bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dispersion interaction between helium pair from hydrodynamic analogy to quantum mechanics
Authors: Shukla G. C. , Easa S. I. 
Published: December 2004

AbstractFrequency‐dependent multipole polarizabilities of the He sequence have been calculated from a hydrodynamic model of quantum mechanics and using an independent‐particle model Hamiltonian. Our present scheme is parallel to the uncoupled Hartree–Fock approximation so our values of polarizabilities, multipole transition energies and dispersion force coefficients between the HeHe pair are comparable with earlier works using the uncoupled Hartree–Fock approximation.
Configuration interaction matrix elements. I. Algebraic approach to the relationship between unitary group generators and permutations
Authors: Wormer P. E. S. , Paldus J. 
Published: December 2004

AbstractMatrix elements of unitary group generators between spin‐adapted antisymmetric states are shown to be proportional to spin matrix elements of so‐called “line‐up” permutations. The proportionality factor is given explicitly as a simple function of the orbital occupation numbers. If one bases the theory on ordered orbital products, the line‐up permutations are given a priori. The final formulas have a very simple structure; this is a direct consequence of the fact that the spin functions have been taken to be geminally antisymmetric.
Calculation of molecular electric polarizabilities and dipole moments. III. BH molecule
Authors: Waltz Carl E. , Kim Hae‐Won , Hameka Hendrik F. 
Published: December 2004

AbstractWe use a previously proposed variation‐perturbation method to calculate the electric polarizabilities and the electric dipole moment at equilibrium nuclear distance of the BH molecule. We obtain 3.56 × 10−24 cm3 for the perpendicular polarizability αxx and 3.22 × 10−24 cm3 for the parallel polarizability αzz. Our result for the electric dipole moment μ0 is 1.734 debye units; there is no reliable experimental result to compare it with.
Incomplete basis set problem. IV. Perturbative corrections to pair energies
Authors: McDowell Keith 
Published: December 2004

AbstractA partitioning of the molecular Hamiltonian into the occupied and virtual orbital spaces and their orthogonal complement is introduced and used to develop a perturbation expansion of the exact ground‐state energy relative to the Hartree–Fock energy computed using an incomplete basis set. The leading perturbation corrections to pair energies due to using the incomplete basis set are considered in detail. Summations of certain classes of pair contributions are discussed and a resummed correction is obtained.
Global analysis of composite particles
Authors: Story Troy L. 
Published: December 2004

AbstractThe theory of vibrations of a composite particle when vibrational amplitudes are not constrained to be small according to the Eckart conditions is developed using the methods of differential topology. A global classical Hamiltonian appropriate for this system is given, and for the case of the molecular vibration–rotation problem, it is transformed into a global quantum Hamiltonian operator. It is shown that the zeroth‐order term in the global Hamiltonian operator is identical to the Wilson–Howard Hamiltonian; higher‐order terms are shown to give successively better approximations to the large amplitude problem. Generalized Eckart conditions are derived for the global classical Hamiltonian; the quantum equivalent of these conditions along with the quantum equivalent of the Eckart conditions are given. The spectrum of the global Hamiltonian operator is discussed and it is shown that the calculation of the vibration–rotation energy states of the system reduces to the same straight‐forward procedure, the solution of a secular determinant, as was carried out for the Wilson–Howard Hamiltonian at a later time by Nielsen.
Ab initio Hartree–Fock–Slater calculations of polysulfanes H2Sn (n = 1–4) and the ions HS and S
Authors: Trsic M. , Laidlaw W. G. 
Published: December 2004

AbstractAn ab initio Hartree–Fock–Slater procedure was applied to the calculation of the electronic structure of polysulfanes, H2Sn (n = 1–4) and the ions HS and S. Charge densities, overlap populations, and deprotonation energies are calculated; the latter appear well correlated with the first and second acidity constants.
Nonadditivity of the SCF interaction energy in the (LiH)3 complex
Authors: Szczȩśniak Małgorzata M. , Ratajczak Henryk 
Published: December 2004

