bluuebunny/crossref_metadata_2025_split huggingface dataset:
The valence electron density distribution of hydrogen bonded systems in the iterative extended Hückel approach. III. The pyrrole–pyridine system
Authors: Almlöf Jan , Mårtensson Olle 
Published: December 2004

AbstractThe valence electron distributions of the hydrogen‐bonded systems C4H4NH … NC5H5 formed from pyrrole and pyridine and C4H4N− … HNC5H+ from pyrrole anion and pyridinium ion have both been investigated using the charge iterative extended Hückel method. The results are presented in the form of contour diagrams showing the charge densities in pertinent sections of the hydrogen‐bonded systems, and also in the corresponding parts of the non‐interacting constituent molecules. The density differences between interacting and non‐interacting systems brought about by hydrogen bonding are also presented. Like earlier investigated hydrogen bond types, an NH … N bond is characterized by an increase of charge density in the covalent NH bond, and a decrease at the acceptor atom. The formation of a rather low maximum in the centre of the H … N bond is also observed. The hydrogen bonding interaction causes a transfer of charge to the donor molecule of the same order of magnitude in both systems considered.
Properties of pyridine and pyrazine from ab initio molecular orbital wave functions
Authors: Ha Tae‐Kyu , O'Konski Chester T. 
Published: December 2004

AbstractVarious molecular properties have been calculated for pyridine and pyrazine from Gaussian lobe ab initio SCF molecular wave functions. Values are compared with available experimental data. In general, agreement is satisfactory with the exception of the rather sensitive asymmetry parameter of the quadrupole coupling tensor. The distributions of total electronic charge, and of selected molecular orbitals have been displayed as plots of the charge density contours in two dimensions.
Modified Gaussian functions and their use in quantum chemistry. I. Integrals
Authors: Gołlebiewski A. , Mrozek J. 
Published: December 2004

AbstractA modified Gaussian function g(u, v, w, a, R) = const
equation image
s(a, R) is considered where l = u + v + w, s (a, R) is a 1s‐type Gaussian function centered at R, a is the coefficient in the exponent of the 1 s Gaussian function and X, Y, Z are components of R. General formulae are derived for overlap integrals, kinetic energy integrals, nuclear attraction integrals, and electron repulsion integrals, valid for any l. The formulae are much simpler than those derived by Huzinaga for Cartesian Gaussian functions.
Modified Cartesian Gaussian functions and their use in quantum chemistry
Authors: Gołlębiewski A. , Mrozek J. 
Published: December 2004

AbstractThe general formula for the electron‐electron repulsion integral in the modified Cartesian Gaussian basis set derived in Ref. [1] is simplified. A general relation between the standard and modified CG functions is given. A possible use of the modified CG functions to quantum chemical calculations which include the correlation factor rij2 is indicated.
A comparative calculation on excited state energies of some conjugated hydrocarbons
Authors: Mukherjee Debashis 
Published: December 2004

AbstractThe energies of the single‐configuration lowest π – π* singlet and triplet states of some conjugated hydrocarbons have been calculated by the MC‐SCF method using the conjugate‐gradient technique of minimisation. The results are compared with those calculated by other methods currently in use, like (a) single‐configuration calculation with VN−1 potential for virtual orbitals; (b) CI calculation involving singly excited states; and (c) TDHF method. It has been concluded that the results for the MC‐SCF method are very good, considering that only a single open‐shell configuration is involved.
Static and dynamic first‐ and second‐order properties by variational wave functions
Authors: Moccia R. 
Published: December 2004

AbstractThe problem of the evaluation of first‐ and second‐order energies by the use of arbitrary variational wave functions is examined in detail for time‐independent perturbations as well as for time‐dependent perturbations. By using a compact formalism the general formulae to be used for the case of a fully optimized set of variational parameters are readily obtained and the most prominent features are examined. The generality of the approach is tested by showing how some widely used methods are obtained by using particular types of variational wave functions. The case of incompletely optimized sets of variational parameters is examined examined extensively and several approaches at different levels of approximation are proposed. Emphasis is put upon the importance of considering, in the calculation of higher‐order energies, the variational parameters which may be of negligible importance, and thus often neglected, in the absence of perturbations.
Spin‐projected EHF method. II. The equations for successive optimization of the orbitals in the many‐electron case
Authors: Mayer I. , Kondász J. 
Published: December 2004

