bluuebunny/crossref_metadata_2025_split huggingface dataset:
Further calculations on the internal rotation spectrum of phenol
Authors: Smeyers Y. G. , Hernández‐Laguna A. 
Published: December 2004

AbstractThe influence of a small deformation of CĈO angle in phenol (tilt), into the rotational far‐infrared (FIR) spectrum is analyzed using several approaches. In all of them, the CNDO/2 method is used to determine the potential energy functions. In a first step, the CO bond and the rotation axis are both supposed to coincide with the C2 symmetry axis of the phenyl group. With this assumption the torsional frequencies are calculated in both the symmetric and asymmetric rotor approximations. In a second step, the tilt of the COH bond is determined theoretically and found to be −3°, measured from the C2 symmetry axis, the COH bond crossing this axis, Using this second geometry, and taking as the rotation axis the C2 axis, the torsional frequencies are again determined in both approximations. An improvement of the calculated transition energies is encountered at each stage of the calculation, when compared with experimental data. Finally the importance of the introduction of a tilt into the FIR torsional frequency calculations is discussed.
Electrostatic potential around halogenated anesthetics. Acidic hydrogen; chlorine vs. fluorine
Authors: Ruelle P. , Sandorfy C. 
Published: December 2004

AbstractMaps of electrostatic potential around a number of halocarbon anethetics have been obtained through Gaussian ab initio calculations. Information is obtained on the mode of association of these compounds with possible receptors. The positive potential found along the extension of the CH bonds substantiates the role of the “acidic hydrogen.” The presence of fluorines on the carbon atom to which it is bound restricts the access to this positive site through the repulsive action of the fluorine lone pairs, an observation that explains the very different behavior of chloroform and fluoroform, for example, with respect to their anesthetic potencies and hydrogen bonding abilities. In the absence of an acidic hydrogen the chlorine atoms can serve as associative sites.
Obtaining self–consistent wave functions which satisfy the virial theorem
Authors: Magnoli Douglas E. , Murdoch Joseph R. 
Published: December 2004

AbstractThe virial theorem has played an important role in applying quantum mechanics to chemical problems. It has served as one criterion of a satisfactory wave function and its consequences on chemical bonding, molecular structure, and substituent effects have been analyzed extensively. A common method of gaining compliance with the virial theorem is to introduce a “scale” factor which adjusts all distances by a factor η. Optimizing the scale factor through the variational principle produces a wave function satisfying the virial theorem. In the present paper it is shown that when this “scaling” procedure is applied to self‐consistent wave functions, the virial theorem can be satisfied, but self‐consistency is lost. Scaling generally has a small effect on the total energy, but the effects on the energy components (T, Vne, Vee, Vnn) can be two to three orders of magnitude larger and in the range of tens to hundreds of kcal. Consequently, for applications where the energy components are useful, it is highly desirable to obtain wave functions which satisfy the virial theorem and are self‐consistent. In the present paper, a simple, inexpensive extrapolation technique is reported which requires one integral evaluation and two SCF cycles to achieve convergence. Applications to atoms and small molecules are reported.
Cluster analysis of the full configuration interaction wave functions of cyclic polyene models
Authors: Paldus J. , Boyle M. J. 
Published: December 2004

AbstractThe first two members of the cyclic polyene homologous series are studied over a wide range of the coupling constant using the Hubbard and Pariser–Parr–Pople model Hamiltonians. The full and various limited configuration interaction (CI) correlation energies and wave functions are calculated exploiting the unitary group approach. The formalism for the cluster analysis of the exact wave function expressed through the unitary group formalism electronic Gelfand states is developed and applied to the full CI wave functions of the cyclic polyene models studied. It is shown that the connected tetraexcited clusters become essential in the fully correlated limit and that their contribution also significantly increases with electron number even for the coupling constant corresponding to the spectroscopic parametrization of the model Hamiltonians used.
Thomas–Fermi term as the simplest correction to the Weizsacker term
Authors: Ludeñta Eduardo V. 
Published: December 2004

