bluuebunny/crossref_metadata_2025_split huggingface dataset:
Phase associated with the single‐particle density of many‐electron systems
Authors: Sukumar N. , Deb B. M. 
Published: January 2005

AbstractIn excited states of atoms and molecules, as well as in time‐dependent situations, the one‐electron density no longer suffices to completely characterize the electronic state; in addition, one now requires information about the electronic phase or the current density. We show that, for a stationary electronic state, the continuity equation of quantum fluid dynamics represents a differential equation for the electronic phase, which must be solved subject to certain periodicity conditions. These periodicity conditions arise from the nodal topology of the wave function and give rise to quantized vortices of current. The consequences of writing an electronic “wave function” for a many‐electron system directly in terms of the single‐particle density and phase have been investigated. We have shown that such a procedure leads to the appearance of an “internal magnetic vector potential.” We also establish the connection between the electronic phase and the geometrical (“Berry”) phase accompanying the adiabatic transport of a quantal system around a closed loop in parameter space. This leads to a generalization of the current density concept and allows us to discuss the geometrical phase in terms of the circulation of this current in parameter space.
Multidimensional wave‐packet analysis: Splitting method for time‐resolved property determination
Authors: Pernot P. , Lester W. A. 
Published: January 2005

AbstractThe splitting method is an approach for preventing unphysical behavior of a wave‐packet at the edges of its coordinate space representation grid. We examine here the possibilities and limits of using this method to perform time‐resolved analysis of the products of molecular processes.
Harmonic oscillator tensors in contact transformation theory
Authors: Palting Pancracio , Villa Maria 
Published: January 2005

AbstractThe application of contact transformation theory to the perturbed harmonic oscillator is reexamined in the light of the harmonic oscillator tensors previously presented. It is found that the recasting of the formalism of this problem in terms of harmonic oscillator tensors results in great simplifications, most of which stem from the introduction of the additional algebraic quantum numbers (l, m). The order of magnitude of each fragment of the Hamiltonian is easily recognizable, and the diagonal and nondiagonal parts contained therein are readily identifiable. The determination of the contact transformation operator is reduced to a simple formula. First, an analysis is made for a single mode of vibration, and it is subsequently extended to a multimode case. The perturbed diatomic vibrator is presented as an example.
A quantum‐chemical study of the LiH + LiH⋅− ⇄ LiH2− + Li⋅ reaction
Authors: Hess B. Andes , Zahradník R. 
Published: January 2005

AbstractParts of the potential energy surface of the title process and related processes have been investigated at the SCF/6‐31G**, SCF/6‐31++G**, and MP2/6‐31++G** levels. The investigated reaction is exothermic (−6.23 kcal/mol, MP4/6‐31++G**//MP2/6–31++G** level, ZPE included): A linear intermediate radical anion, LiHLiH⋅−, is significantly stabilized with respect to LiH + LiH⋅− (−38.74 kcal/mol, the same level as above). The BSSE at MP2/6–31++G**//MP2/6–31++G** amounts to 1.8 kcal/mol. The title process seems to be suitable for experimental study in molecular beams.
A graphical approach to configuration interaction studies in molecules using determinants of nonorthogonal orbitals
Authors: Rettrup Sten , Thorsteinsson Thorstein , Sarma C. R. 
Published: January 2005

AbstractA direct graphical approach is presented for the evaluation of matrix elements of a spin‐free Hamiltonian between Slater determinants of nonorthogonal orbitals. The matrix element contributions are obtained by repeated use of N – 2 electron minors of the overlap matrix that are generated by independent looping of reduced N – 2 electron graphs for the α‐ and β‐strings of the determinants. Because of the successive generation of the matrix contributions, the method is well suited for direct diagonalization schemes.
Two‐electron repulsion integrals over Gaussian s functions
Authors: Gill Peter M. W. , Johnson Benny G. , Pople John A. 
Published: January 2005

AbstractWe present an efficient scheme to evaluate the [0](m) integrals that arise in many ab initio quantum chemical two‐electron integral algorithms. The total number of floating‐point operations (FLOPS) required by the scheme has been carefully minimized, both for cases where multipole expansions of the integrals are admissable and for cases where this is not so. The algorithm is based on the use of a modified Chebyshev interpolation formula to compute the function exp(−T) and the integral Fm(T) = ∫01u2mexp(−Tu2) du very cheaply.
Self‐Consistent‐Field potential‐energy surfaces for hydrogen atom pairs within small palladium clusters
Authors: Anchell James , Gutowski Maciej , Nichols Jeff , Simons Jack 
Published: January 2005

