bluuebunny/crossref_metadata_2025_split huggingface dataset:
New derivation of the Waller–Hartree–Fock spatial wave function
Authors: Pauncz Ruben 
Published: December 2004

AbstractA new derivation is given for the Waller–Hartree–Fock double‐determinantal spatial wave function. One starts from the single‐determinant wave function in which a orbitals are doubly occupied, and decomposes it into a sum of products of spatial and spin functions. The spatial product of the first genealogical spin eigenfunction is a double‐determinantal function. The derivation is based on the simple form of U1ƒ(P) when the representation matrix is obtained from the genealogical spin eigenfunction.
Chemical shifts calculation in a CNDO framework. I. Formulation and analysis of the model
Authors: Riera Josep M. , Mariné Josep 
Published: December 2004

AbstractExpressions for the second‐row nuclei chemical shifts calculation using a CNDO‐SCF wave function are outlined. In collating it with the Pugmire and Grant's equations, a term with a changed sign appears. Both sets of equations are applied to a study of carbon‐13 chemical shifts in alkanes down to C7; results are interpreted with aid of the Lindeman–Adams' empirical equation. Expressions proposed are confirmed for an excellent agreement with experimental data.
Study of potential curves by UHF‐type methods. VI. Spin‐extended Hartree–Fock method
Authors: Klimo Villiam , Tiňo Josef 
Published: December 2004

AbstractAn improved computational procedure of the spin‐extended Hartree–Fock method is presented. The main features of the EHF potential curves for the dissociation of the LiH, BeH, and Li2 molecules are discussed. The results are compared with curves calculated by the perturbation or CI methods using the same basis set.
Singularities of magnetic‐field induced electron current density: A study of the ethylene molecule
Authors: Lazzeretti P. , Rossi E. , Zanasi R. 
Published: December 2004

AbstractThe singularities of the quantum‐mechanical electron current density induced in a molecule by an external magnetic field are analyzed via the theory of the real autonomous systems of differential equations. Group‐theoretical techniques, based on the idea of color symmetry, are developed, which predict the occurrence of singularities about molecular symmetry elements. Plots of induced electron current density have been obtained for the ethylene molecule through accurate coupled SCF calculations. The theoretical magnetic properties of ethylene obtained through this investigation are the most accurate reported so far.
Fourier transforms of atomic orbitals. I. Reduction to four‐dimensional harmonics and quadratic transformations
Authors: Niukkanen A. W. 
Published: December 2004

AbstractA general expression for the Fourier transform of the basis functions of exponential class has been derived. Particular cases of Slater functions, hydrogen‐like functions, Shull and Löwdin functions, Shavitt, Filter, and Steinborn functions have been considered. In many particular cases the Fourier transforms have been shown to reveal some important special properties (reduction to four‐dimensional harmonics, quadratic transformations, etc.) which considerably simplify the mathematical treatment of these functions and lead to new possibilities in the development of calculation methods for multicenter integrals.
The nodal structure of the momentum distributions of molecules
Authors: Simas Alfredo M. , Smith Vedene H. , Kaijser Per 
Published: December 2004

AbstractThe nodal structure of molecular momentum distributions is studied by considering the simplest case of the ground state of the hydrogen molecular ion. By examining the exact expansion of the H2+ momentum distribution, it is shown that an infinite sequence of nodes does exist along the pz axis (z axis parallel to the bond axis) but not nodal planes perpendicular to the pz axis (as is found for the simplest LCAO function). The nodes are those points where nonplanar nodal surfaces cross the pz axis. It is also shown that molecular systems with more than one electron cannot, in the ground state, have nodal surfaces in their momentum distributions. Implications for the directional Compton profiles J(q) are discussed.
Anisotropic electronic intracule densities for diatomics
Authors: Thakkar Ajit J. , Tripathi A. N. , Smith Vedene H. 
Published: December 2004

AbstractThe electronic intracule density, a three‐dimensional contraction of the spinless electron pair density, is the probability density function for an interelectronic vector. A computationally efficient algorithm for the evaluation of the basic two‐electron intracule integral for GTOs is presented. In order to provide an initial understanding of the topography of intracule distributions, anisotropic intracule densities for the X1Σ ground states of the H2 and N2 molecules are reported and analyzed.
Ab initio Hartree–Fock approach to the study of polymers: Application to polyacetylenes
Authors: Dovesi Roberto 
Published: December 2004

AbstractAn ab initio Hartree–Fock method devoted to the study of polymers is presented, truncation criteria for the exchange and the Coulomb series are discussed, and it is shown that the error in the total energy evaluation can be reduced to below 0.0001 a.u./cell with little computational effort. Results for six polyacetylene isomers are presented and compared with previous ab initio data obtained using the same basis set. In agreement with the experimental findings, the alternating all‐trans structure is the most stable.
New efficient numerical method for solving pair correlation equations for diatomic molecules
Authors: Adamowicz Ludwik , Bartlett Rodney J. 
Published: December 2004

AbstractA new method for solving correlation pair equations for diatomics is presented and followed by numerical results for the H2 and LiH molecules. The essential feature of the new approach is a numerical representation of both basis two‐electron functions and Coulomb and exchange potentials in pair equations. It is shown that this significantly simplifies the pair function optimization procedure.
Ab initio studies of negative ion‐molecule(s) clusters present in the atmosphere. III. OH−(CO2)n for n = 1, 2
Authors: Sapse Anne‐Marie , Osorio Louis , Snyder Grace 
Published: December 2004

AbstractClusters formed by the OH− ion and carbon dioxide are investigated using ab initio Hartree–Fock calculations, with 6–31 G and 6–31 G* basis sets. Geometries and binding energies are determined.
Configuration interaction matrix elements for atoms using permutation group algebra
Authors: Dinesha K. V. , Hinze Juergen 
Published: December 2004