AbstractThe nature of the correlation function G(1,2) appearing in the definition of the reduced first order density operator γ′(1,2) = ρ(1)1/2ρ(2)1/2G(1,2) is analyzed. It is shown that when G(1,2) is expanded in terms of plane waves in the context of a single‐determinant approximation to the wave function, the correction to the Weizsacker term in the kinetic energy density expression is the Thomas–Fermi term.
Löwdin partitioning in a Lanczos basis: Applications to scattering states
Authors: Hermann M. R. , Langhoff P. W. 
Published: December 2004

AbstractMethods are reported for construction of closed‐form optical potentials that provide useful L2‐basis‐set approximations to the discrete and continuum Schrödinger states of self‐adjoint Hamiltonian operators. The potentials are obtained employing information from a finite (Lanczos) reference space only, but nevertheless correspond to explicit summation over an infinite‐dimensional remainder space. Connections are indicated between the Stieltjes–Tchebycheff orbital solutions of the resulting optical‐potential Schrödinger problem and previously described corresponding moment‐theory approximations to spectral densities and distributions. Use of a Lanczos basis insures that the orbital eigenvalues are generalized Gaussian or Radau quadrature points of the spectral density, and that their (reciprocal) norms provide the associated quadrature weights. Convergence of the orbitals in the limit of high order is obtained to Schrödinger eigenstates of finite norm in the discrete spectral region, and to scattering states of improper (infinite) norm in the essential portion of the spectrum. In finite orders the spatial characteristics of the Stieltjes–Tchebycheff orbitals correspond to spectral averages in the neighborhoods of the quadrature points over the correct Schrödinger states. Explicit closed‐form expressions are obtained for the spectral content of individual orbitals in terms of orthogonal polynomials without reference to the correct Schrödinger states. A computational application to regular Coulomb l waves illustrates the nature and convergence of the development.
Concept of the “stability” of the quantum mechanical state
Authors: Tachibana Akitomo 
Published: December 2004

AbstractA new concept of the “stability” of the quantum mechanical state is presented. This concept is closely related to the hydrodynamical theory of quantum mechanics. For charged particles, the “stability” against the application of the external electromagnetic field is examined. Under a nonlinear interaction, a new type of the solitonlike phenomenon is shown as a novel, illustrative example of the “stable” state. An extension for the relativistic treatment is also examined.
Recent advances in simple valence‐bond theory and the theory of hybrid bond orbitals
Authors: Herman Zelek S. 
Published: December 2004

AbstractThe history of important developments in the theory of hybrid bond orbitals and its application in valence‐bond theory is reviewed. One of the salient points, the bond strength of a hybrid orbital, is defined as the value of the angular part of the orbital along the bond direction. Characteristic bond angles corresponding to maxima in the bond strength are presented for various basis sets. In order to alleviate computational difficulties in determining the bond strength for complex systems, an approximation to it, the pair‐defect‐sum approximation, is described. The results of an exhaustive test of the validity of this approximation are presented. Applicaitions of these ideas to coordinate chemistry are provided. Finally, the case is made for the continued viability of valence‐bond theory in this age of omnipresent computer terminals.
Computer‐generated formulas for some three‐center molecular integrals over Slater‐type orbitals
Authors: Jones Herbert W. 
Published: December 2004

AbstractExact computer‐generated formulas can be produced for each term of an infinite series that gives the value of three‐center Coulomb integrals over Slater‐type orbitals of the s‐type with equal screening constants. As a specific example, the Coulomb energy of an equilateral triangular arrangement of all 1s orbitals is calculated using seven terms of an infinite expansion to obtain an answer comparable to earlier work in elliptic coordinates. Generalization to all three‐center cases of this implementation of the Löwdin α‐function method is straightforward.
Cumulative BK approximation as a method to select configurations for CI calculations of transition energies
Authors: Nascimento Marco Antonio Chaer 
Published: December 2004