AbstractAn ab initio electronic structure study is presented of hydrogen–hydrogen interactions in an electronic environment perturbed by the presence of palladium atom clusters. In particular, we investigated changes in the interatomic potential when the atomic centers are trapped inside an fcc palladium octahedral hole and when they are separated from each other by a (111) plane of palladium atoms. The (111) plane was modeled with a cluster of three palladium atoms. Self‐consistent field (SCF) level calculations were performed, and palladium atom pseudopotentials were employed to make the systems studied computationally tractable. For pairs of atoms placed within the octahedral hole, various lines of approach were considered [along the (100), (110), and (111) directions]. Lattice deformations and electronic excitations were examined for their effect on the interatomic potential.
A Laplace transform solution of Schrödinger's equation using symbolic algebra
Authors: Clarkson Michael E. , Pritchard Huw O. 
Published: January 2005

AbstractA general solution to the one‐dimensional time‐independent Schrödinger equation is derived using the properties of the Laplace transform. The derivation assumes that the potential function is real and that it can be expressed as a Fourier series with a finite number of terms, which includes, but is not limited to, equations of the type of Hill's or Mathieu's equations. In the case where the Laplace transform of the potential function contains only simple first‐order poles, then a complete closed‐form solution is derived. In other cases, an approximate solution is derived and the symbolic algebra program MACSYMA is used to carry out the inverse Laplace transform. The MACSYMA implementation of this general solution is discussed and is applied to a sample problem, namely, the solution of eigenvalues of Mathieu's equation. Comparison is made between the eigenvalues obtained by this method and those obtained by another analytical solution of this equation, by Hill's method.
Application of an optimization technique to the discretized version of the Griffin–Hill–Wheeler–Hartree–Fock equations
Authors: Custodio R. , Giordan M. , Morgon N. H. , Goddard J. D. 
Published: January 2005

AbstractApplication of the SIMPLEX optimization method to define the mesh of the discretized version of the Griffin–Hill–Wheeler–Hartree–Fock (GHWHF) equations was studied. Improved discretization parameters with respect to the original method were obtained for atomic systems with two or four electrons and for the H2 molecule. For the atomic systems, the following correlations between the discretization parameters and the total energy were found: N = a · In(ΔE) + b; Ω0 = a′ · In(ΔΩ) + a″; and In(ΔΩ) = b′ · ln(N) + b″. These equations provide a systematic procedure to reach a desired degree of accuracy in the energy for the atomic systems studied as well as to fix the basis set to be employed. These equations are similar to those found earlier for eventempered basis sets and permit the establishment of a relationship between the two methods. The even‐tempered method is also an approximate solution of the GHWHF equations. The optimized integral discretized basis is more efficient in representing small basis sets for atoms and the basis for the hydrogen molecule in comparison to the even‐tempered one. The optimization procedure was successfully applied to generate the universal basis for the atomic systems studied.
Electron correlation in pericyclic reactivity: A similarity approach
Authors: Ponec Robert , Strnad Martin 
Published: January 2005

AbstractThe recently proposed topological approach to chemical reactivity in terms of the secondorder similarity index was applied to the detailed analysis of correlation effects in pericyclic reactivity. This analysis implies that (i) the electron correlation is generally more important in forbidden reactions than in the allowed ones; (ii) in contributing to the discrimination between the allowed and forbidden reactions, the Fermi correlation acts in parallel with the Coulomb one; and (iii) the so‐called multibond reactions are more sensitive to electron correlation than are the electrocyclic ones.
On the complex relations between equations describing the dynamics of wave and particle aspects
Authors: Schuch D. 
Published: January 2005

