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  • 1990-1994  (478)
  • 1890-1899
  • 1993  (478)
  • Computational Chemistry and Molecular Modeling  (478)
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 405-423 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A numerical method is developed to obtain sequences of functions converging to the eigenfunctions of a Schrödinger operator in the Hilbert space L2(-∞, ∞), whose norm is used to introduce the criterion of convergence in the norm and we show that it guarantees the accurate computation of expected values of a symmetric operator. The method consists in solving the Dirichlet problem associated to the eigenvalue problem in the interval [-n, n] by the Ritz method, whose convergence to both eigenvalues and eigenfunctions is guaranteed by the compactness criterion. Using the asymptotic perturbation theory in L2(-∞, ∞), we prove the convergence of both eigenvalues and eigenfunctions of the Dirichlet problem to those of the unbounded system when the interval [-n, n] is expanded. The method is applied to the harmonic oscillator, the Mitra potential, as well as to the potential V(r) = r and the Coulomb and Yukawa potentials; in each case, the convergence of eigenvalues and eigenfunctions is shown. © 1993 John Wiley & Sons, Inc.
    Additional Material: 10 Tab.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 425-435 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a relativistic theory for the nuclear spin-spin coupling tensor within the polarization propagator approach using the particle-hole Dirac-Coulomb-Breit Hamiltonian and the full four-component wave function. We give explicit expressions for the coupling tensor in the random-phase approximation, neglecting the Breit interaction. A purely relativistic perturbative electron-nuclear Hamiltonian is used and it is shown how the single relativistic contribution to the coupling tensor reduces to Ramsey's three second-order terms (Fermi contact, spin-dipole, and paramagnetic spin-orbit) in the nonrelativistic limit. The principal propagator becomes complex and the leading property integrals mix atomic orbitals of different parity. The well-known propagator expressions for the coupling tensor in the nonrelativistic limit is obtained neglecting terms of the order c-n (n ≥ 1). © 1993 John Wiley & Sons, Inc.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 47 (1993), S. 437-448 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, we present a symmetrical and adiabatic reaction path for hydrogen transfer from methane by methyl by an ab initio method using the 6-31G** basis set with the MP2 corrections for UHF/6-31G** optimized geometries. The theoretical barrier is fitted with the symmetric Eckart function. We, then, take thermal averages of the rate constant (k) considering the discrete nature of the C-H oscillators and the continuous character of the various low-frequency modes of the composite system. Finally, we present a linear relation between log k and temperature. It is shown that this relation gives a good description of the kinetic results of both thermal and photochemical H-transfer reactions in the region of low temperatures. © 1993 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 151-159 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the regime of the molecular-orbital theory, a new approach beyond of the Hartree-Fock approximations is proposed. By an appropriate introduction of the quantum fluctuation into the molecular electronic ground state, the Hartree-Fock equations plus Gaussian fluctuation are obtained. Basically, the procedure proposed consists of an adequate utilization of the Gaussian approximation of the Ising model on a lattice (K.G. Wilson et al., Phys. Rep. C 12, 76 (1974)). It leads to an effective nonlinear Hamiltonian for the molecule for studying nonlinear problems in the molecular electronic structure. © 1993 John Wiley & Sons, Inc.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 185-200 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We review and update a recently developed one-particle theory of N-electron systems that bears a certain resemblance to density functional (DF) theory and has been put forward by the present author. As distinct from DF theory, the new one-particle scheme applies to any N-electron eigenstate including the ground state. For the latter case, the key equations become identical to those familiar from the Kohn-Sham version of DF theory. The theory given here constitutes the basis for a consistent treatment of normal conductivity and superconductivity, which will be discussed in Part II of this article. © 1993 John Wiley & Sons, Inc.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 161-183 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An explicitly connected commutator expansion for the average value of an observable in the coupled-cluster theory is derived. Specifically, it is shown that the expectation value of an operator for the state Ψ, related to the Fermi vacuum Φ by the exponential Ansatz ψ = eT Φ, is expressed as a finite commutator series containing the cluster operator T and an auxiliary operator S, defined by a linear equation involving again a finite commutator series in T. The above result is applied to derive the explicitly connected commutator form of the order-by-order many-body perturbation theory (MBPT) expansion for the expectation values and density matrices. We also show how the commutator expansion derived by us can be used as a basis for size-extensive infinite-order summation techniques. An operator technique of eliminating the nonlocal, “off-energy shell” denominators from MBPT expressions is proposed and applied to obtain compact commutator formulas for the expectation values of one- and two-electron operators through the fourth and third order, respectively, and for the correlation energy through the fifth order of perturbation theory. © 1993 John Wiley & Sons, Inc.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 201-209 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We show that a consistent N-electron theory of a solid can within the Born-Oppenheimer (BO) approximation lead only to normal conductivity or to an insulating state. If one leaves the BO approximation by turning on electron-phonon interaction, one can establish a lower-lying new ground state that has the characteristic properties of the Bardeen-Cooper-Schrieffer (BSC) ground state. We demonstrate that an (N + Nn)-particle Schrödinger equation that describes the motion of N electrons and Nn nuclei of a solid can lead only to phonon-induced superconductivity. © 1993 John Wiley & Sons, Inc.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 219-224 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The energy levels in sodium atoms with quarks attached to the nucleus are obtained using the configuration interaction method. It is found that the calculated 3p (2P) → 3s (2S) electronic transition in the Na-1/3 quark-atom system is in agreement with that predicted by extrapolation of experimental data. Also, ionization potentials of fractionally charged sodium atoms are discussed. © 1993 John Wiley & Sons, Inc.
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 225-232 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A survey is given of the resemblance theorems associated with various types of orthonormalization procedures having different kinds of symmetry properties appearing in the current literature. It is shown that the proofs for the minimum properties are closely connected with the fact that the Hilbert-Schmidt norm of an operator is always nonnegative and has the lowest value equal to zero. © 1993 John Wiley & Sons, Inc.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 233-238 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electric-field maps of certain anions and cations of adenine and guanine have been studied. The results are compared with those of neutral adenine and guanine. In all cases, the geometries were fully optimized using the MNDO molecular orbital method and the fields were computed using Mulliken charges. Reactivities and hydrogen-bonding pattern predicted by electric fields, including those of cations, are in satisfactory agreement with experiment. © 1993 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 239-247 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, a general method is presented that allows the derivation of the expansion coefficients of the product of two orthogonal functions provided the generating function is known. For the three classical orthogonal polynomials, the Laguerre, the Hermite, and the Legendre polynomials, the coefficients blmn with φmφn = ∑lblmnφl are derived. © 1993 John Wiley & Sons, Inc.
