bluuebunny/crossref_metadata_2025_split huggingface dataset:
Hyperspherical Strumian basis functions
Authors: Avery J. , Herschbach D. R. 
Published: January 2005

AbstractA Sturmian basis set is a set of solutions to the Schrödinger equation, with the potential scaled in such a way that all the members of the set correspond to the same value of the energy. We discuss, in particular, the set of Sturmian basis functions corresponding to solutions of the d‐dimensional hydrogenlike wave equation. These hydrogenlike Sturmian functions are expressed in terms of Laguerre polynomials and hyperspherical harmonics. When they are used as a basis for solving the many‐particle Schrödinger equation, the secular equations take on a simple form [Eq. (59)]. The necessary integrals are evaluated explicitly, and the possibility of combining the hyperspherical technique with dimensional scaling is discussed.
The second‐order energy contribution from the spin–orbit interaction operator to the potential energy curve of Cr2
Authors: Vahtras Olav , Ågren Hans , Jørgensen Poul , Jørgen Hans , Jensen Aa. , Helgaker Trygve 
Published: January 2005

AbstractThe second‐order Rayleigh–Schrödinger energy correction to the Born–Oppenheimer potential energy due to the spin–orbit interaction can be expressed as a linear response function evaluated at zero frequency. We have calculated this energy contribution to the Cr2 singlet ground state X1 Σ+g potential energy function for a multiconfiguration self‐consistent field (MCSCF) wave function. The calculations show that the effect of spin–orbit interaction is small and of the same magnitude for the whole potential energy curve.
Ab initio studies of the electronic structure and density of states of metallic beryllium
Authors: Ross R. B. , Ermler W. C. , Kern C. W. , Pitzer R. M. 
Published: January 2005

AbstractHartree–Fock–Roothaan studies are reported for low‐lying electronic states of metallic beryllium as modeled by a moiety of 135 beryllium atoms. The system corresponds to 16 coordination shells of a central Be with internuclear separations derived from the lattice constants of the bulk metal. The calculations become tractable by use of the full D3h symmetry of the system at both the integrals and self‐consistent‐field stages and by employing ab initio effective potentials for the 1s electrons of each beryllium atom. Ionization potentials, binding energies, orbital energies, electric field gradients, nuclear‐electrostatic potentials, diamagnetic shielding constants, second moments, and Mulliken populations are calculated for selected electronic states. The calculated ionization potential for the lowest state agrees to within 10% of the experimental bulk work function. A density‐of‐states analysis for that state is reported and compared with band structure calculations.
Löwdin α‐function, overlap integral, and computer algebra
Authors: Jones Herbert W. 
Published: January 2005

AbstractA commercial computer algebra program, Mathematica, is used to generate the C matrix that characterizes our implementation of the Löwdin α‐function method as applied to Slater‐type orbitals. An example of a two‐center overlap integral is done to show how the arbitrary precision capability of Mathematica can overcome severe cancellation errors encountered with programming in FORTRAN. This strategy is capable of being generalized to other multicenter molecular integrals. Mathematica programs are included.
CH Bond dissociation of acetylene: Local density functional calculations
Authors: Habibollahzadeh Dariush , Murray Jane S. , Grodzicki Michael , Seminario Jorge M. , Politzer Peter 
Published: January 2005

AbstractThe CH bond dissociation energy of acetylene was computed by both ab initio approaches and density functional theory in a local density approximation (DFT–LDA). Structures and energies for acetylene and its dissociation products (the ethynyl and hydrogen radicals) are presented and compared. Using directly computed HCCH and HCC· energies and the exact H· value, the DFT–LDA calculations are found to yield CH dissociation energies ranging from 129 to 131 kcal/mol, in good agreement with recent experimental and the highest level theoretical results. The DFT–LDA results show little dependence upon the computational procedure used to obtain geometries.
Electronic states and nature of the chemical bond in the molecule CrC by all‐electron ab initio calculations
Authors: Shim Irene , Gingerich Karl A. 
Published: January 2005