AbstractEquations of motion on the levels of classical mechanics, hydrodynamics, and wave mechanics are analyzed and existing connections are shown. The manifestation of particle and wave aspects on the different levels are investigated. Dissipative frictional forces are included and a nonlinear wave mechanical equation for these systems can be found. It follows the discussion of an alternative to the time‐dependent Schrödinger equation (SE) in order to obtain the dynamical information on particle and wave aspects in a different form. This can be achieved by means of a Newtonian equation for a complex variable in connection with a conservation law for a nonclassical angular momentum‐type quantity. With the help of this complex variable it is also possible to develop a Hamiltonian formalism for the wave aspect contained in the SE, the fluctuation, or uncertainty of position and momentum. In this case the Hamiltonian function is equivalent to the difference between the mean value of the Hamiltonian operator and the classical Hamiltonian function. Possible extensions and applications are mentioned.
A fragment multipole approach to long‐range Coulomb interactions in Hartree–Fock calculations on large systems
Authors: Panas Itai , Almlöf Jan 
Published: January 2005

AbstractAn efficient ab initio method for electronic structure calculations on extended molecular systems is presented, along with some illustrative applications. A division of the system into subunits allows the interactions to be separated into short‐ and long‐range contributions, leading to a reduction of the computational effort from the original fourth‐power size‐dependence to one that is approximately quadratic. The short‐range contributions to the Fock matrix are obtained in an essentially conventional fashion, while the long‐range interactions are evaluated using a two‐center multipole expansion formalism. The number of short‐range contributions grows only linearly with the number of subunits, while the long‐range contributions grow as N2. Systematic studies of the computational efforts for systems of up to 99 water molecules organized as one‐stranded chains, three‐stranded chains, and three‐dimensional clusters, as well as alkane chains with up to 69 carbon atoms, have been performed. In these model systems, the overall computational effort grows as NK where 1 < K < 2.
Conformational structure of the tetrapeptide Boc–Aib–Leu–Leu–Aib–OMe–the central fragment of the nonapeptide antibiotic leucinostatin A
Authors: Nandel Fateh S. , Singh Balvinder , Saran Anil 
Published: January 2005

AbstractThe conformational structure of the tetrapeptide Boc–Aib–Leu–Leu–Aib–OMe has been investigated by the PCILO method. The computational results show the formation of two closed β‐turns, both of which are of type III, and the peptide backbone folds into a right‐handed 310‐helical conformation stabilized by two intramolecular 4 → 1 hydrogen bonds. The helix thus formed generates a pore of ∼3 Å along helix axis with hydrophobic amino acid side chains located on the outside of the helix, and this tendency of leucine side chains may enable leucinostatin A to fit into the membrane bilayer. The pore thus formed is cation‐selective, and through this pore, the cation can pass only in a single file.
Quasispin symmetry for the derivation of coupled cluster equations for the Hubbard model of benzene
Authors: Vinette F. 
Published: January 2005

AbstractThe concept of quasispin is applied to a special case of the Pariser–Parr–Pople (PPP) model of the benzene molecule, namely, the Hubbard Hamiltonian. Added to the spin, space, and alternancy symmetries already taken into account in the PPP Hamiltonian, this new symmetry, called quasispin symmetry, has the effect of reducing the size of the CI matrix. Coupled cluster (CC) equations are then obtained after applying the CC approach with doubles as well as its extension that accounts for triexcited clusters (CCSDT‐1). The derivation of these equations following the use of quasispin to the Hubbard model of benzene constitutes the most simple nontrivial example of CC results. In addition, the CC equations can be written in explicit algebraic form using the symbolic computation language MAPLE.
On the calculation of the hyperpolarizabilities of organic molecules by the sum over virtual excited states method
Authors: Morley J. O. , Pavlides P. , Pugh D. 
Published: January 2005

AbstractThe version of time‐dependent perturbation theory commonly used in the calculation of the hyperpolarizabilities of organic molecules through an expansion over electronic excited states is briefly rederived by a direct method, simplified for application to the nonresonant case. The application of the method within the framework of semiempirical quantum theory and singly and doubly excited configuration interaction is described. A particular parametrization of the CNDO/S method (CNDO/VS) that has been successful in estimating molecular β values for polar aromatic molecules is reviewed. Applications of the method to the interpretation of the molecular and crystal second‐order properties of 2‐(α‐methylbenzylamino)‐5‐nitropyridine (MBANP) and to the calculation of the γ tensor for various types of conjugated compounds are given.
Relations among functional groups within a stoichiometry: A nuclear configuration space approach
Authors: Dubois Jacques‐Emile , Mezey Paul G. 
Published: January 2005