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 211-218 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: When second-order Jahn-Teller couplings become strong along “streambeds” on potential energy surfaces, instability reflected in negative curvature along a symmetry-lowering distortion coordinate can take place. The point where such negative curvature sets in is usually not a transition state because the gradient of the potential is usually large there. In this paper, it is demonstrated how to use the local energy, local gradient, local Hessian, and knowledge of how quickly the curvature for the symmetry-breaking mode evolves along the streambed (i.e., the derivative of this curvature) to predict how far to move in the symmetry-breaking mode in search of the desired transition state. It is shown that the Hessian matrix evaluated at the symmetry-broken geometry suggested by this analysis has only one negative eigenvalue. Because this analysis is based on a local approximation to the potential, its predictions are, of course, approximate. As such, they only “suggest” the proper direction and magnitude that one should “step” to move toward a transition state. © 1993 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 249-256 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronegativity perturbation theory up to the fourth order has been used to study the preferred isomer of linear triatomic molecules. Systems with 10-16 valence-electron systems are discussed and our results are in good agreement with the experiment. The only exception is the isomers of PNO. The usefulness of the perturbation theory is briefly discussed. © 1993 John Wiley & Sons, Inc.
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 257-265 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recurrence relations are derived for the evaluation of two-electron repulsion integrals (ERIs) over Hermite and spherical Gaussian functions. Through such relations, a generic ERI or ERI derivative may be reduced to “basic” integrals, i.e., true and auxiliary integrals involving only zero angular momentum functions. Extensive use is made of differential operators, in particular, of the spherical tensor gradient Ylm(∇). Spherical Gaussians, being nonseparable in the x, y, and z coordinates, were not included in previous formulations. The advantages of using spherical Gaussians instead of Cartesian or Hermite Gaussians are briefly discussed. © 1993 John Wiley & Sons, Inc.
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 267-276 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chain-length dependencies of 2Ag and 1Bu excitation energies as well as their unexpected inversion observed experimentally in trans-polyenes have so far been explained satisfactorily only in terms of configuration interactions within the standard Pariser-Parr-Pople (PPP) parametrization scheme with at least double excitations involving prohibitively large computational labor for long polyenes. A simpler calculation allowing nonzero differential overlap and employing restricted (first-order) single-excitation configuration interaction with chain-length-dependent screened potential is shown to provide an adequate alternative for the studies of those basic spectroscopic features of polyene excited states. The screening factor is parametrized in accordance with the chain-length-dependent behavior of the electrical polarizabilities of polyenes derived within the standard PPP approximation scheme. © 1993 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 225-225 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 19
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 189-207 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The atomic statement of the principle of stationary action determines the theorems that establish the mechanics of an atom in a molecule. The atomic torque, power, and current theorems are derived. They, along with the atomic force, virial, and continuity theorems, provide a complete description of the behavior of matter at the atomic level, including the effects of electric and magnetic fields. The atomic power theorem, for example, enables one to determine the atomic or molecular source of the creation and dissipation of power in combustion or in an explosion. An atom in a molecule is an open system and an atomic equation of motion for an observable contains a component corresponding to the flux in its current density through the atomic surface. The differential form of each theorem yields an equation of motion for the corresponding property density in which the same surface flux appears as a divergence of the current density. The differential and integrated atomic theorems complement those for the total system, enabling one to relate the property to its atomic components and ultimately to a local contribution from each element in real space. © 1993 John Wiley & Sons, Inc.
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 21
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 235-250 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In 1940, Kramers derived an expression for the rate constant of chemical reactions in viscous media. In this theory, chemical reactions are modeled as Brownian processes in the presence of potential barriers. The derivation of the rate constant is based on the solution of the Smoluchowski equation, a Fokker-Planck-type equation in position space. Kramers' theory has been confirmed for reactions in systems ranging from nuclear processes to biochemical problems. The reaction rate can be obtained from the diffusion currents appearing in the Smoluchowski equation. A Smoluchowski-type equation in position space, similar to the one applied by Kramers, has been used to find a nonlinear Schrödinger equation (NLSE) for dissipative, frictionally damped systems, taking into account wave-particle duality. For several potentials, the NLSE can be solved exactly, and analytic expressions for the currents, appearing in the Smoluchowski equation and necessary to determine the rate constants, can be obtained. These currents are not only stationary, but can also be time-dependent. It will be shown that, essentially, the determination of the time-dependence of the currents can be reduced to the solution of Newton-type equations of motion. © 1993 John Wiley & Sons, Inc.
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  • 22
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 263-294 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper considers a Husimi representation of quantum mechanics in which the (stationary) state of a system or ensemble is described by a Husimi function and an observable is described by a phase space function or distribution such that the expectation value of the observable is given by an integral over phase space of the product of that function or distribution and the Husimi function. The density matrix, Wigner function, and Husimi function are considered to be alternative ways of describing the state of a system or ensemble, and methods of recovering the Wigner function or density matrix from the Husimi function are discussed. The classical limits of the Wigner and Husimi functions and of the relationship between them are considered. © 1993 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 23
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 24
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 333-347 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The nonlocal weighted density approximation (WDA) to the exchange and kinetic energy functionals of many-electron systems proposed several years ago by Alonso and Girifalco in the context of density functional theory is used to compute, within the framework of the spherical jellium model, the ground-state electronic density, chemical potential, and total energy of neutral and negatively and positively charged sodium clusters containing up to N = 800 atoms. From these calculations, we study the behavior of the total energies per atom, chemical potentials, ionization potentials, I(N), and electron affinities, A(N). These results are compared with analogous calculations using the well-known Thomas-Fermi-Weizsäcker-Dirac (TFWD) approximation for the kinetic (TFW) and exchange (D) energy functionals. Calculations including the local density approximation (LDA) to the correlation energy in both WDA and TFWD functionals are also presented. © 1993 John Wiley & Sons, Inc.
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  • 25
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 363-383 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We derived the necessary conditions to which the vector coupling coefficients (VCC) amn and bmn, describing atomic L, S-multiplets of pMdN and dNs1 configurations (1 ≤ N ≤ 9, 1 ≤ M ≤ 5), should satisfy. It is shown that for two-open-shell systems under consideration the unknown VCC should satisfy not only usual restrictions resulting from the spheric symmetry, but also some additional equation introduced in the present paper in the form of a postulate. VCC obtained were used for the ab initio calculations (by the general SCF coupling operator method) of several transitionmetal atoms and ions with electronic configurations 3d14p1, 3p43d3, and 3p53d3. To check the presented theory, we carried out a detailed comparison between these results and analogous data, obtained by the atomic Roothaan-Hartree-Fock method [2]. © 1993 John Wiley & Sons, Inc.
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  • 26
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    International Journal of Quantum Chemistry 45 (1993), S. 385-390 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Schrödinger equation for an atom or molecule is expressed in terms of hyperspherical coordinates. The eigenfunction is expanded in a series of orthonormal complete sets: Yλ,μ(Ω), eigenfunctions of generalized angular momentum scalar operator, and Lvn, generalized Laguerre polynomials. The recurrence relation of the expansion coefficients are derived and the eigenvalues can be obtained from the secular equation. © 1993 John Wiley & Sons, Inc.