AbstractAll‐electron ab initio Hartree–Fock (HF), valence configuration interaction (CI), and multiconfiguration self‐consistent‐field (CASSCF) calculations have been applied to investigate the electronic states of the CrC molecule. The molecule is predicted as having four low‐lying electronic states, 3∑−, 5∑−, 7∑−, and 9∑−, separated by an energy gap of 0.55 eV from the next higher‐lying state, 1∑−, which is followed by the states 5Π and 7Π. The four lowest‐lying electronic states are due to the coupling of the angular momenta of the 6Sg Cr+ ion with those of the 4Su C− anion. The chemical bond in the 3∑− ground state can be viewed as a quadruple bond composed of two σ and two π bonds. One σ bond is due to the formation of a molecular orbital that is doubly occupied. The remaining bonds, i.e., one σ and two π bonds, arise from valence‐bond couplings. The π bonds originate from the valence‐bond couplings of the electrons in the C 2pπ orbitals with those in the Cr 3dπ orbitals. The σ bond originates from the valence‐bond coupling of the C 2pσ electron with an electron in the Cr 4s, 4p hybrid that is polarized away from the C atom.
Implications of unitary invariance for gradient theory
Authors: Jayatilaka Dylan , Handy Nicholas C. 
Published: January 2005

AbstractThis paper shows how the invariance of the energy to unitary rotations in the occupied orbitals and in the virtual orbitals can be used to determine some elements of the derivative molecular orbitals, which are important for gradient theory. Solution of the usual coupled‐perturbed Hartree–Fock equations for these elements is thus unnecessary, eliminating considerable effort. We also give proofs which show clearly that special conditions are required.
Shape similarity and shape stability along reaction paths: The case of the PPO → OPP isomerization
Authors: Luo Xincai , Arteca Gustavo A. , Mezey Paul G. 
Published: January 2005

AbstractAn analysis of molecular shape has been performed along an ab initio reaction path of the PPO → OPP isomerization. In a first approximation, we have used the fused‐sphere (van der Waals) model to represent shape. The results show that there is no one‐to‐one correspondence between the species defined in terms of shape and those based on an energy criterion. In contrast to the cases of most other isomerizations in triatomic molecules (e.g., the HCN isomerization), for the PPO isomerization there exists a stable intermediate. For the elementary reaction from the linear PPO (C∞v, reactant) to the cyclic P2O (C2v, intermediate), we found no distinct molecular shape type for the transition structure (between the reactant and the intermediate). In terms of shape, the nuclear configuration of the transition structure is more similar to that of the intermediate than that of the reactant. Using the PPO → OPP reaction as an example, in this work we discuss the implications of this result regarding possible measures of shape similarity and shape stability along reaction paths. We have performed a molecular‐shape analysis of the electron density contour surfaces to test the reliability of the fused‐sphere approximation. A comparison between the van der Waals and electron isodensity surface models at critical points shows that the former model can be qualitatively correct for molecular‐shape descriptions along some reaction paths.
Monte Carlo determination of the oscillator strength and excited state lifetime for the Li 22S → 22P transition
Authors: Barnett R. N. , Reynolds P. J. , Lester W. A. 
Published: January 2005

AbstractA recently developed Monte Carlo method is used to compute the 22S → 22P transition dipole moment of Li. This approach employs a guided Metropolis random walk with quantum Monte Carlo “side” walks to sample the required probability distributions. The transition dipole moment is employed to obtain the oscillator strength and excited‐state lifetime. Our most accurately converged calculations yield an oscillator strength of 0.742(7) and excited‐state lifetime of 27.41(35) ns. These results are in excellent agreement with precise experimental measurements of 0.742(1) and 27.29(4) ns, respectively. In addition, single‐state expectation values are computed for both states. Monte Carlo parameters, such as the time step size and the convergence time, are varied in order to study their effect on computed results.
Spline methods for multiconfiguration Hartree–Fock calculations
Authors: Fischer Charlotte Froese , Guo W. , Shen Z. 
Published: January 2005

