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  • 1990-1994  (301)
  • 1993  (301)
  • Atomic, Molecular and Optical Physics  (301)
  • 101
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 393-404 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A justification of Berlin's theorem that concerns the binding force concept in diatomic molecules is presented based on the Hellmann-Feynman theorem and its electrostatic interpretations. The concepts of binding and the binding region first derived by Berlin for diatomic molecules are extended to polyatomic molecules; the virial of forces necessary to hold the nuclei in a molecule fixed is proved to be an exact description of the total binding effect in the molecule. The partition of molecular space into binding and antibinding regions based on the binding function, which is defined in this paper, is proposed for polyatomic molecules. The binding diagrams for some planar molecules are developed as simple applications of our theoretical considerations. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 102
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 375-391 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Earlier experimental data by Friedrich et al. found out, from time-of-flight experiments of collisions between H+ and HF molecules, that most of the available relative energy becomes transferred into internal rotations of the target and that vibrational excitation only begins to be noticeable around and above 30 eV of relative collision energy. The present theoretical and computational model starts from the potential energy function presented in the preceding paper and carries out quantum calculations that explicitly include the coupling of vibrational target states. The results indeed show very few vibrational excitations and rather large rotational energy transfers as the collision energy is increased up to 50 eV. Microscopic features of both the interaction and the dynamics are found to be responsible for such behavior, which brings the present calculations in agreement with experimental findings. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 103
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 104
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 335-373 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The various structures and geometries of the cation FH2+ (fluoronium ion) have been investigated over a broad range of relative orientations between the proton and the HF molecule using ab initio, correlated wave functions and focusing on the lowest-energy channel that can lead to the production of vibrationally excited HF in the collisions with near-thermal H+ atoms. Various quality controls have been carried out on the accuracy of the target molecule description and a reliable, overall structure of the vibrational-rotational potential energy surface is obtained, for the first time extensively enough, to carry out scattering calculations in the direct excitation channels for the rotovibrational modes. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 11 Ill.
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  • 105
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 106
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 239-242 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 107
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 319-323 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A general formula to decompose the p-power of irreducible representations of an arbitrary space group into sum of sets of irreducible representations of such a group, having identical permutational symmetry, is presented. Its proof is based upon a straightforward application of the properties of the generalized projection (shift) operators. © 1993 John Wiley & Sons, Inc.
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  • 108
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 307-317 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: This contribution analyzes the changes in the intrinsic reaction coordinate (IRC) and in the potential energy profiles (PEP) caused by application of a uniform electric field evaluated using standard ab initio MO methods. Two typical organic processes have been studied, namely, the Friedel-Crafts and the Walden inversion reactions, which are modeled by suitable simple systems. The results for the Walden inversion reaction show that the IRC for the field-free and that for perturbed processes are almost coincident; in this case, using the field-free IRC to compute the perturbed energy profile is a very good approximation. On the contrary, for the Friedel-Crafts model reaction, the two IRCs differ slightly, so the energy profiles using the perturbed and the unperturbed IRCs are somewhat different. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 109
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 243-260 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The formulation of a combinatorial theory of the structure of the class-algebra of the symmetric group is pursued. Recurrence relations for reduced class-coefficients involving imbedded cycles are presented. It is pointed out that for bridging cycles one can obtain elimination rules that involve symmetrization over sets of reduced class-coefficients with common cycle-structures but inequivalent index distributions. Consequently, some of these reduced class-coefficients remain individually inaccessible. © 1993 John Wiley & Sons, Inc.
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  • 110
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 111
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 261-305 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: General theory of the long-range intermolecular potentials, resulting from the use of the Rayleigh-Schrödinger perturbation theory at the multipole approximation level and the irreducible tensor formalism, is applied to describe the three-body induction effects in a system composed of two atoms and a linear molecule. Explicit expressions for the leading van der Waals coefficients are derived by employing the symbolic computation language MAPLE. Both the general geometry and the special case of the isosceles triangular configuration are considered. Final equations are presented using the interface between MAPLE and the typesetting language LATEX. Some technical aspects associated with the use of MAPLE in theory of long-range intermolecular potentials are briefly discussed. Explicit expressions obtained in this article should be useful in determination of the nonadditive induction effects in van der Waals clusters composed of two atoms and a diatomic molecule. In particular, they may be helpful in explaining the discrepancies between experiment and theory that are observed in high-resolution microwave studies of the complex Ar⃛Ar⃛HCl. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
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  • 112
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 325-334 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The concept of the electronic-phase transition of the Hartree-Fock solution has been applied to an arbitrarily doped polyacetylene (PA) chain to examine the structural change of the polymer skeletons under such doping regimes. It has been found that the n-doped PA chain tends to suppress the generation of the charge-density wave (CDW) phase, whereas the p-doped PA favors the CDW phase leading to the charged-soliton shape. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 113
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 309-317 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The interaction of Al+ (1S) ions with H2 on the lowest electronic energy surface is studied using ab initio electronic structure methods. A Cs symmetry transition state is located and found to have the geometry of a product AlH+ ion loosely bound to a H atom, consistent with the Hammond postulate for this endothermic reaction. Locating this transition state, beginning at geometries that characterize vibrationally cold H2 and translationally hot Al+, posed special challenges to the commonly used “hill-climbing” algorithm because of regions of geometrical instability along the path thus generated. This instability was found to be a result of second-order Jahn-Teller coupling with a low-lying 1B2 electronic state. In addition to these primary findings, a weakly bound T-shaped Al+  -  -  -  H2 C2v van der Waals complex is found that lies only 242 cm-1 below the Al+ and H2 asymptote, with H—H internuclear separation only slightly distorted from the equilibrium bond distance of H2 and Al—H distance (3.5 Å) much longer than the covalent bond length in AlH+ (1.6 Å). The locally stable but thermodynamically unstable linear HAlH+ (1∑g+) species and, of course, the H + AlH+(2∑+) reaction products have also been identified as critical points on the ground-state surface. Where known, the geometries and energies that we calculte agree well with experimental data. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 114
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 343-354 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We here report the AM1 and PM3 evaluation of proton affinities for phosphorus compounds. The substituent effects, the preferred site of protonation, and the geometry changes produced upon protonation are discussed in terms of the present semiempirical calculations that are compared with experimental data and previous ab initio results. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 115
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 367-373 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Molecular calculations are presented using “space-restricted basis functions,” i.e., basis functions that are nonzero only in selected regions of space. The nature of the calculations their justification, and their possible utility to quantum chemistry are described. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Tab.