AbstractThe role of nonadditivity of the interaction energy between three LiH molecules was investigated within the SCF ab initio framework. The nonadditive part of the interaction energy is more important in the case of a cyclic structure than in a linear trimer, and is stabilizing in both cases. The value of the ratio of three‐body and two‐body terms for different points on the energy hypersurface is discussed.
Nonrigidity effects of fragments of membrane‐active complexons
Authors: Kryachko Eugene S. , Kruglyak Yuri A. , Dalton Bryan J. 
Published: December 2004

AbstractThe nonrigid molecule symmetry group for methylamine is determined as a set of permutations and permutation inversions for two situations. In the first case the internal rotations of the methyl group and NH2 group are included and the second case the transition of a hydrogen atom from the methyl group to NH2 group is also included. In both cases the energy level splitting of the rigid model of the methylamine is found, nuclear statistical weights of the split levels are calculated, and sections rules for the allowed dipole transitions are given.
Stable and efficient algorithms for Xα multiple scattering calculations
Authors: Case David A. , Yang Cary Y. 
Published: December 2004

AbstractSolutions to some practical problems that arise in multiple scattering calculations on large molecules are discussed. (1) Numerical instabilities near the zero of energy can be removed by rescaling the secular matrix. (2) The calculation of structure factors can be made much more efficient by the application of symmetry projection operators. (3) An energy search procedure is described that ensures that no states are inadvertently neglected. Test calculations incorporating these changes illustrate the improved numerical stability, and show decreases in computation time of 30–60%, when compared to previous codes. The procedures suggested here are applicable to both relativistic and nonrelatitistic calculations.
Self‐consistent approximation to the polarization propagator
Authors: Weiner Brian , Goscinski Osvaldo 
Published: December 2004

AbstractA theorem is presented that characterizes approximate states and truncated operator manifolds associated with self‐consistent approximate propagators. This theorem establishes a natural relationship between Hermiticity, stationarity, nonredundance and completeness of operator manifolds, model time evolution, and the vacuum condition. For the case of the polarization propagator we describe algorithms by which we can construct states and manifolds that satisfy this theorem and the vacuum condition.
High accuracy wave functions and eigenenergies for the vibration–rotation states of diatomic molecules by factorization method
Authors: Mohammad S. Noor 
Published: December 2004

AbstractA solution of the wave equation for the nuclear motion of a diatomic molecule with an exponential potential function and the rotational term included has been performed by the Schrödinger‐Infeld‐Hull factorization method. Two different procedures have been followed for the factorization of the wave equation, and in each of them, as type‐B factorization is incapable of producing wave functions, it has been transformed to type‐C factorization, and a complete set of wave equations have been obtained. Of the two procedures, although the former appears to be more direct, the latter is the more accurate and generalized one.
Nonempirical calculations on diatomic transition metals. II. RHF investigation of lowest closed‐shell states of homonuclear 3d transition‐metal dimers
Authors: Wolf Andreas , Schmidtke Hans‐Herbert 
Published: December 2004

AbstractPotential‐energy curves of the 3d dimer series Sc2 through Cu2 are calculated for the lowest closed‐shell states within the nonempirical RHF formalism using limited basis sets of minimal to near‐double‐zeta–plus‐polarization size. Calculated spectroscopic constants are compared to semiempirical results as well as to experimental estimates. The possibility for closed‐ or open‐shell ground states is discussed for each dimer. For diatomic Sc and Cu a detailed study of basis set effects on calculated molecular constants is carried out.
Effective potential model for calculating the energy shift in the ground state of two‐electron atoms
Authors: Jolly Pratibha , Sane Kamalni , Sane K. V. 
Published: December 2004

AbstractAn effective operator for the electron–electron interaction derived earlier has been shown to be successful for calculating the ground‐state energy shift values for 10 members of the helium isoelectronic sequence in terms of a single free parameter. Some possible applications of the modified interaction operator for semiempirical calculations are suggested.
Direct configuration interaction with a reference state composed of many reference configurations
Authors: Siegbahn Per E. M. 
Published: December 2004

