bluuebunny/crossref_metadata_2025_split huggingface dataset:
Variational solutions of a first‐order perturbation equation
Authors: Hamano Hidekazu 
Published: December 2004

AbstractThe variation‐perturbation method for solving a first‐order equation of Rayleigh‐Schrödinger perturbation theory is presented. The method consists of the combination of variational procedures and successive approximations. The recurrence formulas generate successively the higher‐order approximations of the variational solutions. It is possible to obtain the almost exact solution within a few steps. The method is applied to the calculations of the polarizabilities of a one‐dimensional harmonic oscillator and of the hydrogen atom, and the dipole‐dipole interaction energy of two hydrogen atoms.
Novel graph‐theoretical approach to estimating the relative importance of individual Kekulé valence structures. II. Benzenoid systems having four to seven fused benzene rings: “Pseudodegenerate” states
Authors: El‐Basil Sherif 
Published: December 2004

AbstractKekulé structures of 10 nonlinear acenes comprising 83 graphs are studied through the use of connectivities [M. Randić, J. Am. Chem. Soc. 97, 6609 (1975)] of their corresponding submolecules [H. Joela, Theor. Chim. Acta 39, 241 (1975)]. In certain rare cases states were identified to have identical branching indices but different Kekulé indices [A. Graovac, I. Gutman, M. Randić, and N. Trinajstić, J. Am. Chem. Soc. 95, 6267 (1973)]. Such states are termed pseudodegenerate states. A method is described to forecast and another to remedy such situations. The method emphasizes the relation between VB (resonance) and MO theories using graph‐theoretical concepts.
CNDO/2 calculations of relaxation and reconstruction of diamond and silicon [111] surfaces
Authors: Ĺszló I. 
Published: December 2004

AbstractCNDO/2 calculations have been performed on the clusters X4H9 and X4Y9 modeling the [111] diamond and silicon surfaces. The X is either carbon or silicon atom and the Y is a pseudoatom containing one sp3 hybrid orbital. It is shown that in the CNDO/2 approximation in the foregoing pseudoatom models, the charge distribution of the cluster is better than the hydrogen atom, because the electronegativity of the hydrogen differs significantly from the electronegativity of the sp2 orbital of the silicon atom. Using the CNDO/2 parametrization, the electronegativity of the hydrogen is very near to the electronegativity of the sp3 orbital of the carbon atom, thus the hydrogen can be used for the saturation of the carbon clusters.
Critical examination of approximate LCAOMO methods. I. Reasons for the failure of the CNDO and INDO methods in theoretical conformational analysis of conjugated compounds
Authors: Birner P. , Hofmann H.‐J. 
Published: December 2004

AbstractIt is well known that the CNDO and INDO methods fail in the field of theoretical conformational analysis of conjugated compounds. The NDDO approximation, however, can be applied very successfully to this problem. In this paper, a comparative analysis of these three approximations is performed to clarify the reasons for the failure of the former two methods. It is shown that the defects are inherent in their formalisms, and there is no reasonable possibility to compensate them (e.g., by reparametrization) at the level of the CNDO and INDO approximations. The central inadequacy is the oversimplified treatment of the two‐center exchange interaction. In the case of molecules containing heteroatoms with lone‐pair electrons the known defects concerning the description of Coulombic interactions of anisotropic charge distributions can additionally influence the results. To overcome these shortcomings, at least an extension of the formalism to the level of the NDDO method is necessary.
Theory of the electron distribution in compressed atoms at high temperatures
Authors: Csavinszky P. 
Published: December 2004

AbstractTo calculate atom‐atom interactions at high temperatures, one needs the electron densities of the interacting atoms. The present paper outlines an approach by which the temperature‐dependent electron densities of compressed atoms may be obtained. The approach suggested makes use of the temperature‐dependent Thomas‐Fermi (TF) equation, and a variational principle by which this equation may be solved approximately.
Scaled one‐electron hamiltonian model for open‐shell LCAO–MO–SCF calculations: Comparison with the restricted open‐shell method of roothaan
Authors: Banerjee Manas , Bhattacharyya Sankar Prasad 
Published: December 2004