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  • 116
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 319-332 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Structural features of benzocyclobutenes, benzocyclobutadienes, and some of their fluoroderivatives are studied at the HF/SCF niveau employing a 6-31G* basis set. A Mills-Nixon-type of distortion (lengthening of the annelated bonds with concomitant shortening of the adjacent bonds in the benzene moiety) is found in all molecules, with the exception of polyannelated benzocyclobutenes with perflouro-substituted methylene groups. The latter compounds exhibit a weak anti-Mills-Nixon effect. Peripheral double bonds in benzocyclobutadienes enhance the double-bond localization in the benzene ring, but the main effect originates from angular deformations of the σ-skeleton. Salient structural features are rationalized by rehybridization at the carbon junction atoms and variation of Coulson's π-bond orders. Finally, several scales for quantifying the Mills-Nixon effect are briefly discussed. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 117
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 118
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 1-6 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The geometry and electronic structure of peroxynitrous acid (O=NOOH) and peroxynitrite ion (O=NOO-) were studied at the 6-31G** level. The cis-configuration of peroxynitrous acid was found to be more stable by 3.9 kJ/mol than the trans-form, whereas peroxynitrite is more stable by 4.6 kJ/mol in the trans-form. Calculations of the skew forms, which are as defined by an angle of 90° between the ONO/NOO planes, yield energies larger than those of the most stable forms of the acid or anion by 51.4 and 68.1 kJ/mol, respectively, thus providing an estimate of the gas-phase energy barrier. The electronic structure is discussed from the point of view of the reactivities and transformation processes of ONOOH and ONOO-. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
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  • 119
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The carbon dioxide (CO2) to methylamine (CH3NH2) addition reaction is investigated using an ab initio MO method with the aim of obtaining an insight into the relation between transition-state structure and reactivity. The complete potential energy hypersurface was explored and the stationary points representing reactants, transition state, and product were localized. To establish the relevance of different basis sets, calculations were carried out at different levels of theory: STO-3G, 3-21G, 4-21G, 4-31G, 6-31G, 6-311G, 6-31G*, and MP2/6-31G*//HF/6-31G*. A reaction analysis done by correlation of bond-order indices using the More O'Ferrall-Jencks diagram reveals the reaction mechanism as an asynchronous process. The transition state can be described as a four-membered ring. The lone nitrogen electron pair of the amine appears as the driving force of the reaction, where a positive hydrogen rather than a radical is more likely to be transferred. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 120
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 507-510 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 121
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 545-562 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A number of established nonlinear convergence acceleration methods are presented. It is discussed how these methods, and also some new algorithms, can be applied successfully to Fourier series. Some of the new accelerators are tailored for these-series. Others are also useful for other types of series and sequences, and not only for convergence acceleration, but also for extrapolation or summation problems. Applications to the numerical inversion of the Laplace transform and to a Fourier series arising in the Matsubara formalism are presented. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Tab.
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  • 122
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 511-544 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We shall start with very small systems like H2 and H3, computed with very accurate methods (Hylleraas-CI) or atomic systems up to Zn with accurate methods (CI), then move to more complex ones, like C60, but now with somewhat less accurate methods, specifically Hartree-Fock with density functionals, the latter for the correlation energy but not for the exchange energy. For even more complex tasks like geometry optimization of C60, we have resorted to even simpler and parametrized methods, like local density functionals. Then, we could use quantum mechanics either to provide interaction potentials for classical molecular dynamics or to directly solve dynamical systems, in a quantum molecular dynamics approximation. Having demonstrated that we can use the computational output from small systems as input to larger ones, we discuss in detail a new model for liquid water, which is borne out entirely from ab initio methods and nicely links spectroscopic, thermodynamics, and other physicochemical data. Concerning time scales, we use classical molecular dynamics to determine friction coefficients, and with these we perform stochastic dynamic simulations. The use of simulation results from smaller systems to provide inputs for larger system simulations is the “global simulation” approach, which, today, with the easily available computers, is becoming more and more feasible. Projections on simulations in the 1996-1998 period are discussed, new computational areas are outlined, and a N4 complexity algorithm is compared to density functional approaches. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 123
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 124
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 1-11 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
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  • 125
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 51-66 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A review is presented of our recent work which deals with some aspects of the inclusion of connected triple excitations in single reference coupled-cluster theory. We first consider basic aspects of the theory and computational cost, noting the need for accurate and economical approximations to the complete coupled-cluster single, double and triple excitation method (CCSDT). Several such approximations are now available, including the CCSD + T(CCSD) and CCSD(T) methods. We have now efficiently implemented analytical derivative methods for these and related methods for open-shell reference determinants, including restricted open-shell Hartree-Fock for CCSD(T). Some results with large basis sets are presented for the HCO radical which further demonstrate the reliability of these methods for the spectroscopic characterization of small open-shell species. The remainder of this article deals with several applications of the complete CCSDT method, a new implementation of which we have recently completed. Like our previous work, this is for general single reference determinants, although we have now incorporated Abelian symmetry. We present a comparison with full configuration interaction data for CH3; a study of the N2 potential curve using restricted and unrestricted Hartree-Fock references; a comparison of the dipole moments and polarizabilities of BeO; and a study of the harmonic force field of 2Σu+ C3+. These studies also provide assessments of the accuracy of the approximate schemes for including T3. Finally, we present some timing data for some CCSDT calculations with out new implementation. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 126
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 91-92 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
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  • 127
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 619-636 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Solutions of Hartree-Fock equations expressed as Gaussian functions are studied in various spaces: position, momentum, and position-momentum spaces. The use of the wavelet transform allows one to visualize position and momentum characteristics of atomic orbitals on the same drawing. A complementary viewpoint is then obtained on top of usual position and momentum representations. Applications to Gaussian “atomic” orbitals modeled as one-dimensional functions are performed. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 128
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 591-607 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Multiconfigurational second-order perturbation theory is tested for the calculation of molecular structure and binding energies. The scheme is based on the Complete Active Space (CAS) SCF method, which gives a proper description of the major features in the electronic structure, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second-order perturbation theory with the CASSCF wave function as the reference state (CASPT2). The approach is applied to the calculation of equilibrium geometry and atomization energies for 27 benchmark molecules containing first-row atoms (the “G1” test). Large atomic natural orbital (ANO)-type basis sets are applied (5s4p3d2f for Li—F and 3s2p1d for H). It is shown that the CASSCF/CASPT2 approach is able to predict the equilibrium geometry with an accuracy better than 0.01 Å for bond distances and 0°-2° for bond angles. Calculated atomization energies are underestimated with between 3 and 6 kcal/mol times the number of extra electron pairs formed. The error in the heat of reaction for a number of isogyric reaction (no difference in number of pairs) varies between -2.5 and +1.0 kcal/mol. The same type of accuracy is obtained in calculations for excited states. The molecules B2, C2, FO, FOO, and FOOF have also been studied. Results for the first three molecules are in accordance with those of the benchmark molecules. The FO bond distance in FOO is predicted to be 0.02 Å longer than experiment. The heat of formation for FOO is computed to be 2.9 kcal/mol with an uncertainty of ±3 kcal/mol. Preliminary results for FOOF (obtained with a smaller basis set) indicate that the approach yields a somewhat too long FO bond distance (1.64 Å compared to 1.58 Å experimentally). © 1993 John Wiley & Sons, Inc.