AbstractThe configuration interaction method where a single reference state is composed of a linear combination of reference configurations is analyzed in detail. In this method single and double replacements are constructed by applying annihilation and creation operators on the reference state. The analysis is based on the recently derived factorization of the direct CI coupling coefficients into internal and external parts. Using the internal coupling coefficients the integrals are transformed to new entities which are used in the diagonalization step. This two‐step procedure differs significantly from the usual straightforward one‐step direct CI procedure. A number of operations analysis shows that calculations with the present method should be feasible even with a large number of reference configurations in the reference state. Based on first‐order perturbation theory the accuracy of the method is predicted to be close to the accuracy obtained with the usual CI method with many reference configurations.
Biological response as a function of conformation, chirality, and electronic characteristics: A catecholamine study
Authors: Scharfenberg Peter , Sauer J. 
Published: December 2004

AbstractThe hypothesis is introduced, that nonrigid drug molecules interact with biological receptors in a well‐defined but unknown conformation that need not belong to an energy minimum. A verification of this assumption has to include a match of conformation and chirality. Even the different action of pairs of nonrigid enantiomers allow one to recognize uniquely the important role of conformations because flexible molecules are partially able to avoid wrong attachment to the receptor. According to a simple model, a monotonic relation between biological activity and rotational barriers (and/or torque constants) can be expected and, by means of rank correlation analysis of adrenergic activities, has been found, too. In order to apply the latter method, particularities concerning quantitative structure–activity analyses of nonrigid and/or chiral molecules are worked out. We optimized conformations by means of analytically calculated first partials of the energy and a quasi‐Newton method. Using a convenient numerical method, it was shown that coplanar conformations of catecholamines are generally unstable. Moreover, CNDO/2 proved to be a reliable method for conformational studies, in which rotations about conjugated bonds [1] are not considered.
Padé algorithm for the computation of static and dynamic second‐order properties by large‐scale CI expansion
Authors: Bendazzoli G. L. , Evangelisti S. , Ortolani F. 
Published: December 2004

AbstractBy introducing a finite basis, the inhomogeneous equations of perturbation theory are approximated by systems of linear equations. We present and discuss a Padé‐type algorithm suitable for large‐order systems and having better convergence properties than the classical Jacobi procedure.
Fourth‐order diagrammatic MB–RSPT calculations of the correlation energy: N2, CO, F2 and the reaction energy of the process ½F2 + ½H2 = HF
Authors: Kellö Vladimir , Urban Miroslav 
Published: December 2004

AbstractThe correlation energies obtained by the fourth‐order diagrammatic perturbation theory were analyzed for three diatomic molecules: N2, CO, and F2. The results were compared with correlation energies obtained previously for the ten‐electron hydrides HF, H2O, and NH3. The relative importance of contributions which arise from the double excitations, from the quadruple excitations, as well as from the renormalization term was investigated. It is shown that for the diatomic molecules under study these contributions are considerably larger than for the ten‐electron hydrides not only in absolute value but also in percentage: they represent about 3, 3, and 5%, respectively, of the valence shell correlation energy obtained by the perturbation theory up to the fourth order. A careful analysis of the fourth‐order correlation effects is also presented for the reaction energy of the process ½H2 + ½F2 = HF.
Algebraic reductions of the real symmetric 4 × 4 secular problem
Authors: Linderberg Jan 
Published: December 2004

AbstractProcedures for the construction of the eigenvector matrix and the spectrum of 4 × 4 real and symmetric matrices are given. The Lie algebra of the group O(4)† is used as well as the relation to O(3)†. Perturbations are analyzed in terms of the group parameters.
Theoretical study of excitation energies of some CoF6n− complexes
Authors: Miyoshi Eisaku , Takada Toshikazu , Obara Shigeru , Kashiwagi Hiroshi , Ohno Kimio 
Published: December 2004

AbstractAb initio LCAO SCF MO calculations are carried out on the ground state and d–d transition states of CoF64−, CoF63−, and CoF62− complexes with a basis set of more than double‐zeta quality. To obtain a better description for the excitation energy of d–d transition states, a ligand field configuration interaction calculation is performed. The calculation is improved further by taking into account the correlation energy in the central metal atom and it gives good results for the observed excitation energies. Especially, the excitation energies for the states which have the same configuration as the ground state agree with observed ones within about 1 kK. The excitation energies of several charge transfer states in the CoF62− complex are also computed at the level of SCF MO calculation and the assignments are made for the four strong bands observed in the energy region higher than the weak d–d bands.
Additivity of resonance energy in benzenoid hydrocarbons
Authors: El‐Basil Sherif 
Published: December 2004

