital approximation, are examined in more detail, and the decoupled representation of the normal orbitals (NoO) is introduced, in which the rigid hardness tensor becomes diagonal. Illustrative valence-shell NoO contours for the water molecule are given and discussed. The new approximation for the OR FF indices, as the orbital occupation probabilities, is proposed on the basis of the density matrix functional development of Donnely and Parr for natural orbitals, and the relevant expressions for the molecular fragment (collection of orbitals) quantities are summarized.
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  • 54
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    International Journal of Quantum Chemistry 43 (1992), S. 375-392 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two numerical codes, a complex face vector F and a real face vector D are developed for the characterization of square-cell configurations (lattice animals), used for representing the shapes of molecular monolayers and cross sections of molecular surfaces. The real face vector D represents all the intrinsic properties, size, and shape of the lattice animal. The complex face vector F contains complete information about the size, the shape, and also the placement of the particular lattice animal with respect to the lattice. Based on the properties of the face vectors, a method is developed for the classification of similar animals into equivalence classes. The face vector method is proposed for an algorithmic, nonvisual computer analysis of similarity of shapes of molecular monolayers and planar domains of cross sections of molecular surfaces, approximated by lattice animals.
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  • 55
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron-vibrational problem of the general nonadiabatic molecular systems has been solved by means of the quasi-particle transformations. The SCF ab initio solution of the nonadiabatic fermion Hamiltonian yields stabilization of the electronic ground-state energy due to electron-phonon interaction and it also gives the corrections to the one- and two-particle terms. Two two-particle correction yields effective attractive electron-electron interaction, but in the form different from Frölich's effective electron-electron interaction term. In contrast to the standard electron-phonon Hamiltonian of solid-state physics that does not take into account the possible effects of nonadiabaticity of a system, the presented nonadiabatic theory yields also one-particle corrections. The presence of this term in the Hamiltonian might play a crucial role in the theory of superconductivity since the superconductors are nonadiabatic systems.
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  • 56
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An exact solution for the electron-vibrational problem of the nonadiabatic molecular system has been obtained. By the quasi-particle transformation technique, the fermionic Hamiltonian has been derived and solved at the ab initio level. Results clearly and unambiguously show that the gap formation due to nonadiabatic electron-phonon coupling is mediated by the one-particle electron-phonon interaction term, whereas the two-particle one represents just a correction to the correlation energy. The temperature dependence of the gap and electronic specific heat connected with the electron-phonon coupling have also been derived.
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  • 57
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Conditions for low and high Tc superconductors following from the nonadiabatic electron-vibrational theory at ab initio level have been obtained. According to the presented results, the supercurrent is realized by the motion of the ground-state electronic-charge distribution of the fully occupied band. The motion of the electronic-charge distribution is conditioned by the newly arisen, nondissipative degrees of freedom of nuclear, Jahn-Teller-like, microflows. The Meissner effect is also interpreted.
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  • 58
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 59
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    International Journal of Quantum Chemistry 43 (1992), S. 443-443 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 60
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    International Journal of Quantum Chemistry 43 (1992), S. 449-461 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A vibrational coupling model to treat the solvation effects in chemical reaction rate calculations is proposed and applied to the intramolecular hydrogen transfer reaction CH3O· → ·CH2OH in the condensed phase. The effect of solvation is taken into account in two ways: (1) the solvent effect on the activation energy of the reaction is simulated by including 39 surrounding water molecules, represented by fractional charges at the assumed atomic positions, in the potential energy surface calculation; and (2) the vibrational couplings between the 10 nearest solvent molecules and the molecules constituting the reaction system are explicitly included in a vibrational frequency calculation. RRKM theory with Miller's tunneling correction included is employed to calculate the rate constants. The effect of solvation causes a significant change in the chemical reaction rate, mainly through a lowering of the activation energy. The vibrational coupling causes a slight increase of the rate constant in the tunneling region by perturbing the vibrational frequencies of the reactant and transition states, which appear in the rate-constant expression, but has little effect at higher temperatures.