AbstractThe performance of a recently proposed scaled one‐electron Hamiltonian (SOEH) model is tested against parallel sets of restricted open‐shell calculations by the method of Roothaan. It is found that the energy calculated by SOEH model, in general, lies slightly higher than the energy computed by the restricted open‐shell method of Roothaan lending credibility to the application of variational argument to the scaled pseudoenergy functional (Eav) for deriving the SOEH model. The numerical stability of the converged SOEH energy with respect to changes in trial vectors indicates the reliability of the method. The SOEH model is shown to perform well in the calculation of geometries of radicals and ions. The convergence behavior of the SOEH model is compared with that of the restricted open‐shell method of Roothaan.
Permutation‐Group classification and description of the Feynman–Goldstone diagrams
Authors: Kvasnička V. 
Published: December 2004

AbstractA systematic permutation‐group approach for both the classification and description of the Feynman–Goldstone diagrams is presented. It allows us to “algebraize” the topological concepts and notions of many‐body diagrammatic techniques, for example, the connectivity and disconnectivity of diagrams, the automorphism of a diagram, etc.
Long‐Range interaction coefficients between H, He+, He, and Li+
Authors: Easa S. I. , Yousif F. N. 
Published: December 2004

AbstractUsing the new expression for the dynamic multipole polarizabilities in terms of effective oscillator strengths and effective transition energies derived from the variational procedure based on the total energy expansion in terms of induced multipole moments, we have calculated the long‐range dispersion force coefficients and the leading relativistic correction to the long‐range potential between H, He+, He, and Li+. We have in addition calculated the oscillator strength sum rules for the above systems. Our results are comparable with those obtained using both a hydrodynamic model to quantum mechanics and double perturbation theory.
Electronic aspects of enzymatic catalysis
Authors: Ferreira Ricardo , Gomes Marcelo A. F. 
Published: December 2004

AbstractEnzymes are large aperiodic structures and this hinders both ab initio molecular orbital and Bloch‐type band theory of calculations. A frontier orbital perturbation theory of catalysis is applied to enzymes. Reasons are given for proposing that the induced‐fit conformational changes, essential to enzymatic catalysis, leads to an increase in the electronic eigenvalue density at the active site, enhancing the necessary catalytic orbital perturbation.
Ab initio studies on the nonplanarity of a peptide unit: Calculations on model compounds
Authors: Sawaryn A. , Yadav J. S. 
Published: December 2004

AbstractThe nonplanarity of a peptide unit has been studied by the ab initio method (GAUSSIAN 76) considering both the pyramidal structure of nitrogen and the variation of the ω angle for the N‐methyl‐acetamide and N‐ethyl‐acetamide in their cis and trans conformations. Several semiempirical methods have also been used for the sake of completeness of this work. All methods except CNDO and PCILO support a nonpyramidal structure for the nitrogen. The distortions of the planarity of the peptide unit show for both cis and trans conformations of the model compounds a significant asymmetric shift of the minimum on the energy curve for the ω rotation.
Electrostatic effect on catalytic rate enhancement in serine proteinases
Authors: Náray‐Szabó G. 
Published: December 2004

AbstractThe hypothesis that an electrostatic effect of the buried aspartate side chain may contribute considerably to the extreme catalytic power of serine proteinases is presented. This statement is based on an analysis of environmental effects, due to amino acid residues in the vicinity of the Ser‐221…︁His‐64 couple and to hydration in subtilisin BPN'.
Self consistent field MO studies of the vinylguanidinium ion
Authors: Sapse A. M. , Snyder G. , Santoro A. V. , Herzig L. 
Published: December 2004

AbstractAb initio Hartree–Fock calculations are used to predict the effect of substituting one hydrogen on the guanidinium ion by a vinyl group. The results show no overlap between the π‐electron system of the guanidinium moiety and that of the vinyl group.
Convergence behavior of some orbital optimization procedures
Authors: Bhattacharyya Sankar Prasad , Mukherjee Debashis , Bhattacharyya Debendra Madhab , Khan Priyotosh 
Published: December 2004

AbstractSources of occasional convergence problems in our recently proposed algorithm for orthonormality constrained orbital optimization in SCF calculation, as well as in the partly equivalent orthogonal gradient method, are traced and remedial measures are suggested. A simple scaling technique for accelerating convergence in the related orthogonal gradient method is tested.
Bounds to atomic properties: Electric polarizabilities of lithium isoelectronic sequence ions
Authors: Muszyńska J. , Papierowska D. , Pipin J. , Woźnicki W. 
Published: December 2004