AbstractA general scheme is proposed for the study of the interrelations and transformations of flexible functional groups within the configuration space M of all possible chemical species of a fixed overall stoichiometry S. The methods presented provide a concise but sufficiently detailed description of the occurrence of various functional groups in different families of nuclear arrangements, as well as their transformations into one another during chemical reactions and conformational changes. The approach uses experimental information on the importance of various functional groups, topological criteria for the flexibility of these functional groups, and the fundamental pattern of distribution of configurational families within the nuclear configuration space. The resulting schemes have simple, easily programmable representations. © 1992 John Wiley & Sons, Inc.
Ab initio study on the mechanism of cycloaddition reaction of ketene with methylenimine: A new reaction scheme
Authors: Fang Decai , Fu Xiaoyuan 
Published: January 2005

AbstractThe cycloaddition reaction of ketene and methylenimine, leading to 2‐azetidinone, has been studied theoretically by RHF/3‐21G and IRC. This reaction is believed to be nonsynchronous and concerted, taking place through a twisted transition state. Four π orbitals are involved in this reaction, which is a “2 × [1 + 1]”‐type cycloaddition. In the course of the reaction, rotation of the methylene group instead of oxygen in ketene was ascertained. The activated barrier is calculated to be 33.9 kcal/mol. © 1992 John Wiley & Sons, Inc.
The rotation–vibration coupling equations for polyatomic molecules in internal coordinates
Authors: Chen Guang‐Ju , Tang Au‐Chin , Fu Xiao‐Yuan 
Published: January 2005

AbstractAn exact vibration–rotation kinetic energy operator for polyatomic molecules has been obtained on the basis of Sutcliffe's method, in terms of curvilinear internal coordinates and rotational angular moment operators. This operator is derived from the kinetic energy operator in Cartesian coordinates by the successive transformations using the chain rule. This kinetic energy operator can be used not only for the system of any triatomic and tetraatomic molecules and common polyatomic molecules in chemistry, but also for the investigation of the collision problems between two molecules after some modifications. Finally, using this Hamiltonian, the rotation–vibration coupling equations of polyatomic molecules have been derived and discussed. © 1992 John Wiley & Sons, Inc.
The variation of spectroscopic properties with numbers of electrons
Authors: Laurenzi Bernard J. , Gaylord Donald R. 
Published: January 2005

AbstractA formalism that describes the variation of the spectroscopic properties, De, Re, and ke, of homonuclear, diatomic molecules, with the number of molecular electrons has been developed. The theory describes the interrelation of these properties and predicts “critical” behavior in sequences of “isonuclear” and neutral molecules. Detailed calculations are possible with the help of experimental data in lieu of a deeper, dynamical theory of molecular behavior with respect to electron number. The present work points the way toward a first‐principle's theory. © 1992 John Wiley & Sons, Inc.
Molecular conformation of bilirubin from semiempirical molecular orbital calculations
Authors: Shelver Weilin L. , Rosenberg Harry , Shelver William H. 
Published: January 2005

AbstractSemiempirical methods were utilized in the computation of a fully optimized structure of bilirubin. Bond lengths and bond angles obtained using either AM1 or PM3 calculations showed excellent agreement with those obtained by X‐ray diffraction. This indicated that molecular orbital methods satisfactory reproduced the complex conjugation found in bilirubin. Dihedral angles of the crucial “hinge” and the dihedral angles of the propionic acid side chains agreed well with those found by X‐ray diffraction. Calculated hydrogen‐ bond parameters (distance and angles) showed substantial differences from experimental values, probably due to inherent weakness in the parameterization of the molecular orbital techniques. Conformational studies were carried out using AM1 by rotating the C9C10 bond in 5° increments showed that the most stable structure exhibited a minimum at about 125° and exhibited a structure similar to those postulated from X‐ray and NMR experiments. The hydrogen bonds showed remarkable tenacity during rotation of the C9C10 bond and resisted breaking until the molecule was under extreme strain. © 1992 John Wiley & Sons, Inc.
Quantum mechanical parametrization of a conformationally dependent hydrophobic index
Authors: Alkorta Ibon , Villar Hugo O. 
Published: January 2005