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  • 61
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    International Journal of Quantum Chemistry 43 (1992), S. 511-529 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The sulfur mustard, HD, a chemical warfare agent, has been studied by ab initio quantum molecular computations (HF/6-31G* and 6-31G) on its various forms (neutral, ethylene sulfonium, and free carbonium ions). The geometries of these molecules have been completely optimized and the minimal energy conformations determined with their associated charge distributions. We discuss these results on electrostatic properties with respect to the mechanism of DNA alkylation by HD and compare them with our previous study of the nitrogen mustard mechlorethamine.
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  • 62
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    International Journal of Quantum Chemistry 43 (1992), S. 531-550 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new method is presented for calculating the normalization factor for continuous spectrum functions in the L2 basis. One variant of this method is applicable to operators given in the form of a tridiagonal matrix and does not require explicit calculation of the roots of the characteristic equation. The other variant of the method can be used in the general case, where after the basis is introduced, its length is gradually increased.
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  • 63
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    International Journal of Quantum Chemistry 43 (1992) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 64
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    International Journal of Quantum Chemistry 43 (1992), S. 579-589 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four esters of 5-cyano-1,6-dihydro-6-oxo-3-pyridinecarboxylic acid involving substitutions of methyl, isopropyl, tert-butyl, and phenyl groups at the 2-position, which exhibit varying degrees of cardiotonic activities, have been studied using the electric-field and electrostatic potential mapping techniques. The net charge distributions in the molecules were obtained using the MNDO method. Electric-field mapping has been performed in three-dimension in order to obtain information about the possible structure-activity relationships of these compounds. The results show a prominent electric-field region in the methyl-substituted compound extending from near the oxygen atom O6 to near the methyl group. This fact and orientation of the electric field in this region, in view of the Erhardt model, may be related to the greater cardiotonic activity of the methyl-substituted molecule in comparison to those of the other molecules of this class. © 1992 John Wiley & Sons, Inc.
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  • 65
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    International Journal of Quantum Chemistry 43 (1992), S. 647-658 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general scheme is proposed for the study of the interrelations and transformations of flexible functional groups within the configuration space M of all possible chemical species of a fixed overall stoichiometry S. The methods presented provide a concise but sufficiently detailed description of the occurrence of various functional groups in different families of nuclear arrangements, as well as their transformations into one another during chemical reactions and conformational changes. The approach uses experimental information on the importance of various functional groups, topological criteria for the flexibility of these functional groups, and the fundamental pattern of distribution of configurational families within the nuclear configuration space. The resulting schemes have simple, easily programmable representations. © 1992 John Wiley & Sons, Inc.
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  • 66
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    International Journal of Quantum Chemistry 43 (1992), S. 669-676 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cycloaddition reaction of ketene and methylenimine, leading to 2-azetidinone, has been studied theoretically by RHF/3-21G and IRC. This reaction is believed to be nonsynchronous and concerted, taking place through a twisted transition state. Four π orbitals are involved in this reaction, which is a “2 × [1 + 1]”-type cycloaddition. In the course of the reaction, rotation of the methylene group instead of oxygen in ketene was ascertained. The activated barrier is calculated to be 33.9 kcal/mol. © 1992 John Wiley & Sons, Inc.
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  • 67
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    International Journal of Quantum Chemistry 43 (1992), S. 677-699 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two things were done in this paper: (i) A generalization of Schwinger's variational principle to a subsystem was developed within the framework of quantum field theory and applied to the theory of atoms in molecules. This work generalizes the previous derivation given in the Schrödinger formulation. (ii) It is demonstrated that Feynman's path integral, when expressed in terms of the coherent-state representation, can be constructed for a subsystem of a many-electron system if a divergence term, which serves as a variational constraint in the definition of an atom in a molecule, is added to the Lagrangian. This formulation is equivalent to the atomic statement of the variational principle if the divergence term is suitably constructed. It is shown that the path integral can be expressed as a product of the individual atomic contributions to the steps along the paths with the action being determined by a corresponding sum of the atomic contributions to the action integral. © 1992 John Wiley & Sons, Inc.