AbstractUsing the optimal wave functions obtained by means of the superposition of correlated configurations method, elaborated previously, static and dynamic polarizabilities and rigorous lower bounds to them have been calculated for the ground states of three‐electron atoms in the lithium isoelectronic sequence (Li I–C IV). The results can be treated with considerable confidence, especially for higher members of the sequence. Furthermore, the calculated rigorous lower bounds for static polarizabilities enable us to rule out several theoretical and experimental values obtained previously. The results obtained by us are then employed to evaluate the leading unknown terms in the Z−1‐type expansion for the polarizability, extending thereby our treatment to much higher members of the isoelectronic sequence.
Perturbation for a rigid rotator in an electric field
Authors: Requena A. , Peña R. , Serna A. 
Published: December 2004

AbstractThe behavior of a rigid rotor perturbed by an electric field is studied. An hypervirial calculation, to obtain the perturbation series without calculation of the perturbed wavefunctions, is used to determine the Stark‐shifted eigenvalues. A numerical estimation of the m equals; 0 → m = 1 transition for the lowest vibrational state (v = 0) and the first rotational state (J = 1) for the HF and DF molecules using Padé approximants is made.
AGP Propagator calculations
Authors: Aa. Jensen Hans Jørgen , Weiner Brian , Öhrn Yngve 
Published: December 2004

AbstractThe use of the antisymmetrized geminal power (AGP) consistent reference state for RPA polarization propagator calculations is explored. Illustrative applications to the three lowest electronic states (3B1, 1A1, and 1B1) of the methylene radical are reported with comparisons to other theoretical methods and experiment.
Comparative study of the Hartree–Fock and the Hartree–Fock–Slater method
Authors: Wagner A. J. 
Published: December 2004

AbstractThe Hartree–Fock method (standard Roothaan closed‐shell HF–LCAO theory) and the Hartree–Fock–Slater method (restricted HFS–LCAO–DV method developed by Baerends and Ros) have been compared with emphasis on the respective one‐electron equations and on the matrix elements of the respective Fock operators. Using the same STO basis in the two cases, the matrix elements of the Fock operators and of their separate one‐electron, Coulomb, and exchange contributions have been calculated for the same orbitals and density of the ground state of the diatomic molecule ZnO. The effects of methodical (exchange potential) and numerical (DV method, density fit) differences between the HF and HFS methods on the various matrix elements have been analyzed. As expected the methodical effect prevails and is responsible for the higher (less negative) values of the matrix elements of the HFS Fock operator compared to those of the HF Fock operator. Numerical effects are observable also and are caused by the difference in integration procedures (DV method), not by the density fit.
Two‐dimensional fully numerical solutions of molecular Schrödinger equations. I. One‐electron molecules
Authors: Laaksonen Leif , Pyykkö Pekka , Sundholm Dage 
Published: December 2004

AbstractA two‐dimensional fully numerical relaxation approach is presented for the electronic Schrödinger equation of linear molecules. The method is tested on the lowest σ, π, δ, and ϕ states of H and HeH2+. About 10‐figure accuracy is obtained for the orbital energies. Seven‐point numerical formulas are given for first and second derivatives.
Van der Waals molecules: Quantum chemistry, physical properties, and reactivity
Authors: Hobza P. , Zahradník R. 
Published: December 2004

AbstractInteraction energies ΔE accompanying the formation of van der Waals (VDW) systems may be estimated by means of the formula ΔE = ΔESCF + BSSE + ED(r−6, r−8, r−10), where BSSE is the basis set superposition error and ED is the dispersion energy in the expanded form. Twenty one stationary points were located on the potential energy surfaces of hydrogen‐bonded complexes; of those, 18 were local minima and 3 were saddle points. Thermodynamic characteristics calculated on the basis of molecular constants, quantum chemically generated, were obtained for 12 hydrogen‐bonded complexes and for competition equilibria between VDW molecules. The role of the VDW forces in some rate processes and in a prototype of enzyme–substrate interactions is outlined. In connection with enzymic catalysis, the intramolecular discrimination in 1,2‐difluorohydrazine, and the stability of hydrogen‐bonded complexes in a cation field is discussed.
Contracted CI calculations of models for catalytic reactions involving transition metals
Authors: Blomberg M. , Brandemark U. , Pettersson L. , Siegbahn P. 
Published: December 2004

