bluuebunny/crossref_metadata_2025_split huggingface dataset:
Structural features of some small carbonyls in the 1,3nπ states: An mcscf study
Authors: Das Kalyan Kumar , Mukherjee D. , Bhattacharyya S. P. 
Published: December 2004

AbstractStructural features of a series of tetraatomic carbonyls in the 1,3nπ* states are studied by using a recently proposed MCSCF algorithm at the INDO/2 level. The computed geometrical parameters and the inversion barrier heights agree quite well with the available experimental or ab initio theoretical data. The calculated transition energies turn out to be somewhat lower than the experimental counterparts. The specific MCSCF algorithm used is found to have excellent convergence properties.
Calculation of SN 1 + N 2 ⊃ SN1 ⊗ sn 2 and U(n1 + n2) ⊃ u (n1) ⊗ u (n2) subduction coefficients by using spin graph
Authors: Zhenyi Wen 
Published: December 2004

AbstractBy use of the graphical method of spin algebra, simple, and closed expressions for SN1+N2 ⊃ SN1 ⊗ SN2 and U(n1 + n2) ⊃ U(n1) ⊗ U(n2), subduction coefficients are derived.
Slater determinant from atomic form factors
Authors: Frishberg C. 
Published: December 2004

AbstractThe electron densities and expectation values for several physical properties predicted by three single determinant wave functions for the beryllium atom are presented. The wave functions are the Hartree–Fock, the Best Density Matrix, and the X‐ray fitted. The X‐ray fitted electron density comes closest to reproducing the exact density for r < 8.0 a.u. and gives, in most cases, better values for the physical properties.
Composite exciton‐phonon states in molecular crystals
Authors: Singh Jai 
Published: December 2004

AbstractIt is known that the diagonalization of the exciton‐phonon Hamiltonian gives rise to the composite exciton‐phonon energy states. Within the single phonon approximation, two different diagonalized Hamiltonians are obtained for 0–0 and 0–1 phonon transitions in pure molecular crystals. The resulting secular equations are solved, and analytical expressions for the energy eigenvalues corresponding to the two transitions are derived. The thermal broadening due to the composite exciton‐phonon states interaction is also calculated at low temperatures, and the results agree with experiments.
Electron density near an impurity
Authors: Adawi I. 
Published: December 2004

AbstractThe behavior of the electron density n(r) [and potential energy V(r)] near an impurity of charge Z is studied by using the linear response theory of an electron gas at finite temperature and with exchange and correlation effects included. The odd powers series in the expansion of n(r) [and V(r)] are calculated exactly by using asymptotic methods, and the coefficients in the series are given in terms of moments taken over the Fermi–Dirac distribution function. In all linear response theories and at all temperatures, the derivative n'(0) = ‐2Zn0/a0, where n0 is the unperturbed electron density and a0 is the Bohr radius. The effects of exchange and correlation appear in the fifth‐ and higher‐order terms in nodd(r).
Correlation with independent particle orbitals
Authors: Esséan Hanno 
Published: December 2004

AbstractA method is suggested for calculating the correlation energy within an independent particle or SCF approach. It is based on the fact that a correlation factor, depending explicitly on the electron‐electron distances, can be shown to yield an effective potential operator in a Schrödinger equation for the remaining (SCF) factor in the wave function. An explicit correlation factor with certain desirable properties is the basis for the discussion. The resulting effective operator is calculated, and an independent‐pair approximation is made to make it more tractable. Finally, it is argued that the essential effects of the operator is simply given by a short range electron‐electron attraction and electron‐nuclear repulsion. A qualitative justification for this is also given.
Unified treatment of the electronic structure of organic conjugated polymers
Authors: dos Santos M. C. , de Melo C. P. , Brandi H. S. 
Published: December 2004

AbstractTheoretical calculations for polaron‐type defect structures on organic conjugated polymers are carried out by Green's function and transfer function formalism. Firstly, the renormalization concept is applied to mathematically convert the polymers PPP, PPY, and PTP to an equivalent (trans‐PA)‐like form. Then, it is shown that Green's functions and polaron wave functions are readily obtained from the equations for trans‐PA by the appropriate substitution of the renormalized parameters. It is suggested that the existence of defects such as polarons or bipolarons could be invoked to provide a unified treatment of electric conduction in organic conjugated polymers.
Investigation of the hydrogen bonds between formaldehyde and hydrogen halides by pseudopotential Ab Initio method
Authors: Yanbo (Y. B. Ding) Ding , Xiaoyuan (X. Y. Fu) Fu 
Published: December 2004