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  • 129
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 609-618 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A method using the Svartholm iterative procedure to solve atomic Hartree-Fock equations in momentum space is defined and applied to the ground states of Be and B+. The calculated atomic orbital properties follow a monotonic and stable convergence, but with rates of convergence depending on each property. The evolution of the orbitals during the iterations is explained by the combined actions of the variational principle, the Svartholm iterative procedure, and the momentum space representation. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 130
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 219-226 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In our previous work, the stopping properties of metallic and covalent films were investigated. Here we consider an ionically bound film. The energy loss of a proton in an LiF monolayer (LiF-1L) is calculated in orbital fashion, based on kinetic theory. The required momentum density and mean excitation energy are obtained from the local density approximation and local plasma approximation respectively. For comparison, the LiF molecule is treated by use of a large intermolecular distance in the film. We find the stopping cross section of the LiF molecule to be only slightly larger than that for the LiF-1L. The Bragg rule (additivity of stopping for the corresponding atoms) is not valid for the ionically bound molecule nor the corresponding extended system, but may be valid if additivity of stopping of atomic ions is assumed. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 131
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 249-261 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We evaluate binding energies, ionization energies, and second-order energy differences as functions of valence electron number for small spherical clusters of stabilized jellium, using the Kohn-Sham equations with the local-spin-density (LSD) approximation. Cohesive energies are also reported. A comparison is made with semiclassical formulas (liquid drop model and Padé approximant, with surface and curvature coefficients derived from first principles). These formulas nicely average the shell-structure oscillations of the energy, which are found to be almost the same as for ordinary jellium. Spherical clusters with 1, 7, and 9 electrons have binding energies very close to those of the semiclassical predictions. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
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  • 132
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 295-301 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The equilibrium geometries, excitation energies, force constants, and vibrational frequencies for four low-lying electronic states X 2A1, 2B1, 2B2, and 2A2 of the BF2 radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated excitation energy for X2A1 → 2B1 is in agreement with available experimental data. The electronic transition dipole moments, oscillator strengths for the 2B1 → X2A1 and 2B2 → X2A1 transitions, radiative lifetimes for the 2B1 and 2B2 states, and the spin properties for the X2A1 state are calculated based on the MRSDCI wave functions, predicting results in reasonable agreement with available experimental data. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Tab.
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  • 133
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 303-328 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The bands of orbital energies for several polymeric species of alternant hydrocarbon are calculated. From these, the densities of states are graphed. By integration over the bands, the slope of the asymptotic line for the energy is calculated and compared with the energies of members of the same series calculated directly. For some series, the second, constant, term in the asymptotic line can also be calculated theoretically and compared with that derived from the molecular energies. The results for some related series indicate that for long molecules the number of Kekulé structures does not influence the major term in the energy. The extension of the argument to twodimensional arrays of hexagons is indicated and some results reported. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 15 Ill.
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  • 134
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 127-136 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The electrostatic potential (EP) and the polarization correction (PL) on the (EP) were calculated for four aromatic compounds: benzene, chlorobenzene, phenol, and benzoic amide, at the ab initio SCF level within two basis sets: 6-31G** and MINI-1. One calculation was performed using the polarized MINI-1** basis set. The quantity total interaction energy (TEH) defined as -EP + PL can be used as an indicator of the nucleophilic attack preferential position. By reference to 6-31G**, MINI-1 provides very satisfactory results. Moreover, the MINI-1** results are very similar to the MINI-1 ones. It appears that -EP does not provide a reliable tool to study the nucleophilic attack susceptibility, whereas TEH seems to be very well adapted for this kind of approach. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 10 Ill.
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  • 135
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 137-144 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Silicon analogs of aromatic monocyclic ions, (SiH)3+ (4), (SiH)42+ (5), and (SiH)5- (6) have been studied ab initio at MP2(full)/6-31G*. The D3h structure of Si3H3+ is the global minimum, whereas other two ions are nonplanar. The D2d structure of (SiH)42+ is less folded than the carbon analog and possesses a higher stabilization energy. Stabilization energies for the monocharged ions are diminished with respect to the corresponding carbons © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 136
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 159-170 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: All-electron ab initio Hartree-Fock (RHF) calculations have been carried out to investigate the keto/enol equilibrium of phloroglucinol. The calculations predict that the enol form of phloroglucinol, 1,3,5-benzenetriol, is by far the most stable of the two. This is confirmed by NMR spectra taken on phloroglucinol. A comparison of the keto enol form transformation of phloroglucinol with that of the phenol system shows that the keto form of phloroglucinol, 1,3,5-cyclohexanetrion, is more abundant in the phloroglucinol system, and the keto form of phenol, 2,4-cyclohexadien-1-on, in the phenol system. © 1993 John Wiley & Sons, Inc.
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  • 137
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 145-157 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In the present work, we present results of all-electron ab initio CASSCF calculations of nine electronic states of the molecule YN. Also reported are the spectroscopic constants derived on the basis of the calculated potential energies. The predicted electronic ground state is 1∑+, and this state is found to be separated from the excited states 3∑+, 3Π, and 1Π by 5177, 9290, and 9915 cm-1, respectively. The chemical bond in the YN molecule is polar with charge transfer from Y to N, giving rise to a dipole moment of 8.19 Debye at 3.3 au in the 1∑+ ground state is basically a double bond composed of two π bonds. The dissociation energy of the YN molecule has been derived as 4.59 eV. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 138
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 171-181 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: 3-21G RHF calculations on (CF3)2O and (CF3)2OH+ molecules are carried out to assess the changes induced in the molecular properties of ether links in poly (perfluoro ethers) interacting with acid sites at contact surfaces. Geometry of the species, vibrational frequencies, proton affinity, and energetics of the (CF3)2OH+ fragmentation provide a preliminary basis to understand problems raised by lubricant degradation. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 139
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Glycine receptor substrates are molecules potentially involved in tautomerism phenomena. This study is expected to provide information that might help understand their intrinsic reactivities; such physicochemical data would allow us to establish predictive models in a search for better antagonists. Ab initio molecular orbital studies, using 3-21G and 6-31G* basis sets, are reported for the tautomeric equilibrium of 2-pyridone · H2O/2-hydroxypyridine · H2O. The geometry of the transition state has also been optimized. The results show the important effect of the water molecule through the formation of hydrogen bonds. This system will be used as a prototype for the design of antagonists of the glycine receptor, a potential site for the action of new antiepileptic drugs and compounds preventing ischemic brain damage. © 1993 John Wiley & Sons, Inc.
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  • 140
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 141
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 191-210 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Gound-state electronic properties of porphine, phthalocyanine, and their derivatives, photosensitizers with potential use in the photodynamic therapy of tumors, were studied by semiempirical (MNDO) molecular orbital calculations. Geometry, bond orders, electron populations, and net atomic charges were analyzed. The obtained models were applied to study the intermolecular interactions of these molecules utilizing a classical mechanics approach. The dimerization characteristics of different derivatives were obtained by a separate evaluation of the contribution of electrostatic and steric factors. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 9 Ill.