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  • 68
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    International Journal of Quantum Chemistry 43 (1992), S. 701-712 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total (elastic plus inelastic) intensities of 51 keV electrons scattered by H2CO and H2CCO have been measured over a range of K = (4π/λ) sin(θ/2) = 1-9.5 Å-1 and compared with the theoretical intensities calculated with SCF and CI wave functions. Significant discrepancies are found between the experimental intensities and the theoretical ones based on the SCF wave functions. Most of the chemical binding and electron correlation effects observed in the total scattered intensities are reproduced by the theoretical intensities based on the CI wave functions calculated with the basis set including polarization functions on all atoms. © 1992 John Wiley & Sons, Inc.
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  • 69
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    International Journal of Quantum Chemistry 43 (1992) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 70
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    International Journal of Quantum Chemistry 43 (1992), S. 769-782 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The concept of a self-consistent field is developed within the version of density functional theory based on local-scaling transformations. It is shown that in this context there arise two types of consistency: one relating to the charge-consistency within an orbit and another to “orbit jumping.” The latter is analyzed in terms of one-particle equations. The connection with other methods is discussed. © 1992 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 43 (1992), S. 747-753 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the BCH theorem, we express the Hamiltonian of a Morse oscillator as a complete series of powers of the creation and annihilation operators for the harmonic oscillator. In this way, we improve the results of a previous work that uses a Bogoliubov-Tyablikov tranformation to calculate the Franck-Condon factors by means of equivalent harmonic oscillators potentials. © 1992 John Wiley & Sons, Inc.
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  • 72
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The usefulness and reliability of the recent BSSE-free SCF algorithm based on the “chemical Hamiltonian approach” (CHA/F) is demonstrated by calculating potential curves for several hydrogen-bonded complexes with 4-31G, 6-31G, and 6-31G** basis sets. It is concluded that the CHA/F scheme gives results that are numerically close to those of the Boys-Bernardi a posteriori correction scheme but are free from the “overcompensation” characteristic of the latter at smaller distances and given basis sets. © 1992 John Wiley & Sons, Inc.
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  • 73
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    International Journal of Quantum Chemistry 43 (1992), S. 783-800 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report a comparison of theoretical and experimental proton affinities at nitrogen and oxygen sites within a series of small molecules. The calculated proton affinities are determined using the semiempirical methods AM1, MNDO, and PM3; the ab initio Hartree-Fock method at the following basis levels: 3-21G//3-21G, 3-21+G//3-21G, 6-31G*//6-31G*, and 6-31+G(d, p)//6-31G*; and Møller-Plesset perturbation calculations: MP2/6-31G*//6-31G*, MP3/6-31G*//6-31G*, MP2/6-31G+(d, p)//6-31G*, MP3/6-31G+(d, p)//6-31G*, and MP4(SDTQ)/6-31G+G(d, p)//6-31G*. The semiempirical methods have more nonsystematic scatter from the experimental values, compared to even the minimal 3-21G level ab initio calculations. The thermodynamically corrected 6-31G*//6-31G* proton affinities provide acceptable results compared to experiment, and we see no significant improvement over 6-31G*//6-31G* in the proton affinities with any of the higher-level calculations. © 1992 John Wiley & Sons, Inc.
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  • 74
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    International Journal of Quantum Chemistry 44 (1992) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 75
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    International Journal of Quantum Chemistry 44 (1992), S. 1-15 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A comparison is made between the exact classical dynamic theory and the stochastic theory of reaction rates in dense media. The conditions of their equivalence are derived on the basis of a harmonic oscillator model. It is shown in a direct way that in the general case of a multidimensional system the approximate activated complex (transition-state) theory is not identical to the stochastic theory at any value of the viscosity coefficient. © 1992 John Wiley & Sons, Inc.