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  • 27
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have calculated certain dynamic polarizabilities (for both real and imaginary frequencies) for H, He, and H2 and the dispersion-energy coefficients for long-range interactions between them. We have done so in a sum-over-states formalism with explicitly electron-correlated wave functions to describe the states. To be precise, we have determined the dipole (α1), quadrupole (α2), and octupole (α3) polarizabilities of H and He for real frequencies (ω) in a range between zero and the first electronic-transition frequency and for imaginary frequencies (iω) on a 32-point Gauss-Legendre grid running from zero to ħω = 20 Eh, and for H2, we have found the dipole (α), quadrupole (C), and dipole-octupole (E) polarizability tensors for the same real and imaginary frequencies. The dispersion-energy coefficients, obtained by combining the sum-over-states for-malism for the polarizabilities with analytic integration over ω, gave values of C6, C8, and C10 for the atom-atom systems; C06, C26, C08, C28, and C48 for the atom-diatom systems; and C6, C′6 and C″6 for the H2—H2 system. Nearly all the results are considered to be more reliable than those hitherto published and some have been obtained for the first time, e.g., C(iω), E(ω), and E(iω) for H2 and C08, C48, and C48 for the H—H2 system. © 1993 John Wiley & Sons, Inc.
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  • 28
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    International Journal of Quantum Chemistry 45 (1993), S. 403-403 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 29
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 30
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    International Journal of Quantum Chemistry 45 (1993), S. 445-470 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, the efficient evaluation of the atomic integrals I =∫r1ar2br3cr23λr13μr12νe-αr1-βr2-γr3dτ with one or two factors rij-2 is described. These integrals are necessary for a lower-bound calculation for Li-like systems using the method of variance minimization or Temple's formula. © 1993 John Wiley & Sons, Inc.
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  • 31
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    International Journal of Quantum Chemistry 45 (1993), S. 409-431 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Coupled-cluster equations for the calculation of the nonexpanded (fully damped) dispersion energy are derived. These equations are solved in the ring approximation using the Møller-Plesset expansion in terms of the fluctuation potentials WA and WB for the individual molecules. Numerical results of high-order perturbative calculations for the He, H2, LiH, H2O, and HF dimers are presented and compared with the converged results computed using the same basis sets. It is found that the convergence of the Møller-Plesset expansion of the dispersion energy in the ring approximation is very fast. The padé approximants still accelerate this already good convergence. For all complexes studied in this paper, the sum of the corrections through the second-order in WA + WB reproduces over 99% of the converged value. The sum of third- and higher-order corrections in the ring approximation is found to be one or two orders of magnitude smaller than the sum of second-order terms not included in the ring approximation and, therefore, may be safely neglected. Thus, it appears that a second-order calculation, which does not require iterating coupled-cluster equations or solving random phase approximation equations, offers the best compromise between accuracy and computational requirements. © 1993 John Wiley & Sons, Inc.
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  • 32
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    International Journal of Quantum Chemistry 45 (1993), S. 471-484 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The integrity bases for representations of the nuclear permutation groups Sn (n = 3,4) in the space of internal coordinates describing AXn molecules are determined. This allows optimal expressions of the molecular potential energy functions in terms of internal coordinates that are totally symmetric with respect to permutations of indistinguishable nuclei. © 1993 John Wiley & Sons, Inc.
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  • 33
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    International Journal of Quantum Chemistry 45 (1993) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 34
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    International Journal of Quantum Chemistry 45 (1993), S. 491-505 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 35
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    International Journal of Quantum Chemistry 45 (1993), S. 489-490 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 36
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    International Journal of Quantum Chemistry 45 (1993), S. 563-572 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A p-version finite element method has been used to calculate second-order pair and total correlation energies for closed-shell Mg and Ar. Comparison with the best results found in the literature suggests that the present values are the most accurate and that the method should perform comparatively better for heavier elements. © 1993 John Wiley & Sons, Inc.
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  • 37
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some topics in relativistic quantum chemistry are reviewed with special emphasis on 4-currents and 4-potentials. It is shown that, both in molecular quantum theory and in solid-state physics, calculations can include relativistic and magnetic effects by means of 4-currents without an excessive increase in complication, provided that 4-component Dirac spinors are used rather than the Pauli approximation. © 1993 John Wiley & Sons, Inc.
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  • 38
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    International Journal of Quantum Chemistry 45 (1993), S. 587-590 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Consider two orbital sets χk, k = 1…m and η1, 1 = 1…n, which are mutually nonorthogonal. Provided that n 〉 m, at least n - m orbitals of the set {η} can be orthogonalized to the set {χ} by a transformation within the set {η}. The orthogonalization of the remaining orbitals of {η} to the set {χ} requires a transformation in which the χk appear explicitly. The orthogonalization of one orbital set to another is relevant for SCF optimizations in a truncated basis set, in the presence of frozen occupied orbitals. Examples are frozen core calculations, ECP calculations, and embedded cluster calculations, where the cluster is embedded in a frozen environment. A simple orthogonalization scheme, which makes use of a corresponding orbital transformation, is presented. It is suggested that with a small, well-defined extension of the set {η} the complete orthogonalization can be done with a transformation in which the {χ} do not appear explicitly. © 1993 John Wiley & Sons, Inc.
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  • 39
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    International Journal of Quantum Chemistry 48 (1993), S. 67-80 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ground state electron correlation is introduced into the one-particle propagator via coupled cluster theory. This defines a similarity transformation of the Hamiltonian, which leads to the complete separation of the ionization and electron attachment aspects of the propagator. The latter makes it possible to solve for each property independently. Furthermore, the frequency (or energy) dependence which characterizes propagator theory is eliminated by introducing a wave operator formalism. It is shown that this procedure is equivalent to the summation of certain types of terms in the electron propagator perturbation expansion to infinite order. Finally, the resulting equations are found to be equivalent to those of the Fock-space coupled-cluster (or equivalently the equation of motion coupled-cluster) method, which provides an explicit wave function for each state, demonstrating the connection between these different approaches for the calculation of ionization potentials and electron affinities. Understanding this relationship permits new and powerful approximations to be proposed. © 1993 John Wiley & Sons, Inc.
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  • 40
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    International Journal of Quantum Chemistry 48 (1993), S. 155-162 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 7 Tab.
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  • 41
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    International Journal of Quantum Chemistry 45 (1993), S. 71-85 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variational principle for the transition matrix is considered. Conception of the MC SCF (in particular, CASSCF) approach for the transition matrix is proposed as a solution of the variational problem on the optimal evolution of a packet of two stationary states with some additional conditions. An ordinary MC SCF method for a single state is a special case of the proposed approach. Some aspects of the solving of the equations for the optimal transition matrix are treated. The method can be used in atomic and molecular calculations of transition energies, oscillator strengths, and other properties. © 1993 John Wiley & Sons, Inc.
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  • 42
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    International Journal of Quantum Chemistry 45 (1993), S. 105-114 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some operators considered conventionally as Hermitian are not strictly Hermitian for the freeelectron states known especially in the case of solids. The consequences of this fact on the closure property are examined. © 1993 John Wiley & Sons, Inc.