AbstractAn extension of our previous work on the development of a conformationally dependent hydrophobic index is presented. The parameter was computed in four alternative ways based on steric parameters, such as the total molecular surface area or the solvent accessible surface, and on quantum mechanically computed electronic properties, such as the molecular dipole moment, Mulliken charge densities, and a lone–pair index. The method was parametrized based on 110 rigid analogs and all properties required were calculated using the semiempirical methods PM3, AM1, and MNDO. Examples of the application of this method in calculating hydrophobic indices are presented.
Structural and electronic requirements for binding at the Mu–opioid receptor
Authors: Cometta–Morini Chiara , Loew Gilda H. 
Published: January 2005

AbstractA pharmacophore for μ‐opioid receptor recognition based on a study of the fentanyl class of opioids has recently been characterized in our laboratories. To validate this pharmacophore, we have extended our theoretical studies to include four opiate analogs from structurally different classes and with high affinity but varying selectivity for the μ‐opiate receptor. An extensive conformational search of the flexible regions of these compounds has been carried out at two levels of approximations, using the CHARMm force field and the semiempirical molecular orbital method AM1. In a subsequent step, we have determined a series of structural, environmental, and electronic properties for each low‐energy conformer of the analogs studied. All four analogs studied can assume a low‐energy conformation in which at least three of the four stereoelectronic properties identified as modulators of recognition in the fentanyls are present in the same spatial arrangement. These results provide additional evidence for the plausibility of the proposed pharmacophore for μ–opioid receptor recognition. © 1992 John Wiley & Sons, Inc.
Semiempirical calculations of TiO2 (rutile) clusters
Authors: Hagfeldt Anders , Siegbahn Hans , Lindquist Sten‐Eric , Lunell Sten 
Published: January 2005

AbstractThe densities of states for small (TiO2)x‐clusters, x = 1, 3, 6, 9, and 14, have been calculated by means of the INDO method. The shape of the valence bands' density of states (DOS) are discussed in terms of the distribution of coordination numbers. A one‐slab cluster with uniform distribution of the coordination numbers was used to compare our calculations with experimental spectra. The photoelectric DOS and DOS for a cluster with an oxygen vacancy are in very good agreement with experimental findings for the TiO2 (001) surface. O1s core level shifts between a surfacelike and a bulklike oxygen atom have been estimated. It is concluded that the observed surface–bulk shift for the TiO2 (001) surface contains a substantial relaxation contribution. © 1992 John Wiley & Sons, Inc.
Semiempirical calculations of hyperpolarizabilities for extended π systems: Polyenes, polyynes, and polyphenyls
Authors: Matsuzawa Nobuyuki , Dixon David A. 
Published: January 2005

AbstractThe dipole moments (μ), polarizabilities (α), hyperpolarizabilities (β), and second‐order hyperpolarizabilities (γ) of polyenes, polyynes, and polyphenyls have been calculated by a finitefield method with the PM‐3 parameterization of the semiempirical MNDO Hamiltonian at the optimum geometries. These results were compared to experimental values obtained from EFISH and THG measurements. The calculations reproduce the magnitudes of β and γ, as well as the effect of the substituents and the effect of bond alternation on β and γ. The coefficients of the power law, which describes the dependence of β and γ on the number of π centers, were calculated. For β, exponents of 1.5–2.2 and 0.03–0.04 were obtained for polyenes and polyynes, respectively, and for γ, exponents of 3.9–4.9, 2.9–3.3, and 2.5–2.7 were obtained for polyenes, polyynes, and polyphenyls, respectively. These results confirm the efficiency of enhancing γ by insertion of C = C double bonds into a chain. © 1992 John Wiley & Sons, Inc.
The electronic structure of Keggin anion (PW12O40)3− and catalytic properties
Authors: Chen T. L. , Ji J. , Xiao S. X. , Cai T. X. , Yan G. S. 
Published: January 2005

AbstractThe electronic structure of the Keggin anion (PW12O40)3− has been calculated by the DV–Xα method. The nature of the chemical bonds of the Keggin anion and the relationship between electronic structure and catalytic behavior are discussed. At the same time, the difference in electronic structure and the relationship in catalytic behavior between (PW12O40)3− and (PMo12O40)3− are compared. © 1992 John Wiley & Sons, Inc.