AbstractThe earlier numerical multiconfiguration Hartree–Fock atomic structure package was not designed with high‐performance computers in mind. In this paper, some new algorithms based on spline–Galerkin methods are described that are appropriate for concurrent/vector architectures. The goal is to improve the level of numerical accuracy by several orders of magnitude using fewer basis functions than points in a numerical grid. Of critical importance is the robustness of the code: The most serious problems in the numerical implementation were associated with orthogonality constraints. In a spline basis approach, the orthogonality requirements can be integrated into quadratically convergent update procedures. These procedures are evaluated for a number of cases.
A heuristic potential function for intramolecular and intrasupermolecular chemical reactions
Authors: Nagaoka Masataka , Okuno Yoshishige , Yamabe Tokio , Fukui Kenichi 
Published: January 2005

AbstractA heuristic fitting procedure to obtain an analytical potential function for describing a reactive potential energy surface in the neighborhood of the intrinsic reaction coordinate (IRC) has been developed. For discussion, the pairwise potential function form, ∑anr−n, is assumed in order to fit ab initio quantum mechanical calculations of intramolecular (or intrasupermolecular) interaction energies and its use is found advantageous because all the calculation can be carried out by the linear least squares method. Normal modes perpendicular to IRC are utilized to prepare an initial data base for the potential fitting in the neighborhood of IRC. Some trial molecular dynamics (MD) simulations are performed in order to check the fitted potential function and, unless they lead to reasonable energies within the tolerance assumed, their results are utilized to construct an improved data base (the dynamic sampling). The present systematic optimization procedure has been applied to the proton transfer reaction of the formamidine–water (FW) system. The normal mode analysis in both the transition state (TS) and the stable state (SS) regions suggests that the present fitted potential function can reproduce satisfactorily the Born–Oppenheimer (BO) adiabatic surface obtained by ab initio molecular orbital (MO) calculations. We conclude that our procedure works well for the chemical reaction molecular dynamics (CRMD) simulation.
Ab initio calculations of the ground‐state potential energy surface for the CF bond decomposition in n‐fluoropropane within the method for approximation of the frozen molecular fragment
Authors: Roszak S. , Kaufman Joyce J. 
Published: January 2005

AbstractThe recently proposed ab initio method for calculation on many‐electron molecular systems with the approximation of the inactive part of a molecule by a frozen molecular fragment was tested further in a case of the dissociation reaction of the CF bond in n‐fluoropropane. Results from the Hartree–Fock, multiple reference double‐excitation configuration–interaction and second‐order Møller–Plesset methods are presented. The reproduction of potential energy surfaces as well as the reproduction of electron density distribution are in excellent agreement with extended basis‐set calculations. Different choices of fragments to be frozen have been examined.
Molecular quantum similarity measures and N‐dimensional representation of quantum objects. II. Practical applications
Authors: Carbó R. , Calabuig B. 
Published: January 2005

AbstractNewly developed classification and graphical representation techniques are discussed in order to visualize quantum similarity computational results. Some examples of molecular quantum similarity applications involving the fields of (a) structure–property relationships, (b) structure–activity relationships, (c) MO taxonomy, and (d) conformer taxonomy are presented.
A simplified three‐level model describing the molecular third‐order nonlinear optical susceptibility
Authors: Dirk Carl W. , Cheng Lap‐Tak , Kuzyk Mark G. 
Published: January 2005

AbstractA simplified three‐level model for γ, the molecular third‐order nonlinear optical susceptibility, is presented and discussed. The perturbation theory‐based approach suggests that there are three primary avenues to optimizing molecular four‐wave mixing susceptibilities and that with each of these is associated a particular class of molecular electronic structures. The three electronic structure classes consist of (1) conjugated donor–acceptor dipolar molecules with a large secondorder susceptibility, β; (2) even‐member conjugated chains such as ‐enes, ‐ynes, and ene–ynes with large two‐photon dominated susceptibilities; and (3) “charged” odd‐member conjugated chaines with large linear absorption dominated third‐order susceptibilities such as squaryliums (perhaps, more generally, the polymethine dyes). Classes (1) and (2) have been known and investigated in the past, while recent results of ours suggest the existence of the third and perhaps final class.
New algorithm for high‐order time‐dependent hartree–fock theory for nonlinear optical properties
Authors: Sekino Hideo , Bartlett Rodney J. 
Published: January 2005