AbstractA cumulative Bk approximation is examined as a method to select configurations for CI calculations of transition energies where all the matrix elements are computed (full CI). The results obtained by this approach indicate that the transition energies are comparable to the ones obtained at the full CI level. Even for truncation errors of 1 mhartree, the transition energies differ from the full CI ones by less than 0.1 eV.
Transmission mechanisms of spin–spin coupling constants within the CHF approximation: Their study using inner projections of the polarization propagator
Authors: Engelmann A. R. , Contreras R. H. 
Published: December 2004

AbstractA new method for separating differently transmitted components of spin–spin coupling constants is introduced which is suitable when couplings are calculated using the coupled Hartree–Fock (CHF) approximation via the polarization propagator. Contributions transmitted through different electronic subsystems are found choosing different subsets of orbitals with their corresponding virtual spaces for their description. Examples for two different channels of transmission are given, which are very well known among NMR spectroscopists, namely, components trasmitted via π‐electron systems, and “through‐space” interactions. In the first case a set of canonical orbitals is chosen, while in the latter noncanonical ones are used. For both cases, numerical results are given within the INDO approximation. These results are compared with others obtained previously by means of the partially restricted molecular orbitals approach. The possibility of using this method of inner projections of the polarization propagator in decomposing other second orther properties is suggested.
Hindered internal rotations in Van der Waals molecules and molecular crystals
Authors: Briels W. J. , Tennyson J. , Claessens M. , Van Der Lee Th. , Van Der Avoird A. 
Published: December 2004

AbstractThe quantum dynamical behavior of the Van der Waals molecule (N2)2 and that of the ordered α and γ phases of solid N2 have recently been calculated, starting from the same ab initio N2N2 potential. By interpreting the results of these calculations we try to improve our understanding of the libration/internal rotation motions of the N2 monomers and the orientational order–disorder (α–β) phase transition. Some new results are presented and further (mean‐field and libron‐model) calculations are proposed which assess explicitly the intermolecular pair correlation effects caused by the anisotropic interaction potential.
Electron localization in a finite one‐dimensional chain
Authors: Tanaka Kazuyoshi , Nagaoka Masataka , Yamabe Tokio 
Published: December 2004

AbstractThe localization characteristics of the electronic wave functions in a finite one‐dimensional chain with the diagonal or the off‐diagonal disorder of the potentials have been studied. It has been shown that the eigenfuction at the frontier level is relatively “strong” against the temptation to localize caused by the existence of the random potentials. It has also been pointed out that the spatial behavior of the total density reflects that of the diagonal random potentials, but that under the off‐diagonal random potentials the total density is spatially uniform (completely extended).
Importance of exchange effects in the deformation of interacting ions
Authors: Gutowski Maciej , Ka̧lkol Małgorzata , Piela Lucjan 
Published: December 2004

AbstractIn calculations for liquids or solids one often considers a limited cluster of atoms or molecules in the electrostatic field of their environment. In this paper possibilities of improving this model by inclusion of the exchange deformation potential are shown. The reported numerical results for the LiF and NaF molecules as well as for the LiF and NaF perfect crystals indicate the importance of the exchange deformation effects.
Elastic forward scattering of high‐energy electrons and molecular electron density
Authors: Konaka Shigehiro 
Published: December 2004

AbstractThe elastic forward scattering of high‐energy electrons from molecules has been studied in the second Born approximation. An integral transformation has been adopted to evaluate the second Born integrals analytically without explicit use of molecular wave functions. In the high‐energy limit, the differential cross section for the forward scattering is expressed in terms of electric dipole and quadrupole moments, the second moment of charge distribution with respect to the molecular center, and transition dipole moments. All these quantities are shown to be computable from molecular electron densities in the ground state.
On some problems in quantum pharmacology I. The partition functions
Authors: Gomez‐Jeria J. S. 
Published: December 2004