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  • 76
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    International Journal of Quantum Chemistry 44 (1992), S. 59-65 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Accurate lower bounds for the nonrelativistic lowest triplet states of 4He and ∞He and the He isoelectronic series up to Z = 10 were calculated by the method of variance minimization using wave functions with logarithmic terms. Furthermore, the method of Lehmann/Goerisch was extended to self-adjoint operators with an essential spectrum. Both methods were compared for 4He. © 1992 John Wiley & Sons, Inc.
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  • 77
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    International Journal of Quantum Chemistry 44 (1992) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 78
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    International Journal of Quantum Chemistry 44 (1992), S. 81-89 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Closely related structures, like esters and lactones, have vastly different physical properties. This is apparently due to differences in the intermolecular interactions. The intermolecular interactions of methyl acetate, β-propiolactone, ethyl acetate, and γ-butyrolactone have been studied using the AM1 semiempirical method. Some of the “arranged clusters” were also compared to possible covalently bound trimers and tetramers of β-propiolactone and γ-butyrolactone. © 1992 John Wiley & Sons, Inc.
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  • 79
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    International Journal of Quantum Chemistry 44 (1992), S. 123-140 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper provides a brief and systematic presentation of the basic principle and method of the maximum overlap symmetry molecular orbital (MOSMO) model and its application to simplification of molecular orbital calculation and to calculation of molecular structures and properties, together with some new results about the MOSMO calculation and new insights concerning the further extension of the principle and method. It has been shown that the theoretical method of the MOSMO model is very simple, reliable, and useful and can be employed to study the structure-property relation in even very large molecular systems. The numerical results obtained from the MOSMO calculation on various semiempirical molecular orbital approximation levels show that when the same parametrization, such as one of those employed in EHMO, CNDO/2, and HMO methods, is adopted, the MOSMOS are very close to the canonical molecular orbitals obtained from the customary LCAO method and the MOSMO calculation requires less computing time than does the LCAO method. The MOSMO calculation can be used for rapidly obtaining reasonably good molecular geometries, vibrational frequencies, and other properties of molecules by employing a simple improved semiempirical parametrization. Equilibrium geometries, vibrational frequencies, and other results are in good agreement with the experimental data and the results obtained from ab initio molecular orbital calculation. The basic calculational procedure of the MOSMO model can be extended further and has been employed to give some new results, to propose some new theoretical schemes and principles, and to introduce some new interesting theoretical problems that deserve to be studied further.
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  • 80
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    International Journal of Quantum Chemistry 44 (1992), S. 165-179 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: One long-range goal of our work has been the design of cannabinoid analgesics with reduced psychopharmacological liability. We review here the progress we have made toward this goal. When we began our work on the cannabinoids, no cannabinoid analgesic had been reported that did not also possess significant psychoactivity. We approached the cannabinoid problem by calculating molecular reactivity characteristics (MRCS) of template molecules for each activity: (-)-trans-delta-9-tetrahydrocannabinol (Δ9-THC) for psychoactivity, and (-)-9-nor-9-betahydroxyhexahydrocannabinol (9-nor-9β-OH—HHC) for analgesic activity. We then compared the MRCs of each template with the MRCS of a carefully chosen set of other cannabinoids (both active and inactive). Our focus in the calculation of MRCS was on accessible conformations of each subject molecule and on the molecular electrostatic potential generated in key planes by each accessible conformer. Our working hypothesis concerning the molecular basis of psychopharmacological activity in the cannabinoids was that activity is conferred by the set of molecular properties generated (1) by the orientation of the lone pairs of electrons of the phenyl group hydroxyl oxygen and (2) by the orientation of the carbocyclic ring and its C-9 substituent relative to this oxygen. Our working hypothesis concerning the molecular basis of cannabinoid analgesic activity was that activity is conferred by the set of molecular properties generated by the presence and relative location of two negative potential regions in the top half of the cannabinoid analgesic molecule. Our work thus far has revealed important steric and electronic features of the cannabinoids that may be used to enhance discrimination. We have shown that a simple structural feature, a C-9 substituent protruding into the α face of the molecule, can abolish or significantly diminish psychoactivity. We have also demonstrated that a free phenol group is not an absolute requirement for activity. Instead, we have found that compounds that do not possess free phenolic hydrogens, but that do maintain the orientation of the lone pairs of electrons of the “phenolic” oxygen toward the carbocyclic ring, exhibit selectivity toward analgesia.
