bluuebunny/crossref_metadata_2025_split huggingface dataset:
The generalized padé approximants and chemical graph theory
Authors: Cioslowski J. 
Published: January 2005

AbstractThe theory of the Baker–Gammel approximants is applied to the spectral density function associated with the adjacency matrix. As a result, various upper and lower bounds to topological invariants are obtained in a systematic manner. In particular, the (N, M)‐type lower bound to the total π‐electron energy of benzenoid hydrocarbons is derived for the first time.
Eigenlevel calculations for screened coulomb potential systems via an integration‐free algorithm: Space pruning technique
Authors: Baykara N. A. , Demiralp M. 
Published: January 2005

AbstractIn this work, an algebraic method is developed to determine the eigenlevels of radially screened Coulomb potential systems. The method is mainly based on the concept of space pruning. The basis functions are combined in a way such that the resulting approximate eigenfunction and its several images under certain positive integer powers of the system's Hamiltonian (H) remain in the domain of H. Certain structural parameters are introduced into the basis functions to imitate some known behaviors of the true eigenfunctions of the system.The main purpose of this work is not to reproduce the results for screened Coulomb potential systems but to develop a method which does not necessitate integration at any stage and can later be adapted to atomic and molecular systems as easily as possible.
Inclusion of relativistic effects into ZDO methods. III. A. Quasi‐relativistic INDO/1 version
Authors: Boča Roman 
Published: January 2005

AbstractThe quasi‐relativistic CNDO/1 method for molecular orbital calculations has been extended to the INDO/1 version where all monocentric repulsion integrals are taken into account. This version has been applied to calculate molecular geometries of MXn‐type molecules, with the central atom belonging to from the second to seventh period. Properties of hypothetical superheavy elements (Z = 110–117) have been predicted. Transition metal complexes of the MCl‐type (M = Ni, Pd, Pt, and ePt) have been investigated from the point of view of spin‐state and configuration stability.
Theoretical investigation of the Na+ + H2 system. III. Trajectory calculations of vibrationally inelastic collision processes
Authors: Zuhrt Christian , Jeziorek Danuta , Żurawski Bronislav 
Published: January 2005

AbstractOn the basis of an analytical potential energy surface for the electronic ground state of the Na+ + H2 system reported recently, extensive trajectory calculations have been performed to study the collision dynamics of vibrationally inelastic processes at total energies up to ∼3 eV. Special attention is given to the relative efficiacy of translational and rotational energy, respectively, in promoting vibrational energy transfer. Vibrational transitions are found to be substantially enhanced by initial molecular rotation. Furthermore, the applicability of simple models is discussed.
New numerical approaches to the solution of the N‐well Schrödinger equation
Authors: Wierzbicki Andrzej , Bowman Joel M. 
Published: January 2005

AbstractTwo new numerical methods for solving the Schrödinger equation for an N‐well periodic potential are presented. One is a diagonalization method based on Floquet–Bloch formalism, and the other is a renormalized Numerov–Cooley method with periodic boundary conditions. The numerical superiority of these methods over the standard diagonalization technique is demonstrated. The methods are illustrated by applications to internal rotation.
Hydrogen bonding in vitamin B6
Authors: Plant J. S. , Greenwood H. H. 
Published: January 2005

AbstractDouble‐well potential energy surfaces for hydrogen bonding in crystalline vitamin B6 have been associated with molecular environmental effects. New calculations, involving improved representations of a fragmentation model, include the introduction of a second pyridoxinium chloride system within the model in a “dimer‐like” configuration. The new results confirm the double‐well potential and the prediction of the experimentally observed position for the proton as being due essentially to environmental effects.Atomic difference maps are presented for the charge density distributions, which reflect the nature of the bonding as it depends on the proton position. Mulliken populations are examined particularly in relation to the “intermolecular” transfer of electrons.
The energy of a molecule as a sum of terms related to its nuclei when the nuclear motion is quantum mechanically described*
Authors: García‐Sucre M. 
Published: January 2005