AbstractA new formalism and algorithm is developed for solving the general‐order time‐dependent Hartree–Fock (TDHF) problem. It is shown that for any order a generalization of the TDHF equations can be derived where all lower‐order solutions constitute a constant term. This makes it very easy to obtain high‐order solutions. As the space required for the mapping of density matrices to Fock matrices in a problem of a giver order is largely reduced, we can perform the most time‐consuming steps within the core memory of the machine and easily manipulate vector products via optimum routines. The second hyperpolarizability γ is obtained from the secondorder TDHF solution via a 2n rule. The formalism also allows for expressing all terms in the equation diagrammatically, which provides additional physical insight and a more systematic evaluation of terms. To illustrate the method, TDHF results are presented for trans‐butadiene and carbon monoxide for several optical processes, including correlation corrections to their static hyperpolarizabilities obtained via coupled cluster (CCSD) and many‐body perturbation theory. The hybrid TDHF/CCSD method provides excellent agreement with the DC–SHG experiments (χ||(2) = 11.4 x 10−32 esu/mol compared to 12.9 ± 11.4 × 10−32 esu/mol and χ||(3) = 149 compared to 144 ± 4 × 10−39 esu/mol).
Evolution of polarizabilities and hyperpolarizabilities with molecular aggregation: A model study of acetylene clusters
Authors: Augspurger Joseph D. , Dykstra Clifford E. 
Published: January 2005

AbstractAb initio and model calculations on linear clusters of acetylene molecules have been carried out to identify the effect of weak intermolecular interaction on the intrinsic dipole polarizability (‐) and the second hyperpolarizability (γ) and, in particular, to determine the three‐body and higher‐order effects. Ab initio and model calculations on a linear dimer show clearly that the evolution of properties from the separated limit is largely a consequence of the mutual polarization as the monomers begin to interact. This reveals that intermolecular quantum features are not significant. Furthermore, higher‐order response properties play a negligible role in the evolution of lower‐order cluster properties. For instance, the hyperpolarizability γ of the monomer is relatively unimportant in the evolution of α of the dimer; the change in α due to dimer formation is dictated largely by the mutual polarization via the intrinsic α′s of the monomer. With this determination, an electrical interaction model can be used to explore α and γ of long acetylene chains, and this serves as a guide to the vibronic coupling effects on hyperpolarizabilities of longchain species. We find, in particular, that the hyperpolarizability of the subunits of long acetylene chains are dramatically enhanced by intermolecular electrical interaction.
The accurate calculation of dipole moments and dipole polarizabilities using Gaussian‐based density functional methods
Authors: Sim Fiona , Salahub Dennis R. , Chin Steven 
Published: January 2005

AbstractDipole moments and static dipole polarizabilities have been calculated for a number of small molecules using the linear combination of Gaussian‐type orbitals–local spin density method. The effect of augmenting standard orbital basis sets with polarization functions has been investigated. A set of optimum ζd, for use in calculating polarizabilities, has been derived for the first‐row atoms C, N, O, and F. The results of this optimized doubly polarized double‐zeta basis set compare well with results obtained using a double‐zeta basis set augmented by four even‐tempered ζd polarization functions. The results of the optimized basis set, and a basis set augmented with only a single ζd polarization function derived from it, compare very favorably with those obtained from Møller–Plesset perturbation theory and with experimental data. They show a marked improvement on results obtained using standard Hartree–Fock self‐consistent‐field molecular orbital methods where no treatment of electron‐correlation is included.
Electric‐field mapping of some substituted 3‐pyridinecarboxylic acid cardiotonics and the possible structure–activity relationships
Authors: Kumar Anil , Bhattacharjee A. K. , Mishra P. C. 
Published: January 2005