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  • 142
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 231-238 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: By means of ab initio calculations, the rotational constants and dipole moments of HnC3N (n = 1, 3, 5, 7, and 9) species have been calculated at the HF / 6-31G* level of theory. Selected cases have been also calculated at the MP2/6-31G* level and the influence of calculation level on rotational constant values is briefly discussed. Some of these species were discovered in the interstellar medium, while others have still not been detected there, although their existence is very probable. The results given here could help in their detection. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
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  • 143
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 211-225 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Conformational analyses of large molecules as fatty acids and triglycerides are usually amenable by molecular mechanics. A correct evaluation of the electrostatic energy term is thus crucial in determining reliable results. In this contribution, we have considered the most abundant fatty acids in biomembranes, i.e., lauric, stearic, oleic, and elaidic acid, and the corresponding triglycerides, i.e., trilaurin, tristearin, triolein, and trielaidin, and estimated the Mulliken and potential-derived charges both at the semiempirical AM1 and ab initio HF MO STO-3G level. Atomic charges obtained by the Mulliken population analysis do not take into account the full geometry of the molecule. On the contrary, the change of conformation, due to different chains length or the presence of a trans or cis double bond, greatly influences the repartition of the potential-derived charges. A systematic comparative analysis shows that charges calculated by AM 1 are not suitable because as they do not reproduce potential-derived charges obtained by ab initio. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 144
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 239-255 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A potential-derived atomic multipole method called the cumulative potential-derived atomic multipole method is developed, with which electrostatic atomic multipole moments are derived by fitting the molecular electric potential in a cumulative way. It is applied to the hydrides of N, O, F, S, Cl, and methanol and the hydrogen-bonded dimers formed between them. The relationship between atomic multipole moments and molecular charge distributions is found. The structures calculated with Buckingham's electrostatic model are in good agreement with experiments. The phenomena of nonlinear structures of most H-bonded complexes - the deviations of symmetry axes of electron donors from H bonds - and correct distinguishing between two alternative structures are attributed to atomic dipole and quadrupole moments. Compared with other methods, this method has a quantitative and qualitative advantage and simple algorithm. The main conclusion is that the atomic multipole moments play a substantial role, although a potential-derived charge model was deemed sufficient previously. © 1993 John Wiley & Sons, Inc.
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  • 145
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 257-270 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The spatial symmetries of the harmonic oscillator and the recently found irreducible tensors constructed from the associated annihilation and creation operators are exploited to obtain new expressions for the elements of the matrix representatives of several examples of vibrational operators. Since all vibrational operators are expressible in terms of the irreducible tensors, their matrix elements reflect the angular momentum symmetry inherent in them, for the results derived here are in terms of the Clebsch-Gordan coefficients and the isoscalar factors that arise from the couplinig rule of the irreducible tensors. Familiarity with the mathematical properties of these quantities derived from the elementary theory of angular momentum facilitates the evaluation of many vibrational operators that may be of importance in the study of potentials in this basis. In particular, it is shown that the nonvanishing of matrix elements is governed by a law of conservation of angular momentum along the axis of quantization of the nondegenerate harmonic oscillator. © 1993 John Wiley & Sons, Inc.
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  • 146
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 109-118 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A recently developed self-consistent crystal field (SCCF) method is applied to two suggested crystal structures of solid hydrogen fluoride, polar and nonpolar, respectively. Constrained geometry optimizations are performed and the results are compared with experiment and previous theoretical studies. Comparisons are also made with a previous SCCF study on hydrogen cyanide. The Pauli repulsion, dispersion, and “classical” crystal field contributions to the lattice energy are calculated. © 1993 John Wiley & Sons, Inc.
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  • 147
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 55-71 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An SCF analysis has been carried for ensembles that simulate the CuO2 conduction layer in the tetragonal layer crystal T′ - Nd2CuO4 (Fig. 1). In this work, the CuO2 layer is described by a planar macromolecule, (CuO2)n, subject to the crystal field produced by the point charges in the ionic layers of D4h symmetry The computations were carried out using the KGNMOL, HONDO, and KGNGRAF codes in MOTECC -91. The computations were carried out with different oxygen and copper basis sets and energy convergence to less than 10-8 Hartrees. The purpose of the SCF computation was to estimate the cohesive energy of the ensembles, the electron density for the individual molecular orbitals, and the excess correlation energy, to ascertain the nature of the Cu—O bond in the conduction layer. The results indicate the following: (1) The cohesive energy of the ensembles (measured by the SCF energy plus the correlation energy, above the atomic values): ΔUc ≡ ΔESCF + ΔEC = -4.35 to -4.17 Hartrees per CuO bond as n increases from 4 to 9. Further insight was obtained by considering the electrostatic energy contributions to ΔUc; Eelectrostatic (ensemble) → EModeling (infinite lattice) were evaluated by replacing the oxygen and copper atoms by point charges determined by a Mullion population analysis. The larger oxygen basis set (13, 8/5,3) gave consistent results for the different ensembles of ΔEcovalent ≡ ΔUc - Eelectrostatic = -1.1 Hartrees per CuO bond. (ii) The electron density indicates that covalent bonds are formed and that the oxygen atoms play an important role in the structure stability. The covalent bonds formed indicate that nominal ionic valences do not apply. Mulliken population analyses gave valences of the order of one at the copper and oxygen atoms. The CuO bond orders are 0.47 between neighboring atoms and 0.048 for those separated by two atoms. (iv) The covalent nature of the CuO bonds in (CuO2)n was compared to that for the H2 molecule using as a measure the electron density and the excess correlation energy. The excess correlation energy per CuO bond above the atomic values is one order of magnitude lower that that for the H2 molecule and that for the C=C bond in alternant hydrocarbons. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
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  • 148
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 295-317 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A brute-force numerical investigation has been carried out on the hopping of excitons in a three-dimensional molecular aggregate. Possibilities of vibronic decay, rapid chemical reactions of saturated species, radiative decay of overpopulated molecules, and cooperative chemical reactions involving saturated exciton populations on traps of two different types have been considered. Investigation have been performed with two types of initial distribution of excitons - facial and random - and for 10,000 or, sometimes, for 20,000 time steps each of duration 1ps. Several interesting observations have been made from this computer experiment: (1) The total number of occurrences of fast reactions depends upon the initial distribution of excitons. (2) It decreases if other exciton depleting processes are at work. (3) It also depends on the pattern of placement of traps. (4) The location of impurities also affects the rate of occurrence of these reactions. Thus, more reactions occur when the excitons are initially concentrated on one face and traps are suitably located on the path of flow of these excitons. A random initial distribution tends to equilibrate the excitons quickly over all the lattice points, thus giving rise to fewer reactions. (5) The number of reactions need not necessarily increase with the number of reaction centers; in fact, it decreases as more centers are added when the supply of excitons is severely limited. (6) A Complicated dynamics results when different types of additional processes, viz., enhanced fluorescence, radiative emissions, and cooperative chemical reactions are simultaneously allowed. The cooperative process has been clearly found to dominate. A first-order rate constant of about 108 s-1 has been calculated for the occurrence of the cooperative process. This rate is affected when other nonconserving processes are switched on. Observations (1), (4), and (5) are the most important conclusions of our work. They lie outside the scope of traditional models such as the random walk model, the diffusion model, and the lattice model for the migration of excitons in a molecular aggregate. © 1993 John Wiley & Sons, Inc.
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  • 149
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 343-363 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The Complex Coordinate Scattering Theory is reformulated for the general case of a time-independent Hamiltonian. It is applied to scattering of He atoms from a Cu(115) crystal surface by constracting the Green operator for the T-matrix from the eigenvectors of both the complex scaled Hamiltonian and its transposed (“right” and “left” eigenvectors), which are different in this case. The weakly asymmetric corrugation function describing the (115) face of Cu is shown to cause a strong dependence of the calculated diffraction intensities upon the direction of the incident atomic beam. The calculated transition probabilities are in excellent agreement with the experimentally measured ones, previously obtained by Perreau and Lapujoulade. We show that additional information about the gas atom/surface physisorption interaction potential can be obtained if the incident angle of the atomic beam (the angles between the beam and the surface normal) is changed from γ to -γ. © 1993 John Wiley & Sons, Inc.