AbstractWe show that even when the nuclear motion is quantum mechanically described, the energy of a molecular state can be exactly expressed as a sum of terms, each one related to a nuclei of the molecule. This is a generalization of a similar result obtained by Politzer and Parr for the case in which the nuclei are in fixed positions. In fact, the nuclear motion is exactly taken into account through a modified screening function derivative evaluated on each nucleus of the molecule. We discuss how the screening function derivatives corresponding to the fixed nuclei case relate to those which take into account the nuclear motion. A simple relation between these two arises in the so‐called crude Börn–Oppenheimer approximation.
Projection operators and Clebsch–Gordan coefficients for projective representations
Authors: Altmann S. L. 
Published: January 2005

AbstractPrevious discussions of the bases and projection operators for projective representations are analyzed and alternatives are proposed. Detailed proofs are provided for a result which is often assumed or proved within unacceptable limitations, namely that the form of the projection operators which is standard for vector representations is also valid for unitary projective representations. These proofs provide necessary conditions for this result, and they are constructed in terms of the definition given for the bases of projective representations. The calculation of Clebsch–Gordan coefficients for projective representations by means of the projection operators is discussed. Whereas in the method of Dirl the work is carried out entirely in terms of the matrix representations, and the symmetrization of the bases has to be considered in a second step, all the work of this paper is conducted starting from the symmetrization of the bases of the projective representations, so that those two steps are carried out simultaneously.
Spectroscopic and structural features of small thiocarbonyl molecules in low‐lying excited states: Further applications of a variant of the orthogonal gradient method
Authors: Das Kalyan K. , Mukherjee D. , Bhattacharyya S. P. 
Published: January 2005

AbstractStructural parameters of a set of five thiocarbonyl molecules in the lowest nπ* states are calculated by using a generalized orbital optimization algorithm (a variant of the orthogonal gradient method) in an INDO MCSCF framework. Transition energies, singlet‐triplet splittings, planar inversion barriers, and dipole moments in nπ* states of different spin multiplicities are reported. Predicted structural features agree reasonably well with available experimental or theoretical data. Some interesting trends are noted in the computed inversion barrier heights, singlet‐triplet splittings, and dipole moments in nπ* states.
Guessing the state of a quantum system from a limited set of observations
Authors: Eriksson K.‐E. 
Published: January 2005

AbstractA quantum system is repeatedly prepared in the same way and then observed in measurement processes. The following problem is stated and solved: Given the results of a series of observations, what is the best guess for a density matrix describing the ensemble of prepared systems? The corresponding classical problem and its solution are obtained as a special case.
Determination of the potential from experimental data on energies and widths of quasi‐stationary levels
Authors: Fröman Nanny , Fröman Per Olof 
Published: January 2005

AbstractFormulas for the energies and widths of shape resonances, i.e., quasi‐stationary levels for a quantal particle moving in a spherically symmetric potential, valid also close to the top of the potential barrier, have been published by Drukarev, Fröman, and Fröman. On the basis of these formulas, in the present paper the potential is explicitly expressed in terms of experimental data on the energies and widths of the quasi‐stationary states. The treatment is related to previous treatments by Wheeler and by Cole and Good, Jr., which are in turn related to the Rydberg‐Klein‐Rees method.
Correlation of mutagenicity of 1,1‐dimethyl‐3‐(X‐phenyl)‐triazenes with molecular orbital energies and hydrophobicity
Authors: Shusterman Alan J. , Johnson A. Sherwood , Hansch Corwin 
Published: January 2005