AbstractThe weak hydrogen bonded systems H2CO ⃛HX (X = F, Cl, Br, and I) have been studied by means of ab initio MO method with pseudopotential approximation. The stabilization energies of these hydrogen bonds are 8.96, 4.12, 3.00, and 2.21 kcal/mol, respectively. The interaction eneraction energies are farther decomposed according to Morokuma's energy decomposing scheme. It is found that the title complexes are mainly electrostatic, although the contribution of charge transfer is also significant.
Molecular electrostatic potential studies on antiepileptic drug: Troxidone
Authors: Jayasuriya Keerthi , Williams David J. 
Published: December 2004

AbstractAb initio SCF‐MO‐LCAO calculations of the molecular electrostatic potential (MEP) have been carried out on a seven‐member series related to the antiepileptic drug troxidone. The effect of the substituted alkyl groups on the bioactivity of these molecules is discussed. The substituted alkyl groups increase the negative potential found near the oxygen atoms in the molecular plane.
Size‐extensivity in multireference many‐body perturbation theories: A direct comparison between single‐reference and multireference perturbation theories in the nondegenerate case
Authors: Meissner L. , Jankowski K. 
Published: December 2004

AbstractLinkage properties of the diagrammatic representation of the energies obtained in the multireference many‐body perturbation calculations with respect to the incompleteness or completeness of the model space are discussed. The case of not completely degenerate model space is considered for which a comparison with the standard single‐reference many‐body perturbation expansion is possible. The Hose–Kaldor type of graphical representation of the perturbation expansion for the effective Hamiltonian is used in this comparison. It is shown that for an incomplete model space the perturbation expansion is not size‐extensive. In this case, for a truncated expansion of the effective Hamiltonian, the energies obtained by diagonalization of the effective Hamiltonian matrix are represented by both linked and unlinked irreducible contributions. The unlinked ones do not appear when the complete model space is used.
Ab initio calculations of all‐trans octatetraene and eight isoelectronic conjugated chains: Semiempirical heats of formation and stabilization energies
Authors: Younang Elie , André Jean‐Marie , Delhalle Joseph 
Published: December 2004

AbstractIn the framework of our study of the changes of the electric polarizability upon substituting heteroatomic linkages in conjugated hydrocarbon backbones, we have been led to examine the relative stability of eight compounds isoelectronic to the all‐trans octatetraene using the Cox and Pilcher concept of stabilization energy (SE), SE = ΔHa – ΣNABEAB, where ΔHa is the heat of atomization of the species under consideration and the EAB's are bond‐energy terms. Full geometry optimizations at the 4‐31G level have been performed to obtain the equilibrium geometries needed to deduce reliable semiempirical heats of formation from appropriate isodesmic processes. Some of the compounds containing the  CHN and  NN  groups have electric polarizabilities and stabilization energies comparable to octatetrene.
Theoretical study of CO adsorption on the NiCu(110) system
Authors: Castellani Norberto J. 
Published: December 2004

AbstractThe electronic influence of the matrix on several adsorption sites of the CO/NiCu(110) system has been studied using a semiempirical molecular orbital calculation. A negative ligand effect of a copper matrix on monometallic nickel sites and a less important ligand effect of a nickel matrix on copper sites have been found and explained in base on the electronic structure. Bridge nickel–copper sites show an intermediate negative ligand effect within a Cu matrix. The results of the theoretical calculation are compared with the available experimental data.
Correlation energy calculations for infinite systems
Authors: Fulde Peter , Stollhoff Gernot 
Published: December 2004