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  • 43
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    International Journal of Quantum Chemistry 45 (1993), S. 115-117 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 44
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    International Journal of Quantum Chemistry 45 (1993), S. 123-132 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The barriers to internal rotation about the N—C bond of formamide and the formamide-H2O complex have been examined by ab initio quantum chemistry methods. Both self-consistent field and correlated approaches have been utilized to determine the geometries, energies, and local harmonic vibrational frequencies of the minimum-energy and transition-state structures of these two systems. We find that formamide's rotation barrier of 14-15 kcal/mol is increased to 16-18 kcal/mol when a single H2O molecule is attached. This result contrasts with the effect of a single H2O solvent molecule on the barrier to tautomerization of formamide (to form formamidic acid HN—CHOH) for which a barrier lowering of ca. 20 kcal/mol has been observed. The rotation barriers obtained for formamide and for its H2O complex are compared with barriers obtained experimentally in various solvents. © 1993 John Wiley & Sons, Inc.
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  • 45
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    International Journal of Quantum Chemistry 45 (1993), S. 167-176 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new formulation of the eigenvector equations for certain polymeric molecules is given. The electronic bands for the infinite molecules are deduced. For the finite molecules, the use of ghost molecules allows an exact specialization of the infinite to the finite eigenvector equations. The complete solution of the equations depends on the solution of a simple transcendental equation. Approximate solutions of this equation are given that bracket the accurate solutions so that, for molecules with many repeated moieties, the approximations become good. The general theory is illustrated by applications to the chrysene series and to the polyphenyls. © 1993 John Wiley & Sons, Inc.
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  • 46
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    International Journal of Quantum Chemistry 48 (1993), S. 269-285 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We exploit the unitary group formalism in formulating a multiconfigurational single-reference coupled cluster method for cases involving one or two electrons in open shells. The linear version of CCSD theory for the simple open shell case and for low lying singlet states of closed shell systems are considered in detail. The entire formalism is related to the unitary group based CISD method, and explicit expressions for size-extensivity corrections, leading to the L-CCSD formalism, are given. An illustrative example of the minimum basis set model of the BeH radical is also presented. © 1993 John Wiley & Sons, Inc.
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  • 47
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    International Journal of Quantum Chemistry 48 (1993), S. 303-308 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This article gives an improved derivation for U(n1 + n2) ⊃ U(n1) × U(n2), SN1+N2 ⊃ SN1 × SN2, and other transformation coefficients by using a series of elementary Racah transformations. These transformation coefficients can be expressed generally in terms of a product of segments containing 6j symbols and easily calculated. Possible segment factors are given in Table I. © 1993 John Wiley & Sons, Inc.
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  • 48
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    International Journal of Quantum Chemistry 48 (1993), S. 309-319 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio electronic structure calculations, carried out in conjunction with rigorous analysis of the resulting wavefunctions, demonstrate that the electronic factors responsible for the unusual stability of the 1,3-dimethylimidazol-2-ylidene carbene are directly related to the substantial σ-backdonation from the carbenic carbon to the adjacent nitrogen atoms. The π-donation is found to play only a minor role, ruling out the presence of significant stabilization due to ylidic resonance structures. The unusually large proton affinity of the carbene is explained by the extra π-electron stabilization of the corresponding 1,3-dimethylimidazolium cation. The MP2/6-311++G** level of theory is found to be sufficient for accurate predictions of the geometries of the imidazol-2-ylidene carbenes. © 1993 John Wiley & Sons, Inc.
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  • 49
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    International Journal of Quantum Chemistry 48 (1993), S. 331-341 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Vibration frequencies for 2-aminoethanol, neutral glycine, glycolic acid, n-propylamine, n-propanol, and propionic acid were calculated on the ab-initio RHF level with a variety of standard basis sets including polarization and diffuse functions. Experimental frequencies, if available, are compared with these data and basis set suitability is discussed on the basis of this comparison. © 1993 John Wiley & Sons, Inc.
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  • 50
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    International Journal of Quantum Chemistry 48 (1993), S. 355-361 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetic energy component is shown to be a well-defined quantity in the divide-and-conquer method. This kinetic energy converges to the conventional Kohn-Sham kinetic energy when the numbers of the “buffer atoms” for subsystems increase. Numerical results from sample calculations confirm that the kinetic energy component converges to the Kohn-Sham value as the basis set increases. © 1993 John Wiley & Sons, Inc.
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  • 51
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    International Journal of Quantum Chemistry 48 (1993), S. 479-490 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of an ab initio SCF (4-31G) study of the potential energy surface of 4-aminobutanol are reported. Four different intramolecular hydrogen bonds are present in the various local minima: N … H—O, N—H … O, C4—H … O, N … H—C1. These interactions are discussed and compared with those present in the homologues 3-aminopropanol and 2-aminoethanol. © 1993 John Wiley & Sons, Inc.
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  • 52
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Many-body perturbation theory is applied to the nitrogen triple bond for bond distances ranging from the atomic regime to about 0.6a0 shorter than equilibrium. A full-optimized reaction space model is used to compute orbital spaces with an even-tempered gaussian-type basis set and also with a nominal Bagus-Gilbert Slater-type basis set. Conservation of orbital angular momentum in the atomic regime leads to perturbative theory for Hartree-Fock plus proper dissociation. Angular momentum conservation can also be enforced with a scaled Slater-Condon parameter. Third-order dissociation energies and spectroscopic constants approach limits of the chosen basis sets. © 1993 John Wiley & Sons, Inc.
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  • 53
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure of 2,3-diketopiperazine in a crystal phase (P21/c), modeled by a 13-molecule cluster surrounded by point charges, was completely optimized using standard gradient procedures. The (ab initio) MIA approach was used to perform the SCF step, which using a 4-21G basis set, involves calculations using 1092 basis functions. Results are in very good agreement with experimental (X-ray) data, in contrast to a previous study in which a model was used consisting of only point charges. © 1993 John Wiley & Sons, Inc.
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  • 54
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    International Journal of Quantum Chemistry 46 (1993), S. 87-100 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hartree-Fock calculations of the molecular geometry and electronic properties (ionization potentials) of bare Nin clusters (n = 3, 7, 13, and 25) and nickel clusters bearing a chemisorbed CH3 radical are reported. A basis of double-zeta quality on light atoms including carbon and a model core potential along with double-zeta plus polarization auxiliary basis set for nickel atoms have been used. Computed properties serve to assess cluster-size convergence when nickel atoms core electrons are described with [1s-3d] pseudopotentials. In spite of their simplicity, results with these potentials are in good agreement with previously reported theoretical and experimental data on seven-atom clusters. This suggests that the cluster-model approach to surface bonding using simple pseudoopotentials can lead to meaningful results and tractable computationl times when applied to larger chemisorbed organic molecules. Thus, this approach may be adequate to help find out direct experimental evidence of the interfacial bonding for grafted structures obtained under cathodic electropolymerization of acrylonitrile on nickel surfaces. © 1993 John Wiley & Sons Inc.
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  • 55
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    International Journal of Quantum Chemistry 46 (1993), S. 101-107 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: There exist problems in condensed-matter theory that require evaluating infinite Bloch sums of multipolar potential r-l-1Yl,m(θ,ϕ) on a periodic lattice. For an arbitrary multipolar order l, tractable formulas are given for summing such interactions on a two-dimensional Bravais lattice and evaluating their Bloch sums at a point outside as well as inside the plane of the lattice. The approach used is the Ewald method, which consists of separating the original series in rapidly converging sums in reciprocal and real spaces. Computational aspects of the present formulation are briefly reviewed. © 1993 John Wiley & Sons, Inc.