AbstractA simple method is described for the calculation of resonance energies (RE) of linear acenes based on their number of Kekulé structures. The values obtained for the first five linear acenes are used to graph–theoretically calculate RES of a wide variety of benzenoid hydrocarbons. Excellent linear relationships are found between RES and each of A‐II, graph‐theoretical (GT), Hess–Schaad (HS), and Dewar resonance energies (SCF). These relations apply to 42 hydrocarbons and lead to the following equations: A‐II = 0.084RE + 0.080 (0.9999); GT = 0.072RE + 0. 135 (0.9832); HS = 0. 106RE + 0. 169 (0.9889); and SCF = 0.316RE + 0. 166 (0.9899). Correlation coefficients are shown in parentheses. A linear relation also exists between RES and the square roots of the wavelengths of the UV spectra of hydrocarbons of the linear acenes and phene series. Least‐squares analysis of the data leads to the following equation: RE= 0.412(λ)½ −15.479, with a correlation coefficient equal to 0.9903, in which λ is the wavelength of the β band of the UV spectra of these hydrocarbons. The method predicts no resonance energies for both open chain polyenes and the radialenes.
Direct calculation of resonant states in reactive scattering. Application to linear triatomic systems
Authors: Basilevsky M. V. , Ryaboy V. M. 
Published: December 2004

AbstractA direct method for calculating resonant states in reactive scattering is suggested, permitting us to obtain the characteristics of multichannel resonances (partial width amplitudes). The method is based on the construction of a Laurent expansion of the scattering matrix S(ϵ −iΓ/2) in the complex plane. The position of the poles of the S matrix are derived by solving the dynamical problem with complex energy values. The residue at the pole gives all the information concerning the partial widths. The method is applied to a linear triatomic reactive scattering problem. The properties of the resonant states in the H + H2 system are calculated as an example. Two broad resonances are found which have not been reported in previous calculations. The interference of overlapping resonances is shown to have a profound effect on the energy dependence of the transition probabilities.
Some aspects of bonding and binding in polymeric chains
Authors: André J. M. , Brédas J. L. , Delhalle J. 
Published: December 2004

AbstractThe status of theoretical calculations on polymers is illustrated by two specific problems. The first one is the interpretation of ESCA results on polymers by suitable ab initio calculations. The second problem is the very timely one of bond alternation in polymers. The reliability of theoretical calculations at various levels and the possible central role of electron correlation are discussed.
Hydrogen bonding in clusters and molecular crystals
Authors: Karpfen A. , Beyer A. , Schuster P. 
Published: December 2004

AbstractAb initio calculations on the Hartree–Fock level are used to study the effects of cooperativity in ternary complexes and in infinite chains on the example of hydrogen fluoride. SCF energy partitioning demonstrates that polarization forces dominate three‐body energies in these aggregates. The crystal orbital method is applied to investigate the structural differences between dimers and molecular crystals.
Tautomerism of pyrimidines and purines in the gas phase and in low‐temperature matrices, and some biological implications
Authors: Shugar David , Szczepaniak Krystyna 
Published: December 2004

AbstractInfrared spectra in the NH, OH, and CO stretching regions are reviewed for various methylated and halogenated 2(4)‐oxopyrimidines and uracils, as well as other nucleic acid derivatives, in the vapor phase and in low‐temperature matrices. The 2‐oxopyrimidines are predominantly in the enol form both in the vapor phase and in low‐temperature matrices. By contrast, the 4‐oxopyrimidines exhibit comparable populations of the keto and enol forms, with KT ≈ 1–2, and a difference in chemical binding energy between the two forms in the gaseous phase of the order of 1–2 kcal/mol. The observed tautomeric equilibria in the gas phase point to the need for a drastic revision of interpretations of theoretical methods, and simultaneously provide the appropriate quantitative data necessary for testing the results of quantum mechanical calculations. In sharp contrast to other heterocyclic systems, several of the bases found in natural nucleic acids were found to exist predominantly in the keto or amino forms, as in the solution phase. In particular, uracils exist predominantly in the keto form. This has made possible, for this class of compounds, to evaluate the heats of sublimation and vaporization, and to relate these data to hydrogen bonding and stacking interactions in the condensed phases. Examples are presented of base analogs which do exhibit appreciable tautomerism in solution. Some biological implications of the foregoing are presented in relation to the types of heterocyclic bases found in natural nucleic acids, and to concepts of spontaneous and induced mutagenesis.
Estimation of dissociation energy of alkali diatomics using an extended Linderberg relation
Authors: Raina V. , Sane K. V. , Saxena R. P. , Srivastava P. K. 
Published: December 2004