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  • 150
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 365-374 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An implementation of the complex coordinate method is demonstrated that exploits a new technique for obtaining the matrix representation of the complex dilated Hamiltonian. The purpose is to make the complex coordinate method applicable together with standard numerical ab initio codes designed for large-scale calculations on many-electron atoms and/or molecules. No complex intergrals have to be calculated, and no changes of the standard codes are required even in the common case where the kinetic and potential energy components are not stored separately. Instead, two standard (real) CI calculations are used to generate the dilated (complex) CI matrix representation. The performance of the procedure is demonstrated in the context of the GAMESS program and applied to obtain the resonant structure of the Bethe surface pertinent to the absorption spectrum of the helium atom. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 151
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 375-381 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Approximate conditions determining the complex energy resonances of a one-dimensional model potential, originally discussed by Moiseyev et al., are derived within a multiple-transition-point WKB treatment. It is shown that an erroneous energy spectrum is generated if too few transition points are considered. The numerical results of a correct semiclassical treatment of the problem, where several transition points are included, agree well with the, presumably exact, results of Rittby, Elander, and Brändas. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 152
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 391-399 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: It is proved that for a certain class of potentials dilation operators used in the method of complex scaling (CSM) represent a special case of Lyapunov converters. The time evolution of quantum systems is investigated from the viewpoint of this result, with special emphasis on the time evolution of resonant functions. © 1993 John Wiley & Sons, Inc.
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  • 153
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 415-418 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Complex scaling of the Schrödinger equation on the halfaxis with a nontrivial boundary condition at the origin is investigated. A self-adjoint dilation of the corresponding dissipative operator is constructed. The relations between the scattering problems for the operator and it's dilation are clarified. © 1993 John Wiley & Sons, Inc.
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  • 154
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 401-413 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The relations between the Schrödinger and wave evolutions are investigated. A generalization of the Lax-Phillips scattering theory for the case of the Schrödinger equation is constructed. An analog of the wave decay operator is obtained. The general ideas are illustrated by an explicitly solvable model of multichannel scattering on the halfaxis. A Lax-Phillips scattering theory is developed for the model operator obtained by a self-adjoint perturbation of the background operator. © 1993 John Wiley & Sons, Int.
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  • 155
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 156
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 101-101 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 157
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 85-100 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Expansions of STO orbitals with GTOs for the first-row atoms have been obtained by the method of the distance between subspaces. The expansion coefficients and exponential parameters were simultaneously varied when the distance between subspaces, which are generated from STO and GTO functions, is minimized. The ζ; exponents (or scale factors) for the atomic orbitals that are optimized for these atoms are also shown to be almost independent of the number of Gaussian functions. Comparisons carried out with Stewart's least-squares method produce equivalent results when exponents for 2s and 2p functions are different. Some examples and applications for several atomic properties of the first-row atoms are included: energies and expectation values of ri and pi for the several expansions. These new minimal basis sets were tested for diatomic and polyatomic molecules containing these atoms. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
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  • 158
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 103-118 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We describe a simple expansion of the Krylov type for the estimation of the electronic correlation energy of the ground state of closed-shell systems and show its relationship with Brillouin-Wigner perturbation theory. We demonstrate the Brillouin-Wigner perturbation theory can be quickly solved in a noniterative fashion. Examples are presented using the intermediate neglect of the differential overlap model Hamiltonian (INDO/1), which indicates that this scheme captures well over 95% of the configuration interaction over double excitations method (CID) in a fraction of the computational effort. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
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  • 159
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 119-127 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We report calculations of the excitonic spectra for trans-polyacetylene obtained in the Hartree-Fock, Tamm-Dancoff, and random-phase approximations. In the first case, in terms of two-particle propagator theory, the interaction between the excited electron and the hole is neglected. In the latter two cases, this interaction is considered in the first order. In this framework, the interaction between excitations of different bands and k-vectors has been included. We discuss the bandwidths and density of states for π-π*, σ-π*, π-σ*, and σ-σ* excitons. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 160
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 135-144 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A numerical integration scheme is presented for three-dimensional integrals occurring in electronic structure calculations, concentrating attention on the evaluation of the correlation energy through a density-functional expression. The scheme is based on the choice of density-based weight functions that naturally partition the space into “atomic” volumes (in which the integration is performed in terms of spherical coordinates) and “diatomic” volumes (in which the integration is performed in terms of confocal elliptical coordinates). Such a choice is justified on the basis of the analytical behavior of the integrand. The attainable accuracy and the required computational effort within the proposed scheme are discussed in detail in a test application on the C60 molecule in the symmetrical configuration. Finally, a comparison with previously proposed schemes is presented. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 161
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We have determined the dynamic dipole (α1), quadrupole (α2), octupole (α3), and dipole-dipole-quadrupole (B) polarizabilities and the second hyperpolarizability tensor (γ) for the helium atom in its lowest triplet state (23S). We have done so for both real and imaginary frequencies: in the former case, for a range of frequencies (ω) between zero and the first electronic-transition frequency, and in the latter case for a 32-point Gauss-Legendre grid running from zero to ħω = 20 Eh. We have also determined the dispersion-energy coefficients C6, C8, and C10 for the systems H(12S)—He(23S), He(11S)—He(23S), and He(23S)—He(23S) and the C60, C62, C80, C82, and C84 coefficients for the interaction He(23S)—H2(X1∑g+). Our values of the higher-order multipolar polarizabilities and of γ for the 23S state of helium are, we believe, the first to be published. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Tab.
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  • 162
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 145-153 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The SCF interaction energy in He … Li+, (H2O) … HOH, and OC … BH3 complexes is decomposed, using the original and modified-by-the-author Pauli Blockade (PB) method of Gutowski and Piela. The changes of the dipole moment of the deformed constituents are similarly decomposed and discussed as some indices of the deformation. The deformation is due mainly to the electrostatic interaction, but in some cases, the valence repulsion deformation effects are dominant. The comparison of the PB technique with some other decompositions is discussed. The PB methods seem to be more widely applicable, e.g., they are applicable for small intermolecular distances and large basis sets. The modified PB method is less-time consuming than is the original one. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Tab.
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  • 163
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 155-173 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A mutually consistent method to calculate excitation energies and corresponding transition densities is proposed. The method is based on the extended Brillouin's theorem that is derived from the nonstationary variation principle. Within the proposed procedure, the Brillouin's conditions, which appear in this extension, are used as a set of nonlinear equations for molecular orbitals and configuration interaction coefficients of the trial ground- and excited-state functions. The excitation energy is an eigenvalue of the set. To some extent, this procedure is related to the variational treatment of the conventional random-phase approximation within the equation-of-motion method. The basic features of the proposed procedure are discussed and it is illustrated by numerical examples. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 164
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 165
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 185-190 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In the present note, a linked form of spin-paired functions for an N-electron system in spin state S is suggested. This is found to lead to a simple scheme for generating the representation matrices of the elements of permutation group without searching for linkages in the superposition diagrams. The program based on this is found to generate the representation matrices more efficiently than do currently available procedures. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Tab.