AbstractTheoretical models for reductive elimination from transition metal containing molecules have been studied using large scale contracted CI calculations. Four different models were treated, namely, NiH2, PdH2, Ni(CH3)2, and Pd(H2O)2H2, in order to study the effects of adding ligands, exchanging hydrogens with methyl groups, and comparing nickel and palladium. The most interesting result already appeared for the simplest system NiH2. A closed‐shell‐type 1A1 state with a small bond angle of only 57° is bound compared to Ni and H2 with only a very small barrier for formation. The bond distance is short, shorter than in NiH, and the d orbitals are strongly involved in the binding. The hydrogen atoms bind both to nickel and to each other. With methyl groups rather than hydrogens, this double sided bonding situation is destroyed and Ni(CH3)2 has a negative binding energy with the carbon bonds pointing towards nickel. For PdH2 only a weakly bound complex between an essentially unchanged H2 and Pd was found. The bond distance is very long. Adding H2O ligands to Pd shortens the bond distance and significantly opens up the bond angle. The methods used in the investigation and the chemical implications of the results are discussed.
Scattering in view of the Titchmarsh–Weyl theory
Authors: Rittby Magnus , Elander Nils , Brändas Erkki 
Published: December 2004

AbstractThe Titchmarsh–Weyl theory for a singular second‐order differential equation is presented. The equations, however, are arranged such that the theory is immediately applicable to the multichannel formulation. The mathematical basis of the Titchmarsh–Weyl theory (TWT) is outlined and the Titchmarsh–Weyl m function is presented. Relations to the associated Green's function and the spectral function are given. It is shown how some quantities common in scattering theory can be expressed in terms of the TWT. The method of complex rotation is then applied to the TWT to create a complex rotated analog of the Titchmarsh–Weyl theory. This is then extended as exterior complex scaling is introduced. An outline of a proof for the method of exterior complex scaling is presented and some numerical results of the complete theory are given.
Ordinary field‐theoretic methods for self‐consistent wave functions which describe bond formation and dissociation. II. Commutative coupling case
Authors: England Walter B. 
Published: December 2004

AbstractHartree–Bogoliubov–Valatin (HBV) theory may be implemented with Lipkin Hamiltonians to obtain self‐consistent BCS wave functions which describe bond formation and dissociation. These wave functions are in turn vacuua for Nambu's representation of Feynman–Dyson–Goldstone diagrammatic perturbation theory, and hence provide suitable references for the many‐body treatment of correlation. Exact SCF solutions of the HBV equations are equivalent to special even‐replacement MC–SCF solutions. The latter are similar to generalized valence bond theory, and require one Fock operator for each one‐particle shell. The commutative coupling case of HBV theory is realized when the number‐conserving renormalized one‐body and number‐nonconserving pairing operators commute. In this case, a set of orbital equations which involves a single Fock operator may be solved. Since this could prove to be a significant simplification for large systems, the commutative coupling and exact solutions are compared here for the fragmentation of H2 and F2. Results suggest that commutative coupling orbitals will be useful for the aforementioned many‐body theory.
Application of the many‐body perturbation theory by using localized orbitals
Authors: Kapuy Ede , Csépes Zoltán , Kozmutza Cornelia 
Published: December 2004

AbstractDiagrammatic formulation of the many‐body perturbation theory is investigated when both the occupied orbitals and the virtual ones are localized, i.e., they are unitary transforms of the canonical Hartree–Fock orbitals. All diagrams representing ground state correlation energy can be generated through fifth order. For cyclic polyenes C6H6 and C10H10 as model systems, the energy corrections are calculated in the Pariser–Parr–Pople approximation for a wide range of the coupling constant β−1, through fourth order including some fifth order terms. The results are compared to those obtained by other methods: perturbation theory by using canonical orbitals and full CI. The effect of neglecting contributions from orbitals localized into neighboring sites is also studied.
Scaling in pseudopotential theory
Authors: Gáspár R. 
Published: December 2004