AbstractWe performed an analysis of the drug‐receptor equilibrium constant, Ki = {QDiRQDi−1QR−1} exp (−Δεi/kT). It is shown that, for a group of nonrotating molecules we may consider the product of the partition functions as constant if log(mass) = constant for all the molecules.
Gradient expansion correction to the Dirac exchange term in statistical models for the Na atom with shell structure
Authors: Vinszky P. Csa , Vosman F. 
Published: December 2004

AbstractIn the study of the ground‐state binding energy of atoms, relatively more attention has been paid to the gradient expansion of the kinetic energy term of the Thomas–Fermi–Dirac model than to the gradient expansion of the exchange term of this model. Recently Shih and Shih, Murphy, and Wang have focused on this problem and calculated the first gradient expansion correction to the Dirac exchange term by making use of electron densities constructed from Hartree–Fock wave functions. In previous work, aimed to introduce the shell structure of atoms via a variational procedure using the energy density functional formalism, electron densities have been obtained for the Na atom within the Thomas–Fermi–Dirac model with and without the Weizsäcker and Hodges gradient expansion corrections to the kinetic energy term. In this paper we make use of the respective electron densities and calculate the first gradient expansion corrections to the Dirac exchange term. The results show that, for the Na atom, the magnitude of this correction is about 1% of the magnitude of the total binding energy.
The modified Hartree–Fock self‐consistent field equation
Authors: Deng Conghao 
Published: December 2004

AbstractA one‐electron correlation operator is introduced into the Hartree–Fock self‐consistent field equation. The correlation operator is derived from the second‐order perturbation theory. Energies of atomic and molecular systems calculated from this modified Hartree–Fock equation are equal to that from second‐order perturbation of Hartree–Fock equation. The modified equation can also be solved self‐consistently by the LCAO approximation. We also presented the modified expressions for other operators.
Hydrogen‐like atomic orbitals: Addition and expansion theorems, integrals
Authors: Novosadov Boris K. 
Published: December 2004

AbstractAnalytical properties of hydrogen‐like atomic orbitals (HAO) that are used in the MOLCAO approach to the quantum theory of molecules have been studied. Addition and expansion theorems for HAO have been proved, both in coordinate and momentum representations. A close relation has been established between HAO and the reduced Bessel functions of half‐integer indices. New methods are suggested to calculate integrals for atomic and molecular form factors, and multicenter integrals, for the HAO basis in the MO LCAO theory.
Second‐ and higher‐order convergence in linear and nonlinear multiconfigurational Hartree–Fock theory
Authors: Olsen Jeppe , J⊘rgensen Poul , Yeager Danny L. 
Published: December 2004

AbstractWe discuss how the local convergence of Newton–Raphson and fixed Hessian MCSCF iterative models may be rationalized in terms of a total order of convergence in an error vector and a corresponding error term. We demonstrate that a sequence of N Newton–Raphson iterations has a total order of convergence of 2N and that a sequence of N fixed Hessian iterations has a total order of convergence of N + 1. We derive the error terms of a Newton–Raphson and a fixed Hessian sequence of iterations. We discuss the implementation of the fixed Hessian and the Newton–Raphson approaches both when linear and nonlinear transformations of the variables are carried out. Sample calculations show that insight into the structure of the local convergence of Newton–Raphson and fixed Hessian models can be based on an order of convergence and an error term analysis.
Electron–phonon coupling detection in infrared spectra of ion–radical salts based on dibenztetrathiofulvalen
Authors: Starodub V. A. , Drushlyak T. B. , Sleta L. A. 
Published: December 2004