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  • 166
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 175-176 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Ab initio calculations of potential energy, dipole moment, equilibrium OH distance, force constants, and anharmonic frequencies, and correlations between these quantities, are presented for a water molecule and an OH- ion in a uniform electric field of varying field strength. It is explained why a bound H2O molecule in nature always experiences a frequency downshift with respect to the free molecule, and a bound OH- ion either a downshift or an upshift. The frequency-field variation is well accounted for by the expression ΔνOH ∝ -E∥·(dμ∥free/drOH + 1/2 · ∂μ∥induced/∂rOH). A frequency maximum occurs at the field strength where ∂μ∥tot/∂rOH ∼ 0. Two cases can be discerned: (1) the frequency maximum falls at a positive field strength when dμ∥free/drOH is negative (this is the situation for OH-), and (2) the maximum frequency falls at a negative field when dμ∥free/drOH is positive (this occurs for water). In general, for an OH bond in a bonding situation where the intermolecular interactions are dominated by electrostatic forces, the nonlinearity of the frequency shift with respect to an applied field is governed by how close to the frequency maximum one is, i.e., by both dμ∥free/drOH and ∂μ∥induced/∂rOH. Correlation curves between the external linear force constant, kext, and rOH,e are closely linear over the whole field range studied here, whereas the frequency vs. rOH,e and force constants vs. rOH,e correlation curves form two approximately linear, parallel branches, corresponding to “before” and “after” the maximum in the frequency vs. field curves. Each branch of the v vs. rOH,e curves has a slope of ∼ -16,000 cm-1/Å. © 1993 John Wiley & Sons, Inc.
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  • 167
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 191-211 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The validity of the Hund rule in atomic orbitals (AOs) of the carbon atoms inside closed-shell molecules, such as acetylene, ethylene, and ethane, is examined. Electron-pair populations and contributions of the two-electron covalent structures with parallel (⇈) and antiparallel (↑↓) spins are calculated by multielectron population analysis of MO + CI wave functions. Such an analysis, which allows the visualization of various cooperative electronic effects in some target AOs, is extended on the basis of (strictly orthogonal) hybrid orbitals. Although the HF level shows, incorrectly, that the Hund rule is not satisfied, the CI clearly shows a preference for (⇈) spins to those of (↑↓): This holds for both the electron-pair populations [those of (↑↓) spins diminish with the CI more drastically than those of (⇈) spins], as well as for contributions of the two-electron covalent structures [those of (⇈) spins increase with the CI more drastically than do those of (⇈) spins]. The calculation of the correlation functions (or dependent functions) in AO space allows the comparison of repulsive or “attractive” behaviour of (⇈) and (↑↓) spins in various AO couples. Mutual dependence of the two electrons inside the sigma system increases in the series ethane 〈 ethylene 〈 acetylene. Also found is that parallel spins in (pure) AOs of sigma systems are preferred to the antiparallel spins when going from ethane to acetylene. The preference parallel-antiparallel spins in AOs belonging to two different atoms, including hydrogens, is also examined. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
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  • 168
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 385-385 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 169
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 375-384 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The use of the counterpoise method for the mitigation of basis set superposition error at the correlated level is discussed. Evidence is presented to show that the ghost basis plays a dual role in the counterpoise method: The orbitals of the system are improved by the ghost basis but at the expense of a nonphysical increase in the dimension of the virtual space. This second factor has no effect on application of the counterpoise method at the SCF level but it makes the use of the counterpoise method at the correlated level much less straightforward. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
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  • 170
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. v 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 171
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 7-15 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Hydroxyl-stretching frequencies (vOH) were estimated for a number of 4-substituted and 2,6-disubstituted phenols using the AM1 all-valence electron molecular orbital approximation. There was significant correlation between calculated and experimentally observed frequencies with better correlation observed with the gas phase as compared with dilute solution-phase values. Significant Hammett sigma-rho relationships were recorded for the 4-substituted phenols, consistent with experimental observations. The vOH stretching frequency associated with the equilibrium structure of 2,6-di-t-butylphenol was calculated to be 15 cm-1 higher than that of phenol and this increase was attributed to steric interactions that act to effectively shorten the OH bond length. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 172
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 287-301 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A recently developed Full Configuration Interaction method is discussed and used to compute wave functions for N2 and H8 on an IBM RISC 6000 workstation. As a further application of the formalism a cluster and natural orbital analysis of the wave functions is performed for the N2 molecule at various internuclear distances and for the H8 model. © 1993 John Wiley & Sons, Inc.
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  • 173
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 321-330 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Techniques for computing the multicenter integrals required for molecular calculations for orbitals for which the radial factors are given numerically are described. The methods make extensive use of a numerical algorithm for computing spherical Bessel transforms. The feasibility of using these methods is demonstrated by applying them to SCF calculations for the methane molecule. © 1993 John Wiley & Sons, Inc.
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  • 174
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 343-354 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Koga and Thakkar's reoptimized (9s5p) and (12s7p) Gaussian basis sets for the atoms Li to Ne are contracted to [4s2p] and [7s4p], respectively, and their (4s) and (6s) sets for H and He are contracted to [2s] and [4s], respectively. The basis sets are tested by performing self-cosistent-field (SCF) geometry optimizations on LiH, BeH2, B2H6, CH4, NH3, H2O, and HF. The equilibrium geometries of hydrogen peroxide and hydrazine are determined at both the SCF and fourth-order many-body perturbation theory level. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 11 Tab.
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  • 175
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 363-375 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An approximate numerical method of solving the Generalized Master Equation for a many-body problem is presented, with examples of its application. This method involves the construction from the full Hamiltonian (of the system plus the “bath”) of a set of unitary Langevin equations that combine deterministic microcanonical, stochastic canonical (heat bath), and stochastic nonthermal dynamics in a single time-integration scheme. If implemented in a representation that captures the essential physics and repeatedly run from a given initial condition, this method evaluates stochastic representatives from the actual fiber bundle of system worldlines that flow from the initial condition and, hence, numerically evaluates the path integral. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 176
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 399-406 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In recent publications we have presented a general theory for the identification and computation of correlated wavefunctions of a particular class of doubly excited states which constitute a two-electron ionization ladder (TEIL) leading smoothly to the so-called Wannier state at E = 0. In this work, we examine further the properties of these wavefunctions for two-electron atoms of 1S and 1Po symmetry, especially as regards their analysis in terms of hydrogenic basis sets and good quantum numbers. We find that the Herrick-Sinanoglu (K, T) classification loses accuracy as we move toward threshold and we show that, when single as well as double excitations are considered, a better quantum number for the TEIL is F = N - 1 - K, where N,K are not good numbers anymore. The extent of the breakdown of the (K, T) representation depends on the system and on the level of excitation (more serious in negative ions and for high lying states). © 1993 John Wiley & Sons, Inc.