AbstractSimple Gaussian nonlocal pseudopotentials are determined for K shells. Extrapolation formulas are derived for the parameters as functions of the reduced atomic number. Small hydrids are used for the basis function analysis. Comparison is made with the set of molecules in the book by Snyder and Basch, and detailed results are presented for the molecules BH3, CH4, NH3, H2O, HF, N2, CO, BF, CH3F, C2H2, HCN, CHONH2, and CHOOH. Results on energy levels, total energy, total valence energy and dipolmoment and in a few cases some geometry predictions are made.
Exchange, correlation, and band gaps in conjugated linear chains
Authors: André J. M. , Brédas J. L. , Thémans B. , Piela L. 
Published: December 2004

AbstractIn the case of conjugated polyenic chains (polyacetylene), the relationships between the long‐ (or short‐) range nature of the restricted Hartree–Fock exchange interaction, the role of correlation effects, and the size of energy gaps are illustrated.
Survey of structural regularities in molecular properties. I. Carbon‐13 chemical shifts in alkanes
Authors: Randić Milan 
Published: December 2004

AbstractMany regularities in properties among structurally related compounds are known but a systematic approach to investigate all such cases is rare. A plan is outlined for a systematic study of structural regularities that is based on use of selected graph invariants as prime auxiliary quantities. In this paper carbon‐13 chemical shifts in alkanes are examined. Since NMR spectra of paraffins are well understood, the example provides a useful illustration of the approach, introduces basic concepts, and illustrates the kind of the results that the graph‐theoretical scheme generates. Differences and similarities between graph‐theoretical viewing of the problem of structural regularities and customary direct use of additivities are discussed in some detail.
Coordinate rotation studies of H−, He−, Be−, Mg− resonances: Basis set and configuration list dependence
Authors: Chuljian David T. , Simons Jack 
Published: December 2004

AbstractWe have performed coordinate rotated configuration interaction calculations on well‐studied Feshbach resonances of H− and He− and on 2P shape resonances of Be− and Mg−. The focus of our efforts was the dependence of computed resonance energies on both the quality of the atomicorbital basis and the level of treatment of electron correlation. Our results indicate that great care must be taken to guarantee that a basis is adequate; commonly used quantum‐chemistry bases are probably far from satisfactory. Our findings also indicate that a proper treatment of inner‐shell orbitals within coordinate rotation calculations is a formidable task. We are therefore encouraged to look carefully for modified coordinate rotation techniques that focus on the active valence‐level orbitals and may avoid spurious complex energies arising from improper treatment of inner shells.
Theoretical study of degree of covalency in some CoF6(n‐) complexes (n = 4, 3, and 2)
Authors: Miyoshi Eisaku , Obara Shigeru , Takada Toshikazu , Kashiwagi Hiroshi , Ohno Kimio 
Published: December 2004

AbstractOn the basis of a high‐quality LCAO MO SCF calculation, covalency versus ionicity of a Co–F bond in the CoF6(n‐) complexes, where n = 4, 3, and 2, is discussed. The overlap and gross atomic populations, delocalization of certain MOS, and the charge densities in the bond region as well as around F's all indicate that the covalency increases as n decreases or the valency of Co increases in these complexes.
Applicability of non‐degenerate many‐body perturbation theory to quasidegenerate electronic states: A model study
Authors: Wilson S. , Janmkowski K. , Paldus J. 
Published: December 2004

AbstractThe nondegenerate finite‐order many‐body perturbation theory is applied to simple model systems in which the degree of quasidegeneracy can be continuously varied over a wide range. Three ab initio minimum basis set models involving four hydrogen atoms in various spatial arrangements are considered. The results are compared with the exact full configuration‐interaction approach, double‐excitation configuration‐interaction and the coupled‐pair many‐electron theory.
Usage de l'Algèbre de Lie su (n) dans l'Etude des Systèmes Quantiques à n Etats. II. Transformation de l'Espace des Observables, Problèmes d'Evolution
Authors: Tillieu Jacques , Van Groenendael Augugstin 
Published: December 2004

AbstractIn the previous paper we examined, for a quantum system, the relation between its n‐dimensional state space and the su (n) Lie algebra. The present paper is devoted to relations between unitary transformations in the state space and orthogonal transformations in Lie's algebra. Two cases can happen. First, the transformations are independently chosen in the two spaces; this amounts to changing the former relation. On the other hand, the relation is maintained and the unitary operators are then related to some of the orthogonal operators. This second case is used to study the evolution operators.