AbstractFour esters of 5‐cyano‐1,6‐dihydro‐6‐oxo‐3‐pyridinecarboxylic acid involving substitutions of methyl, isopropyl, tert‐butyl, and phenyl groups at the 2‐position, which exhibit varying degrees of cardiotonic activities, have been studied using the electric‐field and electrostatic potential mapping techniques. The net charge distributions in the molecules were obtained using the MNDO method. Electric‐field mapping has been performed in three‐dimension in order to obtain information about the possible structure–activity relationships of these compounds. The results show a prominent electric‐field region in the methyl‐substituted compound extending from near the oxygen atom O6 to near the methyl group. This fact and orientation of the electric field in this region, in view of the Erhardt model, may be related to the greater cardiotonic activity of the methyl‐substituted molecule in comparison to those of the other molecules of this class. © 1992 John Wiley & Sons, Inc.
Variational principle and path integrals for atoms in molecules
Authors: Zou P. F. , Bader R. F. W. 
Published: January 2005

AbstractTwo things were done in this paper: (i) A generalization of Schwinger's variational principle to a subsystem was developed within the framework of quantum field theory and applied to the theory of atoms in molecules. This work generalizes the previous derivation given in the Schrödinger formulation. (ii) It is demonstrated that Feynman's path integral, when expressed in terms of the coherent‐state representation, can be constructed for a subsystem of a many‐electron system if a divergence term, which serves as a variational constraint in the definition of an atom in a molecule, is added to the Lagrangian. This formulation is equivalent to the atomic statement of the variational principle if the divergence term is suitably constructed. It is shown that the path integral can be expressed as a product of the individual atomic contributions to the steps along the paths with the action being determined by a corresponding sum of the atomic contributions to the action integral. © 1992 John Wiley & Sons, Inc.
Internal orbitally resolved charge sensitivity analysis
Authors: Nalewajski Roman F. 
Published: January 2005

AbstractThe intramolecular (internal) charge sensitivity analysis (ICSA) is presented in the molecular orbital (MO) resolution, with diagonal idempotency constraints being imposed on the MO occupations, n, which can then be considered as independent electron population variables of the system for the assumed fixed shapes of MOS. The standard closed‐shell (C.S.) SCF MO framework is adopted with the energy function E(n) approximated by the average energy of a configuration expression to cover fractional MO occupations. The components of the gradient of the auxiliary energy function, including the constraint terms, with respect to n, identified as negative MO internal electron potentials, vanish identically for the ground‐state c.s. configuration, whereas their responses to hypothetical intramolecular charge displacements assure the system stability. An application of the ICSA to the iterative SCF procedure is suggested. Illustrative results for the water molecule are reported and general properties of molecular normal orbitals (eigenvectors of the hardness matrix) are discussed. © 1992 John Wiley & Sons, Inc.
An analysis of molecular electrostatic potentials obtained by a local density functional approach
Authors: Murray Jane S. , Seminario Jorge M. , Concha Monica C. , Politzer Peter 
Published: January 2005

AbstractLocal density functional theory (DFT–LDA) has been explored as a tool for obtaining the molecular electrostatic potential V(r), using the code DMol. We have presented and discussed DFT–LDA electrostatic potentials for a representative series of molecules: ethylene, benzene, formamide, cytosine, and 2,3,7,8–tetrachlorodibenzo–p–dioxin. V(r) results obtained with a double numerical plus polarization (DNP) basis set show the key features that are characteristic of the ab initio potentials of these compounds and suggest that this is a useful approach, especially for large molecules that are difficult to study by ab initio methods.
Modeling solvent effects in molecular dynamics simulations of proteins
Authors: Solmajer T. , Mehler E. L. 
Published: January 2005

AbstractA series of computer simulations has been carried out on bovine pancreatic trypsin inhibitor using various models to mimic the effects of explicit bulk solvent on the structure of the protein. The solvent properties included are the polarization of the solute by the polar bulk solvent and the restraining effect on the motional freedom of the solute due to frictional drag at the solvent–protein surface interface. The former has been included by using a distance–dependent dielectric permittivity to screen the electrostatic interactions, whereas the latter is simulated by adding a limited number of solvent molecules near the protein surface. To achieve the proper mobility of the water molecules, their motion was restrained by adding a harmonic restraining force. It was found that a very small force constant was sufficient to model the static and dynamical behavior of the fully solvated solute, but that it was necessary to include enough explicit waters to occupy the first solvation shell. © 1992 John Wiley & Sons, Inc.