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  • 56
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    International Journal of Quantum Chemistry 46 (1993) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 57
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    International Journal of Quantum Chemistry 46 (1993), S. 271-294 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Structure and the energy of the stable conformations of the two possible stereochemical configurations of 2,4-diphenylpentane are obtained using the ab initio molecular orbital theory. The objective was to mimic the possible structures and determine the corresponding energies of the dyads of syndiotactic and isotactic polystyrene and, consequently, to study the interactions between the phenyl groups. The results of complete geometry optimization showed significant changes in geometrical parameters compared with those expected from the ideal hydrocarbon structure. The steric strain is most pronounced in some of the gauche conformations where large (approximately 40°) distortions of the backbone torsional angles and/or simultaneous phenyl group rotations in the range of 30°-40° away from its global minimum position may occur. In addition to the discussion of the geometrical parameters, the corresponding dipole moments are also calculated and differences related to the various structures and discussed. © 1993 John Wiley & Sons. Inc.
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  • 58
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    International Journal of Quantum Chemistry 46 (1993), S. 319-333 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The eigenvalues of the Schrödinger equation with a polynomial potential are calculated accurately by means of the Rayleigh-Ritz variational method and a basis set of functions satisfying Dirichlet boundary conditions. The method is applied to the well potentials having one, two, and three minima. It is shown, in the entire range of coupling constants, that the basis set of trigonometric functions has the capability of yeilding the energy spectra of unbounded problems without any loss of convergence providing that the boundary value α remains greater than a critical value αcr. Only the computation of the nearly degenerate states of multiwell oscillators requires dealing with a relatively large truncation order. © 1993 John Wiley & Sons, Inc.
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  • 59
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    International Journal of Quantum Chemistry 46 (1993), S. 339-341 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 60
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    International Journal of Quantum Chemistry 46 (1993), S. 119-125 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The lowest-energy N4 is computed ab initio to be the planar C2h(3Bu) open-chain structure 13. The open-chain N4 singlet-state structures dissociate on geometry optimization. The tetraazatetrahedrane Td structure 1 and the tetrazete D2h structure 2 are minima at MP2/6-31G*. However, both are higher in energy than 13 (24.1 and 21.2 Kcal/mol [UQCISD) (T)(full)/6-311+G*//MP2/6-31G* + ZPE(MP2/6-31G)*, respectively]. The energy of 13 is 157.5 kcal/mol higher than that of two N2(1∑g+ molecules [UQCISD(T)(Full)/6-311+G*//MP2/6-31G*] © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993), S. 671-671 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    International Journal of Quantum Chemistry 46 (1993), S. 673-674 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 63
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    International Journal of Quantum Chemistry 46 (1993) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 64
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    International Journal of Quantum Chemistry 48 (1993), S. 815-826 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Many xenobiotics containing N or S heteroatoms are metabolized by the cytochrome P450s, leading to a variety of products which can be toxic, carcinogenic, or detoxifying. Thus, it is important to try to establish molecular criteria that modulate competitive product formation for these types of compounds. In the absence of 3D structures for the P450 isozymes that are responsible for the oxidations of N- and S-containing compounds, we have focused here on the characterization and identification of possible electronic and thermodynamic factors that could be modulators of different types of product formation. Specifically, the competition among N-oxidation, N-hydroxylation and Cα-hydroxylation for three amines were examined. Similarly, three thioethers were studied for their internal competition between S-oxidation and Cα-hydroxylation. The results obtained indicate that the stability of the cation radical intermediate formed by the one electron transfer mechanism is not a determinant of differences in product distribution between the two types of compounds. Rather, relative product stability appears to be a significant modulator of product distribution explaining why S-oxide formation is favored over N-oxide and why Cα-hydroxylation is usually favored over N-oxide formation in amines. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993), S. 711-734 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This work analyses the adequacy of an analytical electrostatic energy formulation within the CNDO framework to predict the stable conformations of large molecular complexes. Comparisons are made with abinitio results for small systems such as water-formamide, methanol-water-imidazole, or guanine-cytosine and with AM1 results for two large systems: a molecular tweezer + the 9-methyladenine complex and a model active site of the α-chymotrypsin and its ligand complex. This approach is efficient in providing reliable confromers for large molecular systems in a very fast way. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993), S. 425-474 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We introduce a method for the explicit computation of the eigenvalue problem of the evolution operator of mixing dynamical systems. The method is based on the subdynamics decomposition of the Brussels-Austin groups directed by Professor I. Prigogine. We apply the method to three different representatives of mixing systems, namely, the Renyi maps, baker's transformations, and the Friedrichs model. The obtained spectral decompositions acquire meaning in suitable rigged Hilbert spaces that we construct explicitly for the three models. The resulting spectral decompositions show explicitly the intrinsic irreversibility of baker's transformations and Friedrichs model and the intrinsically probabilistic characters of the Renyi maps and baker's transformations. The dynamical properties are reflected in the spectrum because the eigenvalues are the powers of the Lyapunov times for the Renyi and baker systems and include the lifetimes for the Friedrichs model. © 1993 John Wiley & Sons, Inc.
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  • 67
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    International Journal of Quantum Chemistry 46 (1993), S. 483-498 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new base pair (called κ-π) of Watson-Crick type, with a H-bond pattern different from that in A-T and G-C base pairs, has been recently synthesized and shown to be stable and incorporable into duplex DNA and RNA by polymerases. This new basepair, which contains three H-bonds, is compared with G-C, in the framework of modern dynamical theory of quantum nonlocality and quantum correlations. Connection with the traditional treatment of proton transfer in DNA base pairs, which uses the adiabatic approximation, is explicitly made. As a result, the dynamics of the H-bond pattern of G-C is shown to exhibit a specific quantum mechanical phase stability, which is clearly missing in the case of κ-π. This finding is discussed and illustrated, also in connection with recent quantum chemical calculations of proton transfers in DNA base pairs. Additionally, certain speculations concerning the “evolutionary advantage” of G-C with respect to κ-π are briefly considered. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993), S. 475-481 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Band structure with a quasicontinuum of electronic states near the Fermi surface (FS) of a solid-state matter indicates overlapping regions of the electronic and phonon spectra: (εP - εQ) ≈ ℏωv. This simple fact has an important impact on the theoretical aspects of the description and a study of an electronic structure and physical properties of a matter since velocities of the nuclei and electrons are on the same scale. The motion of the electrons is not adiabatic and the description of the electronic structure of a system cannot be based on the assumption of the validity of Born-Oppenheimer approximation. For nonadiabatic systems in contrast to the descripton of adiabatic systems [(εp - εQ) 〉 ℏωv], an effect of the finite mass of the electrons, i.e.· nonadiabatic corrections, cannot be calculated by a perturbation theory due to the divergency of a perturbation expansion. Quasiparticle transformation technique has disclosed the unexpected effects of nonadiabaticity on the electronic structure and physical properties of matter. The aspects of superconductivity are discussed. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 70
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    International Journal of Quantum Chemistry 46 (1993), S. 499-504 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is advocated that the H-bond patterns of the standard C-G and A-T base pairs have an evolutionary advantage over any other H-bond scheme accommodated within the Watson-Crick-type geometry. A suggested proof of the statement is given by the Longuet-Higgins' sign-reversing loop argument. The present analysis indicates a close relation with self-organizational principles. © 1993 John Wiley & Sons, Inc.