AbstractThe ground state wave function of a model potential consisting of a zero‐range square well joined to a Coulomb tail is used in a two equivalent centre problem to evaluate the overlap integral (S) and the resonance integral (β). It is shown that an extended Linderberg relation of the form β = K/R (dS/dR) is generally well obeyed. The extended relation has been used within a LCAO framework to obtain dissociation energies of the homonuclear diatomic molecules of the alkali atoms.
Electron correlation described by an extended geminal model
Authors: Røeggen I. 
Published: December 2004

AbstractA scheme for obtaining approximate solutions of the electronic Schrödinger equation is proposed in terms of an extended geminal model. The model is formulated within the context of the method of moments. The advocated approach is nonvariational, but size‐consistent as long as breaking and/or formation of new bonds can be grouped into at most four‐particle systems. The correlation energy, with respect to a separated pair approximation, is obtained as a sum of intrapair corrections and a sum of interpair corrections. An approximate model for calculating interpair correlation energies is introduced. In a test calculation on LiH, it is demonstrated that this approximate model recovers 98.0% of the correlation energy obtainable by a complete configuration interaction calculation.
Improved Hylleraas calculations for ground state energies of lithium ISO–electronic sequence
Authors: Ho Y. K. 
Published: December 2004

AbstractThe 2S ground state of lithium iso–electronic sequence is calculated by the use of Hylleraas‐type wave functions. A 92 term one‐spin wave function was used for lithium atom calculations. The energy obtained was −7.478031 a.u. as compared with the previous best value of −7.478025 a.u. calculated by Larsson. In addition, improved energies for Z = 4 to 8 were calculated by the use of 60 term wave functions. This work thus provides the lowest ab initio ground state energies for lithium sequence to date.
Compton profiles for ejected electrons: Determination of momentum densities
Authors: Dal Cappello C. , Tavard C. 
Published: December 2004

AbstractElectron Compton scattering effects usually lead to the determination of momentum densities in atoms and molecules by cross‐section measurements on scattered electrons. Similar information is also shown to be directly available from the study of ejected electrons. The proposed treatment results from a simple alteration of the impulse assumptions and appears to give a satisfactory representation of ejected profiles for medium and high energy electron scattering. Preliminary investigations are discussed here for hydrogen and helium atoms.
Simple derivation of conditions for instability in the Hartree–Fock and projected Hartree–Fock schemes
Authors: Löwdin Per–olov , Calais Jean–Louis , Calazans Jacques M. 
Published: December 2004

AbstractThe conditions for instability of solutions of Hartree–Fock and projected Hartree–Fock equations are derived in a form involving finite real symmetric matrices. These conditions are also expressed in terms of the Fock–Dirac density matrix, both at the spin–orbital and at the orbital level. The particular variations which give rise to the so‐called singlet and triplet instabilities are described.
Catecholamine interaction with anionic sites—A model study
Authors: ŠOlmajer T. , Kocjan D. , Hadži D. 
Published: December 2004

AbstractInteractions of catecholamines with acidic sites in the biophase are likely to be involved in the control of the receptors activity as well as in storage and transport mechanisms. In view of the importance of the phenomena a model study of ethanolamine–phosphate complexes was made. The electrostatic interaction energy surface was calculated in the charge density multipole expansion approximation with terms up to quadrupole. The consecutive minimal energy conformation shows the importance for the interaction with the biophase of the catechol ring steric and electronic relation to the side chain.