AbstractA set of 18 aryltriazenes and the antitumor drug DTIC, whose mutagenicity has been determined in the Ames test, has been studied using octanol‐water partition coefficients (P) as a measure of relative hydrophobicity and MNDO molecular orbital energies and electron distributions to account for variation in their electronic characteristics. A good structure‐activity relationship was found using log P in combination with either εHOMO, or qHOMO, where the latter is defined as the HOMO electron density on N1 of the triazene. The dependence of mutagenicity on hydrophobicity is similar to what had been found earlier by Venger, Hansch, Hatheway, and Amrein [4] for the aryltriazenes alone using σ+ as a descriptor of electronic characteristics. A consistent role for electronic factors is found using σ+, εHOMO and qHOMO, namely, increased mutagenicity is associated with structural variations that increase the electron donating ability of the triazenes.
Perturbative virtual SCF CI treatment for energy levels of coupled oscillator systems
Authors: Hidalgo A. , Zúñiga J. , Bastida A. , Requena A. 
Published: January 2005

AbstractA perturbative SCF CI treatment to obtain energy levels of coupled oscillator systems is proposed. The method uses the virtual SCF basis set, and the SCF equations are solved by means of a perturbative treatment that provides the diagonal matrix elements involved in the CI calculation. The off‐diagonal matrix elements are calculated using a commutation relationship derived from exact quantum theorems. Numerical results for several systems are obtained and compared with those from others SCF, SCF CI, and variational treatments.
Sur la theorie des equations de Hartree–Fock dependent du temps. V. Etude d'un modele spherique a 2 electrons. Calcul du tenseur d'hyperpolarisabilite d'ordre 3
Authors: van Groenendael F. Smet et A. 
Published: January 2005

AbstractIn order to apply the TDHF equations in the matrix form we established previously, we conceive a model for which the static Hartree–Fock equations are rigorously solved; so we study problems only linked to the TDHF method, independent of any further approximation (for example, of LCAO type). This model is made of a spherical box containing electrons subject to a particular potential. We solve, numerically, the TDHF equations at different orders, and we calculate the third order hyperpolarizability tensor. We then compare our results with the ones given by a variation‐perturbation method.
Shell model calculations: An efficient new algorithm
Authors: Novoselsky Akiva , Katriel Jacob , Valliègres Michel , Chen Jin‐Quan , Gilmore Robert 
Published: January 2005

AbstractWe describe an efficient new algorithm which extends the range of feasible shell model calculations. This algorithm is applicable to single shell and multiple shell configurations, where two or more quantum numbers (e.g., L and S) are required to label the states within each shell. The algorithm proceeds by factoring the shell model Hilbert space into a product of subspaces, one for each angular momentum. N‐particle wave functions are built up recursively from N – 1 particle wave functions. Three kinds of N – 1‐ to N‐particle coefficients are required to carry out the construction of N‐particle electron (or fermion) states from N – 1 particle states. These are (1) coefficients of fractional parentage (CFPs) within a single shell, (2) outerproduct isoscalar factors (OISFs) within a single angular momentum subspace, and (3) innerproduct isoscalar factors (IISFs) which describe how multishell states within the complementary angular momentum subspaces are combined to form totally antisymmetric wave functions. All three types of N – 1‐ to N‐particle coefficients are generated recursively using a single powerful and efficient matrix diagonalization algorithm. Matrix elements of single particle creation and annihilation operators are expressed in terms of single particle CFPs, OISFs, and IISFs. We also describe an efficient algorithm for computing matrix elements of products of creation and anihilation operators by inserting and summing over complete sets of intermediate states. This is the Feynman‐like sum over path overlaps procedure. Timing benchmarks are presented comparing the new Drexel University shell model (DUSM) code with a state of the art shell model code.
Ab‐Initio MRD‐CI calculations for breaking a chemical bond in a molecule in a crystal or other solid environment. II.H3C—NO2 decomposition of nitromethane in a nitromethane crystal with voids
Authors: Roszak Szczepan , Keegstra Phillip B. , O'neal Douglas W. , Hariharan P. C. , Kaufman Joyce J. 
Published: January 2005