AbstractThe electron–phonon coupling constants in a cation radical of dibenztetrathiofulvalen (DBTTF) have been calculated. To calculate the electronic structure of DBTTF in equilibrium and distorted structures use was made of the unrestricted Hartree–Fock method as π‐electron approximation for symmetrical vibrations of DBTTF. The results obtained are in good agreement with the experimental data on IR spectra of DBTTF‐based cation–radical salts.
Symmetry breaking in the Hartree–Fock approximation for binuclear transition metal compounds—a theoretical investigation based on a variable model operator
Authors: Böhm Michael C. 
Published: December 2004

AbstractThe validity of the Hartree–Fock (HF) approximation in bis(π‐pentadienyl)dinickel (1) and in cyclopentadienyl‐allyl‐cyclobutadiene‐dinickel (2) has been investigated by means of the Thouless instability conditions in the computational framework of a variable model Hamiltonian. Singlet, nonsinglet (triplet), and nonreal instabilities in 1 and 2 have been studied as a function of the one‐electron resonance integral β and as a function of the one‐ and two‐center elements of the electron–electron interaction. The one‐center integrals of Coulomb (γ) and exchange‐type (K) have been modified by a multiplicative factor; the two‐center integrals (γ) have been calculated by means of an exponential interpolation formula with a variable decay amplitude. Additionally the Thouless conditions have been studied for nuclear deformations. The stability domain of the HF solution in the model space spanned by the variable INDO Hamiltonian has been analyzed. The nature of the many‐body interactions in the unstable region depends strongly on the parametrization of the model operator. HF instabilities in the high‐density region (long‐range forces) of 1 have their origin in individual particle–hole fluctuations while negative roots for short‐range forces (low‐density region) are similar to collective excitations in many‐body systems (strong off‐diagonal coupling). The opposite behavior is encountered in the Ni complex 2. The physical origin of these different types of correlation processes are analyzed in a simple two‐electron two‐orbital model. The nature of the HF fluctuations in 1 and 2, the importance of spatial and spin correlation, and the coupling of symmetry breaking of the electronic wave function with nuclear deformations are compared with the nature of phase transitions in solid‐state systems.
Stability properties of closed‐shell restricted Hartree–Fock solutions for electronic systems in the framework of the projected Hartree–Fock method and their utilization
Authors: Karadakov Peter , Castan̄o Obis 
Published: December 2004

AbstractThis paper contains a derivation of stability conditions for closed‐shell restricted Hartree–Fock (RHF) solutions in the framework of the projected Hartree–Fock (PHF) method. It is shown that for any projection operator whose choice is consistent with the symmetry of the RHF solution, one obtains four stability subproblems (one “singlet” and three “nonsinglet”). The stability conditions are specialized to several particular choices of the projection operator, including Löwdin's spin projection operator for the singlet case (this choice corresponds to the celebrated spin‐PHF method). The stability of closed‐shell RHF solutions for alternat π‐electronic systems in the framework of the alternant molecular orbital (AMO) method is also discussed. The theoretical considerations are illustrated by PPP calculations for several closed‐shell π‐electronic models. The results for even alternant conjugated hydrocarbons show that the AMO construction is suggested by the structure of the eigenvector pointing into the direction of steepest descent from the pertinent RHF solution in the framework of the spin‐PHF method. The minimization of the spin‐PHF energy expectation value in this direction in a number of cases allows that value to approach rather closely the energy values obtained in a fully variational spin‐PHF treatment and thus provides either a one‐parameter variational method which is shown to be superior to the one‐parameter AMO method or a convenient initial approximation to the solution of the spin‐PHF equations.
Momentum density interpretation of the H(1s)–H+ long‐range force
Authors: Koga Toshikatsu 
Published: December 2004

AbstractThe recently proposed method of using momentum densities for interatomic interactions is applied to the long‐range force between the ground‐state hydrogen atom and the proton, and the results are compared with those from using the position density based on the electrostatic Hellmann–Feynaman theorem. A new physical interpretation of the long‐range force is obtained, which is complementary to that in position space. It is found that some perturbative changes in the position density do not accompany changes in the momentum density.