AbstractA survey is given of ab initio calculations for solids and polymers, which start from an SCF calculation and treat correlations by using local operators. The theory is formulated by employing a projection method. As regards applications, special attention is paid to elemental semiconductors and to polymers like polyethylene and polyacetylene. With the help of reduced Hamiltonians, insight can be gained into various correlation effects. This is demonstrated by a discussion of the problem of dimerization in polyacetylene. Finally, a formalism is described that allows for treating the effects of correlations on energy bands of solids. It is shown that qualitative new aspects of correlations enter when a quasiparticle in, e.g., a semiconductor is considered. The theory is applied to a calculation of the energy bands of silicon.
Electron correlation in one dimension: Coupled cluster approaches to cyclic polyene π‐electron models
Authors: Paldus J. , Piecuch P. 
Published: December 2004

AbstractMethods accounting for correlation effects in quasi‐one‐dimensional π‐electron systems and based on the restricted Hartree–Fock (RHF) single determinantal reference state are briefly reviewed as well as their role in the bond length alternation (or dimerization) problem in polyactylene‐like systems, as described by the semiempirical Hamiltonians of the Pariser–Parr–Pople (PPP) type. Particular attention is given to the π‐electron model of cyclic polyene homologous series CNHN, N = 2n = 4v + 2, v = 1,2,…, which can be regarded as an idealized model of one‐dimensional metallic‐like systems with imposed Born–von Kármán boundary conditions when N ∞. These models provide a useful bridge between the typical aromatic systems, represented by benzene (N = 6) π‐electron model, and long linear polyenic chains, since the properties of these systems hardly change once a sufficiently large N (˜26 or 30) is reached. It is shown that due to the quasidegenerate character of the RHF reference employed, and the related prominence of connected quadruply excited cluster components in the exact wave function, most of the standard technieques suffer a singular behavior break down entirely for large enough systems, particularly in the strongly correlated regime. The approximate coupled pair theory that accounts for quadruply excited clusters (ACPQ method) is shown to be free of these shortcomings and is extended to include also approximately the effects of connected triexcited cluster components. These results are compared with other recently examined approximate methods accounting for the tri‐ and quadruply excited clusters and the proposed ACPQ+T(ACPQ) procedure is shown to provide the best results in the whole range of the coupling constant.
Theoretical prediction of sequence‐dependent DNA superstructures and their implications in recognition mechanisms with proteins
Authors: Boffelli D. , De Santis P. , Palleschi A. , Scipioni A. , Savino M. 
Published: December 2004

AbstractSuperstructures of biologically relevant DNA fragments were obtained from the sequence by integrating the theoretically evaluated local deviations from the canonical B‐DNA structure produced by the differential interactions in the 16 different dinucleotide steps.The theoretical model was tested on 450 multimeric oligonucleotides different for sequence, periodicity, and length, and reproduced their electrophoretic behaviors with surprisingly good approximation. A very good agreeent was also obtained between the theoretical and experimental gel electrophoretic mobility changes following point mutations in a Sv40 DNA tract. The application to the set of the 177 different nucleosomal DNA tracts of Satchwell et al. revealed a very significant distribution pattern of the curvature phases along the sequence, which accounts for the main features of the nucleosome structure. This distribution allowed the localization of virtual nucleosome positions near the Sv40 origin of replication using the correlation function with the corresponding curvature diagrams in good agreement with the experimental map.
Enthalpic and entropic contributions of water molecules to the functional T → R transition of human hemoglobin in solution
Authors: Bulone D. , Palma‐Vittorelli M. B. , Palma M. U. 
Published: December 2004

AbstractGeneralized solvent‐mediated forces contribute to free energy at the functional T → R transition of human hemoglobin A (HbA). Their contribution is here sorted out quantitatively in both its enthalpic and entropic parts, along with the average number of water molecules involved. The latter (about 75 waters in average) must be considered together with HbA as one statistically defined functional unit for oxygen transport. Their configurations are expected to undergo frequent structural rearrangements. Lifetimes of statistically relevant configurations do not need to (although, of course, they may) exceed by more than a factor 5 the normal H‐bond lifetimes of the pure solvent. Compared to the bare protein, the “functional unit” here evidenced for the first time involves a larger, higher dimensionality region of the phase space. The study allows a microscopic view of solvent‐mediated generalized forces and evidences their qualitative and quantitative relevance to biomacromolecular function and stability.
Computational investigation of the role of hydration in nucleic acid structure and function
Authors: Poltev V. I. , Teplukhin A. V. , Malenkov G. G. 
Published: December 2004