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  • 177
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 407-418 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In the Dyson equation, the self-energy describes all relaxation and correlation corrections to Koopmans's theorem. Energy-independent contributions to the self-energy depend on the one-electron reduced density matrix of the reference state. Three formalisms for calculating these terms are considered: perturbation theory, coupled-cluster theory, and contour integral theory. In one method, combinations of coupled-cluster singles and doubles amplitudes are substituted for first-order double excitation coefficients and for second-order single excitation coefficients. Another approach generates a description of reference state correlation through the evaluation of approximate contour integral expressions. Calculations on electron binding energies of closed-shell molecules and anions reveal that the coupled-cluster results for the energy-independent self-energy terms are closer to the perturbative results than to the contour integral values. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Tab.
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  • 178
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 451-466 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Recently we presented an extension of the direct reaction field (DRF) method, in which a quantum system and a set of point charges and interacting polarizabilities are embedded in a continuum that is characterized by a dielectric constant ∊ and a finite ionic strength. The reaction field of the continuum is found by solving the (linearized) Poisson-Boltzmann equation by a boundary element method for the complete charge distribution in a cavity of arbitrary size and form. Like many other authors, we found that the results depend critically on the choice of the size of the cavity, in the sense that the continuum contribution to the solvation energy decreases rapidly with the relative cavity size. The literature gives no clues for the definition of the cavity size beyond “physical intuition” or implicit fitting to experimental or otherwise desired results. From theoretical considerations, a number of limitations on the position of the boundary are derived. With a boundary defined within these limitations, the experimental hydration energies cannot be reproduced, mainly because of the neglected specific interactions. In addition, we found that the description of the solute's electronic states also depends on the solvation model. We suggest that one or more explicit solvent layers are needed to obtain reliable solvation and excitation energies. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 179
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 491-499 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Statistical mechanics Monte Carlo simulations of the proton transfer in [H3N—H—NH3]+ in aqueous solution have been carried out using the combined quantum mechanical and molecular mechanical AM1/TIP3P potential. As in the gas phase, squeezing and stretching motions of the donor-acceptor distance have profound effects on the proton transfer. Solvation increases the activation free energy of the proton transfer by 3 kcal/mol over that in the gas phase. The results suggest that the combined QM/MM potential may be used in models that treat the proton motion quantum mechanically and the solvent dynamics classically. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 180
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 517-526 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We investigate the intramolecular dynamics of the ground state and the first three singlet 1Σ+ excited states of boron hydride (BH) using the split-operator method. Ab initio calculations show that these states have strong nonadiabatic couplings, resulting in a complex topology of avoided crossing regions and double well potentials. We attempt to find a method to populate the second minimum of B 1Σ+ enabling experimental observation of its vibrational states. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
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  • 181
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 13-24 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Compact expressions are presented for the 3n-j symbols, where 1 ≤ n ≤ 4, which feature sums over products of binomial coefficients, and certain integer triangular coefficients. The triangular coefficients in turn can be expressed as products of binomial coefficients. Thus in the formulas presented for the 3n-j symbols, the dependence on numerous factorials, formally as well as computationally, has been completely eliminated. While formulas which incorporate summations over products of binomial coefficients have been known for the 3- j and 6- j symbols, the introduction of the triangular coefficients, and the application of the binomial/triangular scheme to 3n-j symbols with n 〉 2, provide important new results. The new formulas are simpler, and they permit more efficient computations of the 3n-j symbols, both in exact and in floating point format, than most schemes which are currently in use. © 1993 John Wiley & Sons, Inc.
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  • 182
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 655-665 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Hopping conductivity becomes activationless (metallic) when twice the intersite coupling supercedes the reorganization energy. Consistent with this idea, activationless electron pair transfer may be considered as a condition for the appearance of superconductivity. For electron pairs the condition is slightly more complicated than for single electrons. The activation energy depends on the relative energies between ionic states and a valence bond state. The latter serves as an intermediate state, contributing to the disappearance of the activation barrier. In A3C60, that pairing occurs because the 1Ag state of the negative ions C602- and C604- is stabilized by correlation effects and is the ground state. The correlation effects are primarily due to the closeness in energy and different parity of the tlu and tlg orbitals, which are partly occupied in the negative ions, but empty in neutral C60. The small interelectronic repulsion and the polarization energy from the environment also contribute to the stability of the evenly negative ions. © 1993 John Wiley & Sons, Inc.
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  • 183
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 667-685 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In this work, ab initio coupled and uncoupled Hartree-Fock polarizability calculations are carried out on finite and infinite polyacetylene chain models. We describe the polymeric procedures which are extensions to infinite systems of the molecular Sum Over States (SOS) and Random Phase Approximation (RPA) methods. The latter does include the field-induced electron reorganizational effects whereas the former does not. The inclusion of these effects is particularly important in computing the longitudinal component of the dipole polarizability tensor of polyacetylene chains at an accurate level since the ratio of the coupled/uncoupled results is between 2 and 4 for the conjugated systems studied. In order to evaluate the asymptotic longitudinal polarizabilities per unit cell we have used both finite oligomeric calculations followed by extrapolation procedures and direct polymeric methods. The difficulty of using the extrapolation procedures due to the unknown analytical form of the evolution of the longitudinal polarizability per unit cell illustrates the convenience of using our direct method treating infinite polymeric systems. We obtain a good agreement between our infinite chain results and the largest oligomeric values. We also discuss the choice of atomic basis set, the possibility of using scaling factors, and the crucial role of using a correct bond length alternation value. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 184
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 469-483 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Rayleigh-Schrödinger-type perturbation theory is developed for the nonlinear Schrödinger equation, which contains a linear perturbation operator. Typical applications involve the self-consistent reaction-field model of solvent effects. Consideration of external perturbations, like electric fields, allows. Calculation of the response properties of solvated species. The general formulae allow development of a consistent Møller-Plesset perturbation theory of solvated molecules. © 1993 John Wiley & Sons, Inc.
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  • 185
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 1-14 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The small and subtle effects of electron correlation in the Rydberg-like 31D and 33D states of the helium-like ions are studied using very accurate explicitly correlated wave functions. In particular, the effects of electron correlation on one-electron densities, probability distributions for the interelectronic distance, and their moments are examined with the help of density and Coulomb holes. Statistical electron correlation is examined with radial and angular correlation coefficients. The different methods of analysis provide complementary insight. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
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  • 186
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 15-48 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Detailed working equations are derived for the ionization part of the single-particle Green's function within the coupled cluster Green's function (CCGF) framework. The CCGF method is applied to the calculation of vertical ionization potentials (IPs) of a number of small molecules, notably, HF, N2, CO, F2, CS, C2H4, H2O, and H2CO. The results for the outer-valence IPs, with an average error of 0.12 eV, compare favorably to third-order equation-of-motion calculations within the same basis set (average error 0.28 eV) and outer-valence GF (OVGF) values taken from the literature (average error 0.17 eV). Ground-state properties that derive from the CCGF are compared to expectation values obtained in the related normal coupled cluster methods (NCCM) approximation from a formal point of view. Correlation energies obtained in CCGF are compared to CCSD results for the above series of molecules and, in addition, the so-called true correlation energy density as obtained from the CCGF is compared to the result from an accurate MR-CI calculation for a highly correlated system: the HF molecule at large internuclear separation. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 187
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 49-58 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In the present paper, the electronic energy-band structures of the LnBa2Cu3O7 (Ln = Pr, Nd, Gd, and Dy) were investigated employing the EHMO approach based on the tight-binding method. The results show that the substitution of Nd, Gd, and Dy for Y in YBa2Cu3O7 complicates the shape of the Fermi surface and results in an increase in the number of energy bands crossing the Fermi level, Ef. Compared with those of YBa2Cu3O7, the electronic densities of states at Ef, N(Ef), for NdBa2Cu3O7, GdBa2Cu3O7, and DyBa2Cu3O7 increase greatly, whereas those of PrBa2Cu3O7 are very small. The results given by the present calculations explain why the substitution of Nd, Gd, and Dy for Y in YBa2Cu3O7 can still maintain the high transition temperature Tc, while PrBa2Cu3O7 is a semiconductor rather than a superconductor. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
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  • 188
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 449-467 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A new approach to analyze the electronic structure around the local defects in a polymer chain is developed based on the cluster-series model. In this approach, by extending the cluster molecule to both sides of the defect, the size effect on electronic structure can be estimated systematically and efficiently. Moreover, through the extension process, the periodicity of electronic structure can be extracted in the form of the periodic pattern of stationary orbitals, which are molecular orbitals unaltered by the extension of the cluster. Such periodic orbitals characterize the bulk states of periodic polymer and the energy band structure can be reconstructed from them. Illustrative analyses of some kinds of local defects in all-trans-polyacetylene are presented at the ab initio STO-3G level. The effects of local defects can be detected by the deviation from periodic bulk states. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 12 Ill.