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  • 81
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    International Journal of Quantum Chemistry 44 (1992), S. 203-218 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An extension of our previous work on the development of a conformationally dependent hydrophobic index is presented. The parameter was computed in four alternative ways based on steric parameters, such as the total molecular surface area or the solvent accessible surface, and on quantum mechanically computed electronic properties, such as the molecular dipole moment, Mulliken charge densities, and a lone-pair index. The method was parametrized based on 110 rigid analogs and all properties required were calculated using the semiempirical methods PM3, AM1, and MNDO. Examples of the application of this method in calculating hydrophobic indices are presented.
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  • 82
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    International Journal of Quantum Chemistry 44 (1992), S. 235-250 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A pharmacophore for μ-opioid receptor recognition based on a study of the fentanyl class of opioids has recently been characterized in our laboratories. To validate this pharmacophore, we have extended our theoretical studies to include four opiate analogs from structurally different classes and with high affinity but varying selectivity for the μ-opiate receptor. An extensive conformational search of the flexible regions of these compounds has been carried out at two levels of approximations, using the CHARMm force field and the semiempirical molecular orbital method AM1. In a subsequent step, we have determined a series of structural, environmental, and electronic properties for each low-energy conformer of the analogs studied. All four analogs studied can assume a low-energy conformation in which at least three of the four stereoelectronic properties identified as modulators of recognition in the fentanyls are present in the same spatial arrangement. These results provide additional evidence for the plausibility of the proposed pharmacophore for μ-opioid receptor recognition. © 1992 John Wiley & Sons, Inc.
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  • 83
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    International Journal of Quantum Chemistry 44 (1992), S. 263-275 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An extensive exploration of Met-enkephalin in its zwitterionic form has been carried out, in order to characterize the different low-energy conformational domains accessible to this pentapeptide. The study builds on previous studies carried out in our lab for the neutral molecule, which provided the initial geometries from which the conformational space of the charged molecule could be scanned. The initial conformations were subjected to a series of high- and lowtemperature molecular dynamics simulations. Snapshots along each trajectory were taken, minimized, and used as starting points in further MD trajectories until no lower-energy conformers could be characterized. The CHARMm force field was used throughout the study for this purpose. The same search strategy was used in these studies simulating two different environmental conditions, a distance-dependent dielectric ∊ = r and a high constant dielectric ∊ = 80. In the low dielectric environment, the formation of the salt bridge dominates the structure. In the high dielectric environment, the screening of the electrostatic interactions results in weaker intramolecular interactions. In both cases, the Gly2-Gly3 β-turn-type structures are preferred over the Gly3-Phe4 turns, in marked contrast to what is found for the neutral molecule. The lowest-energy structures from both environmental conditions were reoptimized in the presence of a cluster of explicit water molecules. Reoptimization of the structures considering explicit water structures did not result in significant conformational changes for the structures characterized with the ε = r or ε = 80 environments.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    International Journal of Quantum Chemistry 44 (1992), S. 301-313 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The calculations for biuret (B) and its two sulfur analogs, thiobiuret (TB) and dithiobiuret (DTB), were carried out using the ab initio LCAO-MO method at the Hartree-Fock level. The molecular structure of B was fully optimized using standard 3-21G and 6-31G* basis sets, while geometry optimizations of the TB's and DTB's were performed at the 3-21G* and 6-31G* levels. Only trans-isomers were found to be minimum structures on the HF/3-21G* potential-energy surfaces, whereas cis-forms are the first-order transition structures. The relative energies of the cis-conformers at MP 2/6-31G**//HF/6-31G* + 0.9 zero point energy (ZPE) level are equal to 47.4, 51.7, and 59.7 kJ mol-1 for TB, B, and DTB, respectively. Because of relatively large dipole moments, cis-conformers are predicted to be stabilized in polar solvents.