AbstractThe results of the Monte Carlo Metropolis simulation of water structure and of the hydration of nucleic acid fragments, complementary base pairs and mispairs, base pair stacks, and duplex fragments have been summarized. Systematic investigations suggest some general conclusions: (1) the hydration shell structure of the major and minor grooves of the duplex depends significantly on DNA conformation (or stack configuration) and nucleotide sequence, while global hydration characteristics (average energy, the number of water–water and water–base H‐bonds) are only slightly dependent on these factors, (2) hydration economy takes place in the B–A transition due to an increase of the number of water molecules forming hydrogen bonds with two or more atoms of bases (water bridging), and (3) the hydration of the duplex could contribute to nucleic acid functioning via water‐bridged mispair formation and stabilization of specific conformations.
Some approaches to the multiple‐minima problem in the calculation of polypeptide and protein structures
Authors: Scheraga Harold A. 
Published: December 2004

AbstractVarious methods are used to surmount the multiple‐minima problem that is encountered in the multidimensional conformational energy surface of a polypeptide. A summary is given here of two of these methods: (i) The build‐up procedure that is modified to include statistical data on the positional frequencies of occurrence of amino acid residues along the chain, and (ii) the diffusion equation method that smoothes out the potential surface, leaving only the potential well containing the global minimum.
Computational studies on aspartic proteases. I. Active‐site protonation and hydration in the substrate‐free crystalline state
Authors: Turi László , Náray‐Szabó Gábor 
Published: December 2004

AbstractWe performed semiempirical molecular orbital calculations on the free and hydrated states of the HCOO−…HCOOH couple in crystals of an aspartic protease, rhizopuspepsin, in order to gain information not available by conventional X‐ray diffraction studies. The reliability of the MNDO/PM3 method was proven for the model system HCOO−…HOH, for which we could reproduce geometry and energetics of the complex, obtained by sophisticated ab initio calculations, with astonishing accuracy. Comparing the geometries of the active‐site models to experiment and their relative energies, we suggest that in the substrate‐free crystalline state the carboxyl–carboxylate–hydroxonium triad exists most probably in the neutral form. The carboxyl–carboxylate dyad attracts a hydroxonium ion better than does ammonium, indicating that the nonhydrogen atom, located by X‐ray crystallography near the active site, is oxygen and not nitrogen. We located the most probable hydrogen positions by geometry optimization.
Ab initio CI calculations on free‐base porphin
Authors: Yamamoto Yuichi , Noro Takeshi , Ohno Kimio 
Published: December 2004

AbstractAb initio configuration interaction (CI) calculations were carried out on low‐lying singlet and triplet π–π* states and ionized states of free‐base porphin. We take into account single and double excitations from σ and π electrons in the CI calculations. The composite natural orbitals were employed in order to reduce the size of orbital set to be used in the CI. The calculated excitation energies were in good agreement with experimental values. The use of split‐valence‐type basis and the inclusion of correlation effects of σ electrons were proved to be important to describe the low‐lying π–π* states, especially the Soret band.
Study on some metal–metal quadruple bonds
Authors: Li Ming 
Published: December 2004

AbstractIn the present paper, the role of (n − 1)ƒ orbitals in metal–metal quadruple bonds was studied. It was shown by the calculations that the probabilities for finding the σ‐, π‐, and δ‐electrons between two metal atoms, under the influence of the ƒ orbitals on the metal–metal quadruple bonds, increased while their mean kinetic energy components along the metal bond axis decreased. In addition, the effects of the ƒ orbitals upon the σ, π, and δ metal–metal bonds were different. In general, σ < π < δ.
Representation of square‐cell configurations in the complex plane: Tools for the characterization of molecular monolayers and cross sections of molecular surfaces
Authors: Walker P. Duane , Mezey Paul G. 
Published: December 2004