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  • 189
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 59-72 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Sets of nonlinear equations for the cluster amplitudes of the valence-universal coupled-cluster (VU-CC) method have been solved to obtain physically meaningful multiple solutions for Be, B+, and C2+. The wave operator is taken in Lindgren's normal ordered exponential form and the completeness of the model space is postulated. The cluster operator is restricted to its one- and two-electron components that are represented in terms of radial amplitudes defined by the configurational excitations (VU-CCSD/R method). Five solutions giving rise to 10 approximate energies of four 1S states are obtained and discussed. These are the first multiple solutions documented for a nonmodel system. Some attention is paid to the problem of the efficiency of various methods in obtaining alternative solutions and to some consequences of the availability of alternative solutions. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
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  • 190
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 191
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 73-88 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We have computed ab initio HF/6-31+G* electrostatic potentials and average local ionization energies on the molecular surfaces of the Group V-VII hydrides and corresponding anions of the first three rows of the periodic table. The surfaces were defined to be specified contours (0.002 or 0.001 au) of the molecular electronic density. The most negative potentials, VS,min, and lowest ionization energies ĪS,min, were located and determined. Their magnitudes separately satisfy limited correlations with gas-phase protonation enthalpies and aqueous pKa values. Our results indicate that VS,min, and ĪS,min are complementary, the former reflecting electrostatic factors and the latter being related to charge transfer/polarization. More general relationships for protonation enthalpies are obtained when both VS,min and ĪS,min are explicitly included. Solution-phase and gas-phase acidities are shown to correlate very well if electrostatic effects are explicitly taken into account. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 10 Ill.
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  • 192
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 109-123 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Given a manifold of parameters associated with a trial many-electron wave function to evolve in a fictitious time, the Lagrangian of the corresponding classical system of the parameters' degrees of freedom is constructed explicitly for the case of the closed-shell Hartree-Fock approximation. The relevant equations of motion of a non-Newtonian type are derived. The formulae for the masses of the parameters' degrees of freedom are obtained in the limit of small velocities. The Newtonian dynamics described by the Car-Parrinello Lagrangian is discussed. © 1993 John Wiley & Sons, Inc.
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  • 193
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 125-134 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Explicit expressions for the eigenvalues of the single-cycle class-sums [(p)(1)n-p]n of the symmetric group (Sn), with p ≤ 20, are constructed. A new algorithm is used that makes no use of representation-theoretic data. The expressions obtained consist of polynomials in the symmetric power-sums over the “contents” of the Young diagram specifying the irreducible representation, with coefficients that are polynomials in n. On the basis of the results obtained for p ≤ 20, a conjecture is proposed concerning the general form of the four leading terms in these polynomials. © 1993 John Wiley & Sons, Inc.
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  • 194
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 89-108 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In this work, we study theoretically the thermal decomposition of weak dative complexes (namely, the complexes between BH3, BeH2, or LiH and ammonia or water). The most accurate calculations have been carried out at the MP4sdtq/6-311++G(3df,2p) level on MP2full/6-31G** fully optimized geometries. The transition structures for hydrogen evolution are described. The rate constants were obtained using conventional transition-state theory. Stability conditions for the complexes were considered. Water complexes are less stable than are the corresponding ammonia derivatives. Lithium complexes seem to be very unstable. It seems that beryllium compounds could be synthesized. Borane derivatives are known experimentally. We find, as observed, that the first hydrogen evolution occurs only above room temperature. Iminoborane seems not to be easily accessible under pyrolysis conditions. Other reactions require less activation energy. Finally, the early stages of the ammonia-borane pyrolysis and paths to open-chain polymers as well as to cyclic structures are described. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 195
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 196
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 147-147 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 197
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 135-146 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: It is demonstrated that the topological arrangement of atoms can guarantee the existence of stable electronic eigenvalues. These levels are stable in the presence of off-diagonal disorder. Graph theory is used to describe the topological structure of the Hamiltonian. The actual applications are presented in tight-binding approximation. It was found that graph theory can be used even if the atoms have more than one atomic orbital. Also, the localization properties of the topologically determined eigenvalues are studied. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 198
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 97-104 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Hydration to the aldehyde group in nitro-substituted benzaldehydes was studied theoretically by using an AM1 method. The calculated values of activation energy and heat of reaction showed good correlations with the experimental values of rate constants and equilibrium constants, respectively. To examine the hydration mechanism, localized molecular orbitals were calculated. Furthermore, expression of the molecular orbitals of supermolecular complexes in terms of occupied and vacant orbitals of the component molecules elucidated that the interaction between the lonepair orbital of water and the antibonding C—O π-orbital of aldehyde was important. 2,6-DNBAl was concluded to be less reactive toward water than was 2.4-DNBAl. This is consistent with the experimental result that the mutagenic activity of 2,4-DNBAl depends not only on the reduction of the nitro group by a bacterial enzyme but also on the acetyl conjugation, but that in the case of 2,6-DNBAl the mutagenic activity is manifested through the reduction by a bacterial enzyme. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
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  • 199
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993) 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 200
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    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 133-166 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We have extended the range of systems to which the half-projected Hartree-Fock (HPHF) method has been applied, including the triplet state of the wave function. In our implementation, DIIS overcomes the convergence difficulties reported in earlier studies. HPHF allows generation of a symmetry-broken wave function in regions of the potential energy surface where the RHF wave function is triplet-stable. The fractionally occupied natural orbitals (FONOS) of the HPHF wave function are good starting vectors for CAS-SCF calculations. A CAS-SCF in the space defined by the HPHF FONOS should be used instead of the unrestricted natural orbital CAS-SCF method in regions of triplet stability and for small active space problems. We draw extensive comparisons between the results of both the UNO-CAS and HPNO-CAS methods and those of full CAS-SCF calculations. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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