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    International Journal of Quantum Chemistry 44 (1992), S. 213-227 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We compute a number of properties for the 11S, 21S, and 23S states of helium as well as the ground states of H2 and H3+ using Variational Monte Carlo. These are in good agreement with previous calculations (where available). Electric-response constants for the ground states of helium, H2 and H3+ are computed as derivatives of the total energy. The method used to calculate these quantities is discussed in detail. © 1992 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 44 (1992), S. 181-205 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this article we discuss several principles and tools which should expedite description of the electrostatic potentials and electrostatic interactions of molecules, and show that these also lead to some rather remarkable results in the theory of the irreducible representations of the full rotation group SO(3). First, by representing a molecule's charge-density matrix over a basis of atomic-like orbitals (on the various atoms), we observe that outside its charge distribution the molecule's electrostatic potential is exactly the same as if that charge distribution were merely a sum (and in the case of a finite orbital basis, this is a finite sum) of point multipoles on each of the atomic centers and line multipoles on the line segments joining each of those atomic centers. Possible methods of approximating the field of these line charges and line multipoles, as if they were due to point charges and point multipoles, are discussed. The calculation of the interaction of point multipoles of high order, as is necessary for this procedure to successfully calculate the interaction of arbitrarily oriented molecules, motivates our second topic. Here we present a differential operator which, when acting on the 3-dimensional delta function, produces the source density for a scalar field that is exactly an (l,m) multipole field. Using the Hermitian adjoint of this operator, we express the interaction of this (l,m) multipole with an external scalar field as the result of this differential operator acting on that external field at the location of this multipole source. Irreducible representation matrices of the full rotation group are then used, together with these relations, to simplify the interaction of two arbitrarily oriented multipoles of any orders. Finally, we use the representation of the Condon and Shortley “raising and lowering” relations on eigenstates of the z-component of angular momentum, in an orientation that is not aligned with its fundamental basis states, to generate recursion relations that allow simple calculations of the irreducible representation matrices of the full rotation group, SO(3), and the special unitary group, SU(2). From these recursion relations we display some useful symmetry properties of our parameterization of these matrices, that allow the entire matrix to be very simply generated from an explicit calculation of only about 1/8 of its elements. © 1992 John Wiley & Sons, Inc.
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  • 88
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    International Journal of Quantum Chemistry 44 (1992), S. 359-370 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We formulate the current-density functional theory for systems in arbitrarily strong magnetic fields. A set of self-consistent equations comparable to the Kohn-Sham equations for ordinary density functional theory is derived, and proved to be gauge-invariant and to satisfy the continuity equation. These equations of Vignale and Rasolt involve the gauge field corresponding to the external magnetic field as well as a new gauge field generated entirely from the many-body interactions. We next extend this gauge theory (following Rasolt and Vignale) to a lattice Lagrangian believed to be appropriate to a tight-binding Hamiltonian in the presence of an external magnetic field. We finally examine the nature of the ground state of a strongly nonuniform electron gas in the presence of this many-body, self-induced gauge field. © 1992 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 89
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    International Journal of Quantum Chemistry 44 (1992), S. 333-345 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In recent work, we have provided a rigorous physical interpretation for the exchange energy and potential (or functional derivative) as obtained within the local-density approximation via the Harbola-Sahni formulation of many-electron theory. In this article, we analyze the gradient-expansion approximation (GEA) for these properties from the same physical perspective. The source charge distribution in this approximation is the GEA Fermi hole to O(▽3). This charge distribution is unphysical, so that the resulting force field and work done cannot be defined in a physically meaningful manner, and the exchange energy is singular. Thus, when viewed from the perspective of a source charge, the existence of the gradient expansions for the potential and energy is questionable. We next discuss the conventional method of employing a screened-Coulomb interaction to eliminate the singularities due to the GEA source charge, and show that it leads to inconsistent results. These inconsistencies are also intrinsic to a proof of the inequivalence of the Harbola-Sahni and Kohn-Sham exchange potentials within the GEA. Thus, although the inequivalence of these potentials has been established by other analyses, this proof is shown not to be rigorous. Finally, we demonstrate that when the physics of the GEA exchange source charge is corrected by the satisfaction of sum rules, the modified charge distribution then leads to a well-behaved local exchange potential and exchange energy density, and to a finite exchange energy. The consequences of our analysis on the gradient expansions for the correlation and exchange-correlation potential and energy are also noted. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 90