AbstractThe extended Hartree–Fock equations of the spin‐projected scheme are derived in a form suitable for the construction of a surely convergent method of solution using successive optimization of the individual orbitals. The derivation is based on a specific form of the generalized Brillouin theorem.
Hartree‐Fock calculation of cohesive energies and equilibrium lattice constants for solid Li and Be
Authors: Stoll H. , Preuss H. 
Published: December 2004

AbstractA method is described for calculating cohesive energies of solids in the single‐determinant approximation including the full Hartree‐Fock exchange. The method involves (1) the construction of a rapidly convergent series in vectors of the direct and reciprocal lattice for the Fock matrix, (2) a decoupling procedure for the k‐dependence of the Fock matrix, which works even in the case of strong interatomic overlap. An application to Li and Be is given. Agreement with experiment to 10% is achieved for the cohesive energies and to 5% for the equilibrium lattice constants.
Effect of the rotational motion of molecules on their optical and magneto‐optical activity
Authors: Svendsen E. NØRby , Hameka Hendrik F. 
Published: December 2004

AbstractIn the theoretical descriptions of natural and magneto‐optical activity it is customary to average classically over molecular orientations. We develop a theoretical description of the two phenomena that includes the correct quantum mechanical representation of the rotational motion in order to find out whether or not the customary procedure is correct. We find that the inclusion of the quantum mechanical description of the rotational motion leads to the same result in the theory of optical activity and magneto‐optical activity as the naive classical averaging over molecular orientations.
Wechselwirkungen gerichteter ladungsverteilungen. I. Irdo (intermediate retention of differential overlap)‐berechnungen zum einfluß des einsamen elektronenpaars am stickstoff auf die relative acidität des β‐picolins
Authors: Birner P. , Köhler H.‐J. , Weiss C. 
Published: December 2004

AbstractIt is pointed out by using the IRDO treatment of Jesaitis et al. that the failure of the CNDO/2 method in predicting the acidity difference between toluene and β‐picoline has its reason in the incorrect handling of the nitrogen lone pair. To predict correctly the relative acidity of β‐picoline, it is necessary and sufficient to include only those NDDO terms into the IRDO calculation which contain interactions with the nitrogen. The most important interactions are those between the nitrogen and the three centers of the heterocycle which take over a large part of the negative charge in the corresponding β‐delocalized carbanion.
The relationship between the Rayleigh‐Schrödinger and asymptotic perturbation theories of intermolecular forces
Authors: Whitton W. N. , Brown W. Byers 
Published: December 2004

AbstractThe relation of the straightforward Rayleigh‐Schrödinger perturbation theory for the interaction of two atoms to the asymptotic exchange theory is described. The one‐electron case of a hydrogen atom perturbed by a nucleus is examined in detail. It is shown that the asymptotic theory contains an infinite summation of terms in the Rayleigh‐Schrödinger series. The nature of the branching between states and its implications for convergence is elucidated.
Clebsch‐Gordan coefficients for chains of groups of interest in quantum chemistry
Authors: Kibler M. R. , Guichon P. A. M. 
Published: December 2004

AbstractThe algebra of the representation of the special unitary group SU(2), the universal covering of the proper rotation group SO(3), is studied in a nonstandard basis. We are using a basis adapted to a chain of type SU(2) ⊃ …︁ ⊃ G″ ⊃ G′ ⊃ G. The introduction of such a chain enables us to label, at least partially, the elements of the irreducible tensorial sets under SU(2) with irreducible representations of G, G″ G″, …. We are thus led to introduce the restriction SU(2) → …︁ → G″ → G′ → G in the Wigner‐Racah algebra of the group SU(2). The physical interest of this machinery lies in the fact that the double group of any point symmetry group belongs, up to an isomorphism, to the considered chain. The formalism described in this paper thus appears to be useful in molecular and solid‐state calculations. It is particularly efficient in the fields of vibrational‐rotational and electronic spectroscopy of molecules. In Appendix A the master formulae, principally the Wigner‐Eckart‐Racah theorem, for the Wigner‐Racah algebra of a chain of compact topological groups (discrete or continuous) are briefly discussed. Lastly, a programme for computing Clebsch‐Gordan coefficients for a chain SU(2) ⊃ …︁ ⊃ G″ ⊃ G′ ⊃ G and numerical results for chains isomorphic to SU(2) ⊃ O′ ⊃ D′4 ⊃ D′2 are described in Appendix B.
Influence de la Structure de Diverses Fonctions sur l'Evaluation de l'Energie de Correlation
Authors: Et André Julg Alain Pellegatti 
Published: December 2004