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  • 71
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    International Journal of Quantum Chemistry 46 (1993), S. 505-510 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some elementary properties of a general linear operator, which is defined on a Hilbert space but is not necessarily self-adjoint, are briefly reviewed: the classical canonical form, the bi-orthogonality theorem, the bivariational principle, the spectral resolution of the resolvent, and the properties of exponential forms. © 1993 John Wiley & Sons, Inc.
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  • 72
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    International Journal of Quantum Chemistry 46 (1993), S. 515-518 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Squeezed states are normally studied in configuration space or using the Glauber formalism for the coherent states. Here we show that the use of the Fock-Bargmann space can lead to more general solutions, especially in the case of squeezed states associated with the square of field amplitude. The relations that we obtain are completely general and several well-known, approximate results can be obtained as the limit cases of our approach.© 1993 John Wiley & Sons, Inc.
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  • 73
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    International Journal of Quantum Chemistry 46 (1993), S. 535-576 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An ab initio method for the calculation of atomic and molecular electronic wave functions is presented. The “Optimized-Basis-Set Multiconfiguration Spin-Coupled” (OBS-MCSC) method may be viewed either as a multiconfiguration generalization of the spin-coupled approach or as a nonorthogonal variant of the MCSCF method. In addition, the OBS-MCSC method optimizes the basis-set exponential parameters simultaneously with all other variational parameters, through a second-derivative minimization procedure. Explicit analytic expressions for the required first and second derivatives of the energy with respect to all variational parameters are obtained. Test calculations prove the capability of the method to yield compact yet accurate electronic wave functions.© 1993 John Wiley & Sons, Inc.
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  • 74
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    International Journal of Quantum Chemistry 46 (1993), S. 583-587 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Colle-Salvetti second-order reduced density matrix (2-matrix) is an approximation to the 2-matrix obtained from a wave function that is a product of a reference wave function containing little or no correlation times a product of correlation factors that are functions of the coordinates of pairs of electrons. A formal proof is given for the non-N-representability for the Colle-Salvetti 2-matrix using the nonnegativity condition of the 2-matrix. The nonnegativity condition of the particle-hole overlap matrix (G matrix) is also not satisfied. The proof is valid for Colle-Salvetti 2-matrices obtained from both the Hartree-Fock and small multiconfigurational-self-consistent-field wave functions. Even though the Colle-Salvetti 2-matrix is not N-representable, it does satisfy the Pauli principle component of the G-matrix condition because it reduces to an N-representable first-order reduced density matrix. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    International Journal of Quantum Chemistry 46 (1993), S. 593-607 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The unrestricted complete active space self-consistent field (UCASSCF) function is defined, and a proof that a UCASSCF eigenfunction of the spin operator S2 is a CASSCF function is given. The spin-contamination for an unrestricted Hartree-Fock (UHF) function is evaluated by using Araki angle operators, and the UHF function is then projected on the restricted open-shell Hartree-Fock (ROHF) space. The present analysis has deep consequences since it implies that the only non-spin-contaminated UHF functions are the ROHF functions. This is illustrated in a calculation of the spin density of He2+. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993), S. 609-622 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The hydrogen atom in a semi-infinite space limited by a hyperboloidal boundary, with the nucleus at a focus, is investigated as a model of an atom on the surface of a solid. The energy eigenvalues, hyperfine structure, and electric dipole moment of the system are evaluated for different focal distances and eccentricities of the boundary. It is shown that the system tends to become infinitely degenerate at the ionization threshold as the boundary closes in approaching the nucleus. This model includes as special cases the corresponding models in the literature with plane and paraboloidal boundaries. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993), S. 623-634 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure and energetics of the complexes formed between SCO/He and SCO/N2 were investigated using ab initio wave functions at both the SCF and correlated levels of theory with a medium-sized polarized basis set. The energy of the T-structure for SCO⃛He was found to be lower than the energy of the corresponding linear complexes with He bonded to the oxygen or sulfur atoms. Two linear structures for SCO⃛ He were found to be nearly isoenergetic. There is only a small difference in energy between the T-structure for SCO⃛N2 and the OCS⃛N—N colinear structure, with the T-structure being the lowest. The electron correlation contributions to the interaction energy were calculated using Möller-Plesset perturbation theory at the MP2, MP3 and SDTQ-MP4 levels. Analysis shows the importance of the triple excitations in these complexes. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 46 (1993), S. 635-649 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three hierarchically ordered topological factors, i.e., atom connectivity, centrality, and cyclicity, were found to control the number of self-returning walks (SRWS) associated with every atom in the molecule. The reversal of their order was observed in a few cases where the central location of atoms had a stronger influence than did their connectivity on the number of SRWS. Three atomic topological indices, i.e., the Morgan extended connectivity, the Balaban, Mekenyan, and Bonchev hierarchical extended connectivity, the Randić atomic path code, were found to closely match the ordering of atoms in molecules determined by their number of SRWS. New atomic graph invariants fi = lim SRWin/SRWn and ti = fi · SRW2 were specified and may find application in QSAR and QSPR. The ti indices are nonintegers close to atomic valence. The fi indices represent the limit of the number of SRWS of length n for the atom i, SRWin, normalized by dividing it by the total number of SRWS for the molecule. In the case of Hückel MO considerations, these invariants were shown to be numerically equal to the partial electronic charges of the lowest occupied molecular orbital (LOMO). A new class of isocodal atoms (atoms having the same number of SRWns) was observed, i.e., atoms that become isocodal only at n ≥ 1. A number of open questions following from these findings were formulated, including the possibility for a topological modeling of electron correlation. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 47 (1993), S. 213-223 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In a numerical experiment, the dynamics of natural decay of a metastable state is shown to be strongly affected by the presence of an external time-varying electric field of high intensity. The metastable state, depending upon its level width, is shown to be stabilized by a near-resonant electric field. The applied field may even cause population transfer to a neighboring bound level that is not directly populated by the natural decay process. The effects of continuous and pulsed perturbations are both analyzed numerically, and perturbation, theoretically. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 47 (1993), S. 225-229 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The previously described optimization of small Gaussian expansions, to be employed for the computation of many-center two-electron integrals over Slater basis sets, has been extended, with some refinements, up 4d orbitals. © 1993 John Wiley & Sons, Inc.
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  • 82
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    International Journal of Quantum Chemistry 47 (1993), S. 177-183 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An attempt has been made to study the effect of a nonuniform positive background on the ground-state energy of a nonmagnetic Wigner lattice. The background is modified with a variable “ripple” parameter in the background charge, represented by a Yukawa-type charge distribution. Our calculation shows that a Yukawa-type background leads to a lower ground-state energy value compared to a Gaussian-type and constant positive background. © 1993 John Wiley & Sons, Inc.