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    International Journal of Quantum Chemistry 44 (1992), S. 393-399 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spin-polarized LSD calculations for MoO, both all-electron and model core potential, are reported and compared with the recent spectroscopic data. The properties of the 5Π ground state and four quintet excited states are evaluated, their agreement with the experiment was found to be quite satisfactory. One of the states, 5Σ-, is approached theoretically for the first time. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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  • 91
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    International Journal of Quantum Chemistry 44 (1992), S. 401-408 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The pairwise intercorrelation between 12 topological indices is studied in benzenoid hydrocarbons with up to six rings. The functions used in the intercorrelations were linear, quadratic, cubic, power low, and logarithmic relationships. The last two relationships failed to intercorrelate the selected topological indices. The results obtained suggested that there are basically three subclasses of independent topological indices in the benzenoids studied. When contrasted with the previous findings in alkanes, our results suggest that the cyclicity leads to the stronger intercorrelation of topological indices than the branching of a molecule. © 1992 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 92
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    International Journal of Quantum Chemistry 44 (1992), S. 421-426 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: High-level ab initio studies using the 6-31 IG(3df, 2p) basis set with electron correlation included at the second-order Møller-Plesset perturbation theory are reported. At this level, full geometry optimization was performed, followed by calculation of the vibrational IR spectrum. Predicted molecular parameters (bond lengths, bond angles, dipole moment, rotational constants) and vibrational IR spectra (harmonic wavenumbers, absolute intensities) of formaldehyde and its deuterated species agree very well with the experimental data. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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  • 93
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    International Journal of Quantum Chemistry 44 (1992), S. 435-441 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We study the multiphoton absorption of the diatomic HF by time propagating a wave packet on the electronic potential under the action of a monochromatic laser field. We apply the pseudo-spectral splitoperator method for the propagation. We use different approximations for the potential and for the dipole function. We find that the quantitative details of the absorption spectrum are sensitive to the form of the dipole function. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 94
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    International Journal of Quantum Chemistry 44 (1992), S. 465-474 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Transitions originating from excited states in copper-like ions have been studied. Fine-structure oscillator strengths obtained with both the quantum defect orbital (QDO) and its relativistic (RQDO) counterpart are reported for 4p 2P - nd 2D (n = 5, 6), 5p 2P - nd 2D (n = 5, 6), and np 2P - 6s 2S (n = 4, 5 ) absorption transitions. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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  • 95
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    International Journal of Quantum Chemistry 44 (1992), S. 475-486 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We examine the effect on calculated oscillator strengths for electronic transitions caused by reintroducing the nodes of valence orbitals in effective core methods through a simple Schmidt orthogonalization. This refinement is then tested within the Intermediate Neglect of Differential Overlap (INDO) model, a valence orbital only model, in both configuration interaction and Random-Phase Approximation (RPA) calculations. It is shown that the differences in oscillator strengths calculated using the dipole-length and dipole-velocity formulations are reduced somewhat for π → * transitions and significantly for n → π* transitions. The oscillator strengths calculated from the dipole-length formalism by the RPA model are in best accord with experiment. © 1992 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 96
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    International Journal of Quantum Chemistry 44 (1992), S. 497-504 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several computational approaches, including Gaussian-2 (G2) and nonlocal density functional theory (DFT-GGA ), have been used to calculate the energy requirements for (a) H2N—NO2 dissociation (through N—N bond scission), (b) inversion of the amine group, and (c) isomerization through the nitro-nitrite rearrangement. Taking zero-point energies into account, the G2 predictions are 53.6 kcal/mole for the dissociation energy and 1.5 kcal/mole for the inversion barrier. The corresponding DFT-QGA values are 48.4 and 0.9 kcal/mole, and an activation energy of 48.7 kcal/mole for the nitro-nitrite rearrangement. The DFF-OGA results indicate that dissociation and rearrangement should be competitive for H2N-NO2. The same conclusion was reached earlier by Saxon and Yoshimine on the basis of MRCISD/6-3lG* calculations, although their computed energy requirements differ from the present ones by approximately 8 kcal/mole. © 1992 John Wiley & Sons, Inc.