AbstractThe influence of various correlation functions, multiplying the monoelectronic space function, is studied and applied to the Helium atom and its isoelectronic series. We used Slater‐type basis orbitals and the ground and first excited states have been studied, taking into account the virial and cusp conditions. In the ground state, a very good value for the correlation energy is obtained, using a function of the type

For the excited states, this type of function overestimates the assumed value of the correlation energy.
A finite‐difference approach to the electron correlation problem
Authors: Fimple W. R. , Unwin M. J. 
Published: December 2004

AbstractA variant of the transcorrelated method of Boys and Handy employing finite differences is presented. It is based upon the following two properties of the transcorrelated Hamiltonian operator C−1HC: (1) C−1HC possesses an energy eigenvalue spectrum which is identical to that associated with H itself; and (2) if \documentclass{article}\pagestyle{empty}\begin{document}$$ C \equiv \begin{array}{*{20}c} \pi & {e^{r_{ij} /2} } \\ {i > j} & {} \\ \end{array} $$\end{document} then C−1HC is free of the singularities of H at the points where the interelectron separation rij is zero. A bivariational principle for approximating the eigenvalues and the left and right eigenfunctions of C−1HC is introduced and the resulting set of coupled integro‐differential equations are solved in finite‐difference form by means of a coupled self‐consistent field, Newton Raphson algorithm. As a preliminary test of the method, a calculation of the ground‐state energy of the helium atom is presented.
A characterization of pairs of subspaces for quantum chemical applications of the Galerkin–Petrov method
Authors: Jankowski K. 
Published: December 2004

AbstractA method is proposed for characterizing pairs of finite dimensional subspaces of the Hilbert space. The method might be helpful for the proper selection of coordinate and projective spaces leading to reliable realizations of the Galerkin–Petrov method for use in quantum chemistry. In order to illustrate the method some numerical results are presented.
Modified one‐electron hamiltonian method in the MC SCF theory
Authors: Kuprievich V. A. , Klymenko V. E. 
Published: December 2004

AbstractFor orbital optimization within the MC SCF theory a modification of the OEH method is proposed with the direction of descent determined according to the Fletcher–Reeves gradient method. The combined method developed on this basis ensures the convergence of the iterative process when the Hessian singularities occur. The convergence properties of the method proposed are studied by performing the ab initio water molecule calculations using two types of multiconfigurational wave functions.
Charge distribution analysis in terms of Berlin's binding and antibinding regions for Li2 and F2
Authors: Zuvia Marilena , Ludeña Eduardo V. 
Published: December 2004

AbstractThe MCSCF wave functions of Das for Li2, and Das and Wahl for F2, have been analyzed in terms of Berlin's binding and antibinding regions. The effect of electronic correlation on the charge distributions in these regions is studied. The process of bond formation in going from separated atoms to the correlated molecule is also discussed with reference to Berlin's regions.
Spin‐projected EHF method. IV. Comparison of potential curves given by different one‐electron methods
Authors: Mayer I. 
Published: December 2004

AbstractSome aspects of the computer realization of the spin‐projected extended Hartree–Fock (EHF) method at the ab initio level are briefly discussed for the algorithm of solution developed in the previous papers of this series. Calculations have been performed for the BH molecule by using a small basis of contracted Gaussian lobes with the purpose of comparing the potential curves given by the different one‐electron methods RHF, UHF, UHF with subsequent spin projection, and EHF. It is concluded that the UHF and, in particular, the EHF methods give a qualitatively correct shape of the potential curve; the RHF method shows the known incorrect dissociation behavior while the potential curve obtained by subsequent spin projection of the UHF wave function exhibits spurious extrema at intermediate internuclear separations.
Approximate fourth‐order perturbation theory of the electron correlation energy
Authors: Krishnan R. , Pople J. A. 
Published: December 2004