AbstractRecently we extended our strategy for MRD‐CI (multireference double excitation‐configuration interaction) calculations based on localized/local orbitals and an “effective” CI Hamiltonian for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystal or other solid environment. Our technique involves solving a quantum chemical ab‐initio SCF explicitly for a system of a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD‐CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD‐CI calculations are carried out for breaking a bond in the reference molecule. This method is completely general. The space treated explicitly quantum chemically and the surrounding space can have voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc. We previously applied this procedure successfully to the H3CNO2 bond dissociation of nitromethane in a nitromethane crystal with extensive testing of the number of molecules that have to be included explicitly in the SCF and how many molecules have to be represented by more distant multipoles. The results indicated that it took more energy to dissociate the H3CNO2 bond when the nitromethane molecule was in the crystal than it did to dissociate that bond in the free nitromethane molecule. In this present study we have investigated the effect of voids (both in the nitromethane molecules treated explicitly in the SCF and those in the environment represented by multipoles) on the calculated H3CNO2 bond dissociation energies.
A quantal approximation to the study of the collisional resonances in the NA–N2 system
Authors: Campos‐Martinez J. , Delgado‐Barrio G. , Villarreal P. , Miret‐Artes S. 
Published: January 2005

AbstractExperimental works suggest the existence of an oscillating complex in the collisional quenching of Na with N2 molecules. Until now, however, few efforts have been dedicated to the study of these possible complexes. In this work, a diabatic and diabatic‐configuration interaction method are proposed in order to investigate the existence of these quasibound levels. The results and their comparison with other available data are presented and discussed.
Quantum mechanical calculations on vibronic activity in the MCD spectra of carbonyl compounds
Authors: Marconi Giancarlo 
Published: January 2005

AbstractThe magnetooptical properties (B terms) vibronically induced have been calculated for a series of carbonyl compounds in the region of the first absorption band. The rules deduced experimentally for the signs and sizes of the B terms induced by vibrations of different symmetry are generally confirmed by these calculations.
Orthogonally spin‐adapted coupled‐cluster equations involving singly and doubly excited clusters. Comparison of different procedures for spin‐adaptation
Authors: Piecuch Piotr , Paldus Josef 
Published: January 2005

AbstractA particularly compact form of the orthogonally spin‐adapted coupled‐cluster equations involving all singly and doubly excited clusters is derived for the general case of a non‐Hartree–Fock closed‐shell reference determinant. The diagrammatic approach based on the graphical methods of spin algebras is applied. The relationship of different diagrammatic procedures for spin‐adaptation, employing both bare and spin‐adapted two‐electron interaction vertices, is discussed. A comparison with the results obtained with algebraic spin‐adaption approaches is also given.
Fractal displays of genomic DNA. I. Eco RI fractal lattice of buffalo rat
Authors: Finkel David L. 
Published: January 2005

AbstractA complex multidimensional DNA structure is obtained when treating the elements of the Eco RI restriction set of buffalo rat as a cascade of self‐similar units. Fifteen of the 16 (about 94%) Eco RI repeat families, ranging in length from 2300 to 83 nucleotides and in frequency from 500 to > 106 units, are sorted into a three‐tiered well‐ordered fractal/Fibonacci lattice. On the first level, 11 fractal dimensions are defined as unique collinear collections of repeat DNA families in a “graph‐invariant” plot of log(fragment length):log(frequency). On the second level, the individual dimensional arrays are clustered as groups, each sharing, at last, one common vertex. On the third level, the entire display is scaled along a stepwise log to the base 6 expansion. This study represents the first successful attempt to unify an “entire” set of repeat DNAs into a single motif, in effect confirming the contention that select repeat DNAs provide a framework for the discrete packaging and unpackaging of genomic information.
Bonded tableau unitary group approach to the many‐electron correlation problem
Authors: Li Xiangzhu , Zhang Qianer 
Published: January 2005