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  • 97
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    International Journal of Quantum Chemistry 44 (1992), S. 517-526 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A multireference Møller-Plesset (MR-MP) perturbation method, at the second-order level, is applied to the potential energy curve of the ground state of N2, for comparison with a variety of standard ab initio methods. In spite of the drastic simplification, the MR-MP results are very reliable. The energy errors are almost independent of geometry, allowing unbiased treatment of potential energy curves. The potential efficiency and accuracy of the MR-MP approach are emphasized. © 1992 John Wiley & Sons, Inc.
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  • 98
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    International Journal of Quantum Chemistry 44 (1992), S. 527-541 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A detailed investigation has been made of the methodological dependence of computed structures, binding energies, and some vibrational properties of selected hydrogen-bonded complexes. Structures and vibrational frequencies have been computed using four different levels of theory: HF/6-31G(d), HF/6-31+G(d, p), MP2/6-31G(d), and MP2/6-31+G(d, p). Correlation energy effects on computed binding energies and the basis set dependence of these energies have been evaluated using the Dunning correlation-consistent polarized valence double-, triple-, and truncated quadruple-split basis sets (cc-pVXZ, with X = D for double, T for triple, and Q́ for truncated quadruple), and these same basis sets augmented with diffuse functions on nonhydrogen atoms (cc-pVXZ+). The results of these studies suggest that the methodology of MBPT(4) = MP4/cc-pVTZ+ electronic energies without the counterpoise correction computed at MP2/6-31+G(d, p) geometries is an appropriate theoretical model for investigating complexes RCN ⃛ HX, with R = H or CH3 and X = F or Cl. This model has been applied to investigate the structures, binding energies at absolute zero, binding enthalpies at room temperature, and vibrational frequencies of these complexes. Trends are noted, and comparisons are made with available experimental data. © 1992 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 99
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    International Journal of Quantum Chemistry 44 (1992), S. 551-561 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present an INDO/S molecular-orbital investigation of organic molecules containing a barrelene moiety that provides a rigid link between an aromatic donor and a maleic ester acceptor group. Molecules of this type have recently been synthesized and characterized spectroscopically. We discuss the ground state and various excited states both in vacuo and in solution. Solvent effects are incorporated by use of an electrostatic cavity model which is not restricted to a spherical cavity, but allows for a cavity shape that is adapted to the solute molecule. The calculations indicate low-lying charge-transfer (CT) excitations in the region of the first aromatic transitions, even in the gas phase. © 1992 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 100
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    International Journal of Quantum Chemistry 44 (1992), S. 587-604 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction of CO with rhodium clusters of different sizes is studied by means of Intermediate Neglect of Differential Orbital (INDO) calculations. Interactions of both σ and π type are well-known to contribute to the metal —CO bond formation. The relative importance of the repulsive Rh—5σCO interaction with the metal and the back-bonding to the π* CO empty orbital in the stabilization of the bond is analyzed as a function of the cluster size and the coordination geometry. It is found that the shift of charge in the larger metal clusters brings about a reduction of the Rh—5σ CO repulsion, and this redistribution is important to change the optimal CO bonding geometry from bridge to on top. © 1992 John Wiley & Sons, Inc.
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