AbstractTwo numerical codes, a complex face vector F and a real face vector D are developed for the characterization of square‐cell configurations (lattice animals), used for representing the shapes of molecular monolayers and cross sections of molecular surfaces. The real face vector D represents all the intrinsic properties, size, and shape of the lattice animal. The complex face vector F contains complete information about the size, the shape, and also the placement of the particular lattice animal with respect to the lattice. Based on the properties of the face vectors, a method is developed for the classification of similar animals into equivalence classes. The face vector method is proposed for an algorithmic, nonvisual computer analysis of similarity of shapes of molecular monolayers and planar domains of cross sections of molecular surfaces, approximated by lattice animals.
Nonadiabatic theory of electron–vibrational coupling: New basis for microscopic interpretation of superconductivity
Authors: Svrček Michal , Baňacký Pavol , Zajac Anton 
Published: December 2004

AbstractThe electron–vibrational problem of the general nonadiabatic molecular systems has been solved by means of the quasi‐particle transformations. The SCF ab initio solution of the nonadiabatic fermion Hamiltonian yields stabilization of the electronic ground‐state energy due to electron–phonon interaction and it also gives the corrections to the one‐ and two‐particle terms. Two two‐particle correction yields effective attractive electron–electron interaction, but in the form different from Frölich's effective electron–electron interaction term. In contrast to the standard electron–phonon Hamiltonian of solid‐state physics that does not take into account the possible effects of nonadiabaticity of a system, the presented nonadiabatic theory yields also one‐particle corrections. The presence of this term in the Hamiltonian might play a crucial role in the theory of superconductivity since the superconductors are nonadiabatic systems.
Superconductivity–the phenomenon conditioned by nuclear microflows in nonadiabatic systems
Authors: Svrček Michal , Baňacký Pavol , Zajac Anton 
Published: December 2004

AbstractConditions for low and high Tc superconductors following from the nonadiabatic electron‐vibrational theory at ab initio level have been obtained. According to the presented results, the supercurrent is realized by the motion of the ground‐state electronic‐charge distribution of the fully occupied band. The motion of the electronic‐charge distribution is conditioned by the newly arisen, nondissipative degrees of freedom of nuclear, Jahn–Teller‐like, microflows. The Meissner effect is also interpreted.
The modified all‐valence INDO method with the inclusion of spin–orbit coupling
Authors: Roszak S. , Lipiński J. 
Published: December 2004

AbstractThe all‐valence INDO method has been modified for the inclusion of spin–orbit coupling effects. In the method presented, the Hamiltonian includes spin–orbit coupling and the basis set constitutes the singlet and triplet determinental wave functions constructed from molecular orbitals resulting from nonrelativistic calculations. Eigenvectors obtained are later used for the evaluation of transition probabilities among different states. The results presented include lifetimes of different states of organic molecules and transition energies for halogen molecules and they are in a good agreement with experimental results. © 1992 John Wiley & Sons, Inc.
Relativistic quantum defect calculations on the copper isoelectronic sequence
Authors: Lavin C. , Barrientos C. , Martin I. 
Published: December 2004

AbstractThe quantum defect orbital (QDO) method and its relativistic (RQDO) counterpart have been employed in this work to compute oscillator strengths for several fine‐structure transitions in members of the copper isoelectronic sequence up to Z = 92. The RQDO results are found to be in quite good agreement with the best estimates derived from other sources, whenever the comparison is possible. © 1994 John Wiley & Sons, Inc.
Theoretical evolution of the third‐order molecular polarizabilities as a function of chain length in thiophene and pyrrole oligomers
Authors: Beljonne D. , Shuai Z. , Brédas J. L. 
Published: December 2004

AbstractOn the basis of INDO (intermediate neglect of differential overlap)/MRD‐CI (multireference determinant–configuration interaction) calculations, the SOS (sum‐over‐states) formalism is applied to calculate the third‐order polarizabilities, γ, in thiophene and pyrrole oligomers. For both types of oligomers, the chain‐length dependence of γ can be divided into three regimes: Describing γ as a power law of the number of rings (N), we observe that the power value first strongly increases with N, than reaches a nearly constant value (power regime), and, finally, decreases toward one, indicating the appearance of a saturation regime. Very good agreement with the experimental evolution has been found in the case of the oligothiophenes. The comparison of the results obtained for the two types of oligomers indicates a lower polarizability in the oligopyrroles. © 1994 John Wiley & Sons, Inc.