AbstractAn approximate fourth‐order expression for the electron correlation energy in the Møller–Plesset perturbation scheme is proposed. It takes into account all the contributions to the fourthorder energy neglecting only those of the triple‐substituted determinants. It is size consistent and correct to fourth order for an assembly of isolated two‐electron systems. Illustrative calculations are reported for a series of small molecules.
Iterative hypervirial‐scaling method for obtaining wave functions and eigenvalues
Authors: Castro Eduardo A. 
Published: December 2004

AbstractA procedure to improve trial wave functions is given in terms of the off‐diagonal hypervirial theorem. This procedure is closely related to the optimum scaling method which is valid for the diagonal hypervirial theorem. The second excited state of the one‐dimensional oscillator model is employed as an example, and improved wave functions and expectation values are calculated. The results are compared with previous values based on the diagonal hypervirial theorem.
Energy band structure of (SN)x chain: Unrestricted Hartree–Fock and charge density wave solutions
Authors: Kertesz M. , Koller J. , Ažman A. 
Published: December 2004

AbstractThe ab initio Hartree–Fock crystal orbital method is used for the calculation of the energy band structure of a one‐dimensional model of (SN)x. Two energy band structures are described corresponding to the self‐consistent spin density wave (SDW) and the self‐consistent charge density wave (CDW) solution, respectively.
Some aspects of the model potential method
Authors: Höujer Germund , Chung Johnson 
Published: December 2004

AbstractThe theoretical justification of the model potential method is studied in some detail. The correct equations within the framework of Roothaan's open‐shell scheme are derived and the approximations necessary to get a workable method are discussed. Analysis of the local part of the model potential suggests a new analytical form for it. The new expression is theoretically more consistent than the original one, and it can be determined in a more straightforward way. A basis set approximation, which is particularly suitable for approximate evaluation of two‐electron integrals when only valence orbitals are involved, is discussed and tested with encouraging results. The ideas are tested on the Fe and I atoms.
Polarization model representation of hydrogen fluoride for use in gas‐ and condensed‐phase studies
Authors: Stillinger Frank H. 
Published: December 2004

AbstractThe polarization model, originally developed to describe deformable and ionizable water molecules, has been extended to hydrogen fluoride. Since electronic polarization is explicitly included, the interaction energy in aggregates of molecules (with or without ions) is nonadditive. The model properly describes the structure of (HF)2, including off‐axis bending of the proton acceptor molecule. Calculations are presented to illustrate elementary gas‐phase reactions involving proton transfer between HF and F−, and H2F+ and F−.
Some symmetry aspects of the local‐mode description of vibrational structure
Authors: Gelbart William M. , Stannard Paul R. , Elert Mark L. 
Published: December 2004

AbstractIn the local‐mode description of vibrational structure, the nuclear coordinates—unlike the usual normal coordinates—do not reflect the molecular symmetry. Accordingly, the local‐mode product wave functions must be symmetry adapted. We discuss this simple procedure in light of its implications for the localization of vibrational energy and the dissociation limits of molecular excited states. These properties are in turn related to the separability of the total nuclear Hamiltonian and to the different ways in which the vibrational anharmonicity is “absorbed” by the local and normal coordinates.
Binding of an electron to a molecular dipole: BeF
Authors: Seiders Barbara A. B. , Luken William L. , Jordan Kenneth D. 
Published: December 2004

AbstractResults of extended‐basis SCF calculations indicate that BeF may exist as a metastable species. Comparison of results obtained from SCF calculations on neutral BeF2 with those of BeF shows that the orbital occupied by the electron of BeF is well approximated by the lowest unoccupied molecular orbital of neutral BeF2.
Theoretical study of [F−; e+] and [CN−; e+]
Authors: Kurtz H. A. , Jordan K. D. 
Published: December 2004

AbstractAb initio calculations are presented for the [F−; e+] and [CN−; e+] complexes. Positron affinities of 4.99 and 3.79 eV are obtained for F− and CN−, respectively. The excitation energies to the low‐lying excited states of the positron complexes are also calculated.
Interrelations among x‐ray scattering, electron densities, and ionization potentials
Authors: Hyman Arthur S. , Yaniger Stuart I. , Liebman Joel F. 
Published: December 2004