AbstractA spin‐free symmetry‐adapted valence bond (VB) state, named bonded tableau (BT), is deduced from the classical bonded function and labeled by an at most two‐column Weyl tableau. The complete set, which is composed of the BT basis or canonical bonded tableau (CBT), can be constructed from an overcomplete set of BT states. CI CBT and VB CBT are two kinds of complete sets that are constructed in this paper. They can be used, respectively, in the CI and VB theory. It is shown that there is a one‐to‐one correspondence between the labeling scheme for CI CBT and the Gelfand–Tsetlin (GT) basis. This relationship enables an efficient generation and compact representation of the BT basis if one desires to use the known global representation scheme for the GT basis. Effective algorithms for the matrix element evaluation of unitary group generators and products of generators between BT states are presented. In the formulation, the action of a generator on a BT state yields another BT state times a coefficient, so that the matrix elements of an arbitrary multiple product of generators are reduced to a calculation of the overlaps between BT states. The evaluation of the overlaps leads to a simple factorization into cycle contributions, whose values are given explicitly and only depend on the length parameters of the cycles. It is hoped that the presented formalism can facilitate the procedures for handling of the many‐electron correlation problem.
The multireference constant denominator perturbation theory for one‐particle systems and its application to the anharmonic oscillator
Authors: Lan Zhida , Cullen John M. 
Published: January 2005

AbstractIn this paper a multireference constant denominator perturbation theory (CDPT) is developed to reduce incomplete basis set errors arising when solving the Schrödinger equation with a finite basis set. The advantage of this method is that very few basis functions are needed, and all calculations if carried out to high enough order in the perturbation treatment effectively use a complete basis set. As a first step the theory has been restricted to one‐particle Hamiltonians and applied to the anharmonic oscillator to study the convergence properties. For perturbation calculations carried out to fifth order, results from Pade approximates show an improvement in accuracy of between one and three orders of magnitude.
Two‐center molecular repulsion integrals over slater functions
Authors: Maslen E. N. , Trefry M. G. 
Published: January 2005

AbstractFor the general two‐electron two‐center integral over Slater functions, use of the Neumann expansion for the electron‐electron interaction term yields the standard auxiliary functions. These are expanded and integrated explicitly by two independent methods. The resulting simple analytic formula for the total integral is completely general, requiring only the Slater function quantum numbers and exponents and the internuclear separation. Hence all two‐electron hydrid, coulomb, exchange, and one‐center integrals are considered. The efficiency of calculation of this expression is compared with those of other methods, indicating an order of magnitude improvement in speed over recursion for the exchange integral.
Scalar nonlinearities in physics and chemistry
Authors: Heimsoeth Bettina 
Published: January 2005

AbstractScalar nonlinearities are operators of the form N(u) = 〈Au, u〉Bu, where A, B are linear operators, and 〈·,·〉 is the inner product in a Hilbert space ℋ︁. This paper reviews applications of scalar nonlinearities. We show that operators of the form N(u) are found in equations that describe phenomena of classical mechanics, open systems of quantum mechanics, and chemical physics.
Comparability graphs and molecular properties III. C9 and C10 alkanes
Authors: Bonchev D. , Kamenska V. , Mekenyan O. 
Published: January 2005

AbstractThe graph‐theoretical approach recently developed for comparing and ordering isomeric chemical compounds is extended and applied to the calculation of various molecular properties of C9 and C10 alkanes (critical densities, volumes, and pressures; heats and entropies of vaporization; standard entropies in liquid and gas state). The key to the approach is in specifying subsets of comparable isomers which are located in the same paths in the so‐called comparability graphs. Optimized samples for structure‐property correlations are thus obtained.
Ab initio studies of arginine and deaminoarginine
Authors: Sapse Anne‐Marie , Strauss Rosalind 
Published: January 2005

AbstractL‐Arginine and Deaminoarginine were studied via quantum chemical ab initio calculations using the STO‐3G basis set for arginine and the STO‐3G and 3–21G basis sets for deaminoarginine. It was found that the most stable conformations are the ones featuring the carboxyl group slightly twisted versus the rest of the molecule, which adopts an extended conformation. It was also found that the cyclic zwitterion is less stable than the neutral cyclic conformation and that there is a barrier to the proton transfer from the carboxyl's oxygen to one of guanidine moiety's nitrogens.