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  • 83
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    International Journal of Quantum Chemistry 47 (1993), S. 231-238 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The helical region of the potential energy surface of blocked α-aminoisobutyric acid (Aib) dipeptide has been studied by using ab initio and semiempirical quantum mechanical methods, as well as force-field-derived methods. Depending on the method, an α-helix or a 310-helix is found to be the energy minimum. The conformations obtained from computations performed at the ab initio quantum mechanical level, as well as by using the AMBER force field, are in excellent agreement with X-ray data. Semiempirical results display some important differences with regard to experimental data. On the other hand, the CVFF force field predicts no energy minimum in the helical region of the Aib potential energy surface. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 48 (1993), S. ix 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 85
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    International Journal of Quantum Chemistry 48 (1993), S. 213-230 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metropolis Monte Carlo and Poisson-Boltzmann calculations were done to quantitatively assess the conditions under which counterion condensation (CC) theory could be considered valid. The fundamental prediction of condensation theory, that the number of counterions bound to a polyelectrolyte molecule can be predicted by a single parameter describing the linear charge density of the charged system, was shown to be quantitatively correct for a range of conditions. To define the number of counterions bound, it was necessary to use an energy-based criterion by which ions that interact with the polyion with an energy less than -kT were considered bound. Using this criterion, Monte Carlo calculations on systems consisting of charged cylinders and a neutralizing number of counterions in a dielectric continuum showed that the number of bound counterions was quantitatively predictable by the CC relation (1-1/ξ) for systems with a linear charge density and dimensions approaching those of duplex or triplex DNA. Poisson-Boltzmann (PB) calculations on cylinders with different linear charge densities and radii have been done to assess the limits of the CC prediction that the number of counterions bound is a constant even as the bulk concentration of electrolyte in the surrounding is increased. As in the case of the MC calculations, the validity of the CC prediction is seen to increase with increasing linear charge density of the charged cylinder. The agreement between PB and CC is seen to be very good for highly charged cylinders. The results described here provide justification for the use of CC theory for interpreting experimental data on polyelectrolytes of the approximate dimensions and linear charge density of duplex or triplex DNA. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 48 (1993), S. 25-32 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    International Journal of Quantum Chemistry 48 (1993), S. 43-50 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Supermolecular model of the intermolecular interaction energy is studied. The correlation energy components are calculated by using the many-body perturbation theory (MBPT) in the canonical representation as well as in the localized one. The components of the corrections of the different order are analyzed individually. The effects of the localization procedure and the basis set are also investigated. An H2O trimer is used to study the two- and three-body components of the interaction energy. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 48 (1993), S. 163-173 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have used the ab initio technique of UHF/LANL 1 MB to study the interactions between absorbed oxygen atoms and molecules with their host nickel lattice. The host lattice is approximated by a small cluster of nickel atoms that are arranged as face-centered-cubic, except for a tubular void described by the absence of three or four lattice elements. The oxygen absorbates are placed, one at time, within this cavity. We conclude that, for tubular cavities, the interaction between the nickel lattice and the oxygen absorbates is stronger than the interactions between pairs of oxygen atoms, suggesting that the absorbates are mostly atomic (as apposed to diatomic) in character, while the (energetically) most preferred number of oxygen atoms to reside inside an n-site, tubular hollow of nickel is simply (n - 1). © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 48 (1993), S. 175-186 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An ab initio perturbed ion study using X-ray diffraction data has been carried out for ZrSiO4 (zircon), ZrO2 (monoclinic zirconia, baddeleyite), and SiO2 (α-cristobalite) crystal lattice structures. The different substitutions of V4+ for Zr4+ and Si4+ occurring in these host lattices have been analyzed. Geometry optimizations have been performed with the aim of determining the relative stability, cell parameters, and force constants of radial displacement associated with the local relaxation for pure and doped structures. Numerical results are confronted against experimental data and compared with previous results. The geometrical cell parameters of different structures obtained by computer simulation and the results of the X-ray diffraction studies agree with previous experimental data. For the zircon lattice, the substitution of V4+ for Zr4+ at an eightfold-coordinated site is energetically favorable while the substitution of V4+ for Si4+ at a fourfold-coordinated site is unstable. For ZrO2, the substitution of V4+ for Zr4+ is energetically favorable while the substitution of V4+ for Si4+ in SiO2 is energetically unfavorable. There is less sensitive influence of the crystal lattice parameters for substitutions occurring at the eightfold-coordinated ion site in ZrSiO4 and SiO2 structures. The doping process produces a decrease of force constant (k) values associated with the breathing fundamental vibrational mode for all structures. The k associated with the radial displacement in dodecahedral substitution in the ZrSiO4 structure is especially high. The force constants for this movement in tetrahedral substitution in the ZrSiO4, ZrO2, and SiO2 structures have a lower value. The differences between ionic radii reported by Shannon and Prewitt of the species concerned in the doping process are not capable of explaining the relaxation of crystal lattice parameters in the ZrO2 and SiO2 structures. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 48 (1993), S. 227-243 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of first-principles, density-functional, LMTO calculations on quasi-one-dimensional systems are presented. Special emphasis is put on the problems related with studying the properties of real materials by means of electronic structure calculations. As examples we discuss undoped and doped trans polyacetylene, an MX chain compound, CaNiN, and a comparison between polycarbonitrile and hydrogen cyanide. It is argued that single-chain calculations on undoped polyacetylene predict a bond-length alternation, whereas this may be suppressed in multichain calculations. For an MX chain we demonstrate the importance of including the full ligand structure, whereas many properties of CaNiN can be understood as intrinsic to single chains. A comparison between the covalently bonded polycarbonitrile and the hydrogen-bonded hydrogen cyanide reveals differences between delocalized and localized electrons but also inaccuracies in the relative total energies. Finally, results for doped trans polyacetylene indicate relatively large electronic dopant-polymer hopping integrals as well as problems in describing electron transfers for weakly interacting systems when using a local-density approximation. In total it is demonstrated that reliable and useful informations are obtained in most cases, but also that one has to be careful in choosing the idealized system that is to represent the real material and that certain aspects of the local-density approximation should be improved. © 1993 John Wiley & Sons, Inc.
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  • 91
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    International Journal of Quantum Chemistry 48 (1993), S. 263-268 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nonlocal density-functional and correlated ab initio methods have been used to compute singlet-triplet separations for the Pd atom and Pd2, as well as the dissociation energies of the latter in its ground and first excited states. The results are in good agreement with the available experimental data and with other high-level calculations. Single-point local density-functional computations were carried out for Pd clusters up to Pd22 (1012 electrons). The interaction energy per Pd—Pd bond appears to be approaching a limiting value for Pd22. © 1993 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 48 (1993), S. 33-42 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Statistical electron correlation coefficients provide overall measures of the difference between the electron pair density and the product of one-electron densities. Explicitly correlated wave functions are used to compute radial and angular correlation coefficients for 29 low-lying states (n1S, n3S, n1P, n3P, n1D, and n3D with n 〈 7) of the two-electron ions from helium through Ne8+. This data base of 1566 correlation coefficients permits a systematic study of trends with respect to variation of nuclear charge, degree of excitation, angular momentum, and spin multiplicity. © 1993 John Wiley & Sons, Inc.