AbstractFrom the general formalism of elastic x‐ray scattering and a few meaningful assumptions we have shown that for an atom, ∫ρ2dτ is an experimentally measurable quantity related to the intensity scattered by an element. We have labeled this quantity (p), the “average electron density.” If ψ obeys the virial theorem, within the Thomas–Fermi approximation we show (within a multiplicative constant) that 〈ρ〉 is a lower bound to (∑ ionization potentials )3/2. Thus, the scattered intensity of x rays is related quantitatively to the energy of the scattering atoms. Inequalities have been developed to express these relationships and have been confirmed for the more exact Hartree–Fock wave functions.
Monte carlo simulation of water solvent with biomolecules. Glycine and the corresponding zwitterion
Authors: Romano Silvano , Clementi Enrico 
Published: December 2004

AbstractMonte Carlo calculations for clusters consisting of 200 water molecules surrounding glycine in the neutral and zwitterionic forms were carried out at 300 K; all the relevant interaction potentials have been obtained by means of quantum–mechanical calculations. Water–water and amino acid–water energies were calculated, and the zwitterion was found to be strongly favored with respect to the neutral molecule, as expected. A detailed analysis of the energetic results yielded some information on the special extension of the solute‐induced perturbation. The structural results were found to be in reasonable agreement with predictions that can be obtained by analyzing isoenergy contour maps, calculated for the two‐body amino acid–water potential.
Electronic factors affecting receptor binding of dibenzo‐p‐dioxins and dibenzofurans
Authors: Cheney B. Vernon , Tolly Timothy 
Published: December 2004

AbstractThe electronic structure of a series of 25 chlorinated dibenzo‐p‐dioxins and dibenzofurans has been characterized by means of the ab initio molecular fragment technique. This information has been employed to investigate some of the factors affecting the affinity of the molecules for the hepatic cytosol binding species described by Poland, Glover, and Kende. A quantitative structure–activity relationship involving electronic and steric parameters could be established from the data. It appears that the toxins act as electron acceptors in a charge‐transfer complex with the receptor.
Some aspects of the DNA hypochromic effect theory
Authors: Volkov S. N. 
Published: December 2004

AbstractThe expressions for the polymer absorption band hypochromism and oscillator strength are studied in frames of the first‐order perturbation theory. The physically justified approximations for a polymer hypochromism calculation are indicated. The uniform interpretation of the effect origin is given independently on the approximation used. In frames of the perturbation theory the DNA hypochromism formula is obtained, from which the known experimental dependences of hypochromism on chain length and polymer helicity degree follows directly. The analytic expression of the DNA hypochromism dependence on AT pairs content is obtained. It is established that in most cases for natural DNA the nucleotide sequence does not influence in practice the value of the hypochromic effect.
Theory of chemical reactions in condensed media and its applications to biological processes
Authors: Christov S. G. 
Published: December 2004

AbstractThe general aspects of a theory of dense‐phase reactions, based on an accurate quantum‐mechanical formulation of the rate equation, are considered. Using an adiabatic one‐frequency oscillator model, the theory is applied to several important biological processes at low temperatures: the photinduced oxidation of cytochrome C by bacteriochlorophyll, the electron transfer from primary to secondary acceptors in bacterial photosynthesis, and the ligand rebinding of carbon oxide and β‐chain of hemoglobin. A very good agreement between theory and experiment is found making use of no more than one or two (or even without any) adjustable parameters.
Molecular structure–biological activity relationships on the basis of quantum‐chemical calculations
Authors: Sklenar Heinz , Jäger Joachim 
Published: December 2004

AbstractDetailed quantum‐chemical calculations by means of semiempirical all‐valence electrons methods and a generalized (multivariable) rank correlation analysis are the fundamentals of a novel strategy of search for QSAR within homologous series of compounds. The set of molecular parameters (describing the electronic and conformational properties as well as potential interactions of the drugs) is calculated theoretically. Owing to the rank correlation method, no linear model (like LFER) for the dependence of the biological activity upon the molecular parameters is presumed. The computed correlation coefficients are valued by carefully determined levels of statistical significance. Significant correlations are used to predict unknown activities in terms of ranks relative to the basic sample.