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  • 93
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 81-89 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A serious problem in the ab-initio Hartree-Fock calculation for extended systems is a singularity in the exchange contribution to the band energies (the “exchange pathology”). In the homogeneous electron gas this pathology is exactly cancelled by the correlation contribution arising from the sum of all time-ordered ring diagrams (RPA approximation). In this article we show that this cancellation is more general and does also occur in periodically extended systems. © 1993 John Wiley & Sons, Inc.
    Type of Medium: Electronic Resource
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  • 94
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 101-109 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this article we calculate the asymptotic structure of the exchange potential for the Fermi level electron in the vacuum region outside a metal surface, and show it to be the image potential. In this asymptotic region the exchange potential due to the Fermi level electron is equivalent to the optimized potential of exchange-only density-functional theory. Thus, the asymptotic Kohn-Sham effective potential at a metal surface is the image potential, and arises soley due to correlations which result from the Pauli exclusion principle. © 1993 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 95
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 93-100 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In many situations, the most long-ranged parts of the exchange and correlation holes surrounding an electron cancel one another. Apparently for this reason, local spin density and generalized gradient approximations are more accurate for exchange and correlation together than for either alone. A study is made of the ability of these density functionals, and also the unmodified second-order gradient expansion, to describe various short-range effects in atoms: the correlation contribution to the interacting kinetic energy, the antiparallel-spin correlation energy, and the correction to the random phase approximation. Generalized gradient approximations, constructed with no adjustable parameter from the electron gas of slowly varying density, are found to give results of useful accuracy for real atoms. Prospects are discussed for use of the new functionals to improve the accuracy of electronic-structure calculations. © 1993 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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  • 96
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 131-146 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structural and electronic properties of polysilane, (SiH2)x and polysilene, (SiH)x will be compared with their carbon-based analogues polyethylene, (CH2)x, and polyacetylene, (CH)x, as investigated by the ab initio crystal orbital method using several, partly highly polarized, atomic basis sets of increasing size at the Hartree-Fock (HF) level and by including electron correlation effects up to the third order of Møller-Plesset perturbation theory. The single-particle energy bands have also been corrected for correlation effects applying the electron polaron quasiparticle (QP) method. Electron correlation plays an essential role in stabilizing the gauche conformation of polysilane against the trans form. While the gauche-trans energy difference is about 0.25 mH at the HF level, it will be reduced to about -0.8 mH due to correlation. Also the energy bands and band gaps will be substantially modified in going from the HF to the QP description, resulting in ΔEgap = 9.5 and 11.8 eV for trans and gauche polysilane at HF, and 5.12 and 5.56 eV at the QP level, respectively. Polysilene turns out to be a semiconductor (similar to polyacetylene) for which bond-alternation energetically stabilizes the semiconducting phase (as compared to the equidistant, metallic one) by 1.4 mH at the HF and by 0.6 mH at the MP2 levels, with bond alternations of ΔR = 0.1327 and 0.0865 Å, respectively. © 1993 John Wiley & Sons, Inc.
    Additional Material: 7 Tab.
    Type of Medium: Electronic Resource
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  • 97
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, methods of correlation corrections for 1D and 2D periodic systems as well as for 1D nonperiodic polymers are reviewed. These procedures include not only the correlation correction of the total energy per unit cell, but also the recalculation (using the generalized electronic polaron model) of the band structures themselves in the periodic case, as well as the density of states (DOS) in nonperiodic chains. In both cases the inverse Dyson equation was used in an iterative way in the MP/2 and MP/3 level. The programming of the same procedure for the coupled cluster method is in progress. In a number of examples (ground state properties of (SN)X, the gap of alternating trans-polyacetylene, the exciton spectrum of diacetylene, polyglycine and polyalanine, of a cytosine stack, the bulk modules of polyethylene, the hopping conductivity of insulin), we have obtained quite good agreement with experiment, if a good basis set ab initio calculation was supplemented of the correlation correction at the bands or DOS curves, respectively. In the second part of the paper a new method is developed to take into account the simulataneous effect of a static and a periodic time-dependent electric field on the one-electron wave functions and on the total electronic energy of a periodic polymer. The method uses a variational technique (coupled Hartree-Fock equations) instead of perturbation theory for the time-dependent one-electron wave functions. The proper treatment of the potential caused by the field, which in this way does not destroy the periodicity of the polymer, as well as the inclusion of correlation at the MP/2 level of the t-and ω-dependent quasienergy per unit cell is part of the method. Finally, it is shown how one obtains in the usual way the different polarizabilities, second- and third-order hyperpolarizability tensor elements with the help of the derivatives of the quasienergies according to the different field (both static and time-dependent) components. © 1993 John Wiley & Sons, Inc.
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  • 98
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 187-194 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Due to suggestions that Self Interaction Corrections (SIC), gradient corrections, and short-range electron-electron interactions in the Local (Spin) Density Approximation (L(S)DA) scheme may significantly influence the computed electronic structure for the Mott-insulating (MI) transition metal oxides (TMOS), a comparative study has been made of Hartree Fock (HF) and L(S)DA computations for NiO. Since HF lacks electronic correlation, it overestimates band width (in conductors) and/or band gaps (in insulators). It gives the band gap for NiO two times larger than that in experiment, while LSDA gives the gap one order of magnitude smaller than the experimental value. We demonstrate that the HF results are consistent with some previously believed to be well-understood experiments, while the L(S)DA results are not. It is suggested that HF may offer a better reference state for the development of a LSDA scheme. © 1993 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 99
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 213-217 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Self-consistent one-electron state calculations are carried out, for the first time, for a gallium vacancy in the GaAs1-xPx alloy. The cluster model, within the framework of the multiple-scattering Xα theory, is used to calculate several charge states of the vacancy. Suitable clusters have been considered in order to simulate the phosporous concentration x in the alloy and how the nearest-neghbors configurations affect the vacancy-related levels. It is found that the defect introduces a deep energy level into the band gap which follows the host-valence band edge as x varies. The same trend in donor and acceptor levels is observed, with a very small Mott-Hubbard potential energy. © 1993 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
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  • 100
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 245-248 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We propose a number of rules based on topological and structural considerations for the synthesis of intrinsically conducting polymeric materials. These rules can be summarized as follows: (i) The polymer has to have a glide plane or a twofold screw axis when infinitely extended; (ii) the unit cell must contain 4N + 2 valence electrons; (iii) the packing into the solid is restricted by symmetry rules that conserve the twofold degeneracy caused by rule (i). These rules are necessary (but not sufficient) for superconductivity. The inorganic polymer poly(sulfur-nitride), which is a metallic superconductor with a critical temperature of Tc = 0.33 K, is identified as a striking example of these rules. © 1993 John Wiley & Sons, Inc.
    Type of Medium: Electronic Resource
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