AbstractA procedure is described for the efficient evaluation of the energy matrix elements necessary for atomic configuration‐interaction calculations. With the orbital configurations of an N electron system in spin state S written as the irreducible representations [21/2N−S, 12S] of the permutation group S( N ), it is possible to evaluate readily the energy matrix elements of a spin‐free Hamiltonian expressed in terms of the generators of the unitary group. We show how the use of angular momentum ladder operators permits the effective generation of a basis of eigenstates of ℒ︁2, ℒ︁z as well as 𝒮2 and 𝒮z, for which the energy matrix elements may be evaluated with ease.
Molecular integrals arising in the linear response theory of van der waals forces
Authors: Ishida Kazuhiro 
Published: December 2004

AbstractGeneral formulas are derived for molecular integrals which arise in the linear‐response theory when it is applied to the van der Waals interactions between molecules. These integrals are essentially the matrix elements of the Fourier transformed double Coulomb operator between any Cartesian Gaussians. The computational aspects of these formulas are also discussed.
On a semiclassical interpretation of inter‐ and intramolecular interactions
Authors: Bonaccorsi Rosanna , Ghio Caterina , Tomasi Jacopo 
Published: December 2004

AbstractThe semiclassical models considered here are composed by charge distributions coming from ab initio quantum‐mechanical calculations on actual molecular systems. These charge distributions interact with one another according to the laws of classical electrostatics. This article describes some results of a systematic examination of the performances of this model in a variety of cases, with the aim of putting in evidence the usefulness and the limits of this inherently approximate representation of chemical interactions. Intermolecular interactions are examined first; the test cases are interactions of neutral molecules with H+, Li+, and C1−, and the formation of H‐bonded complexes. Attention is paid mainly to the energetics of the processes; each interacting molecule is considered as a unique entity and classical molecular reactivity indexes (electrostatic potential V, polarization term P) are introduced to compute the interaction energy, to interpret the details of the interaction process, and then to elaborate on less expensive computational procedures. Intramolecular interactions are considered. Attention is paid to the question of defining chemical groups starting from SCF molecular wavefunctions. The transferability and conservation degree of groups derived from localized orbitals of actual molecules is examined in detail, taking as tests their ability to reproduce charge distribution, one‐electron observables, and energy. The effect of classical fields on these groups is then examined, taking into consideration external fields originated either by a point charge or by a solvent, and internal fields deriving from substitution of chemical groups. The intergroup analysis is then extended to the case of bimolecular reaction acts by considering the whole system as a supermolecule. Approximate computational procedures able to reproduce the main features of these interactions are proposed and tested. All through the article the performances of the classical models are compared with ab initio SCF calculations (mainly of low or intermediate quality).
Remarks on the concept of an atom in a molecule and on charge transfer between atoms on molecule formation
Authors: Parr Robert G. 
Published: December 2004

AbstractDensity functional theory provides a natural and rigorous definition of an atom in a molecule in its ground state: The molecular electron density is the sum of atomic densities, the atoms have the same chemical potential as does the molecule, and the atoms are minimally promoted from their ground states. These atoms in general are not spherical, and in general they bear nonintegral charges. Charge transfer on molecule formation is thereby uniquely defined. Calculations by Palke and by Guse are reviewed, in which the hydrogen atom is identified in the hydrogen molecule.
The concept of the chemical bond
Authors: Julg A. 
Published: December 2004

AbstractThe concept of the chemical bond is very old. Many interpretations have been proposed. Nevertheless, the problem is far from being solved. At the present time, the chemist has the choice between two models: the Lewis model (electron pairs) and the quantum model. In spite of a current opinion, both models are not logically equivalent. The localization of molecular orbitals is only a mathematical operation which does not involve any physic localization of the electrons. The loge theory is not more satisfying owing to the fact that loges with minimal fluctuation do not exist in all molecules. The theory of orbital domains seems to bring a solution. Nevertheless, its interpretation can be obtained only outside the strict framework of quantum mechanics.
Structure and function of protamines: An investigation of clupeine properties by nuclear magnetic resonance
Authors: Paolillo L. , Andini S. , Ferrara L. , Wurzburger S. 
Published: December 2004

AbstractIn this paper a detailed study of the structural properties of the three main fractions YI, YII, and Z of clupeine (protamines) has been performed with the aid of nuclear magnetic resonance methods. The investigation includes chemical shift data of protons and C‐13 nuclei and measurement of the T1 relaxation times of the C‐13 nuclei. These data demonstrate that the flexibility of the three fractions is different and that in the presence of inorganic phosphate fraction YI exhibits structural changes more relevant than the other two fractions. The measurement of the relaxation times, under the assumption of a rigid ellipsoid model, gives an estimate of the rough molecular size of the clupeine fractions. Different side chain mobilities that complement the previous observations based on chemical shift data are determined. A further extension of this work concerns the interaction of the clupeines with natural mononucleotides. An appreciable interaction takes place between fraction YI and purinic nucleotides while a simple electrostatic interaction seems to exist with the other two fractions. A possible explanation of this different behavior seems to involve the formation of large aggregates solely in the case of fraction YI.
Soft agonist receptor interactions: Theoretical and experimental simulation of the active site of the receptor of sweet molecules
Authors: Temussi P. A. , Lelj F. , Tancredi T. , Morelli M. A. Castiglione , Pastore A. 
Published: December 2004

AbstractThe structure‐activity relationship of sweet molecules is chosen as an example to illustrate a mechanistic approach of soft agonist‐receptor interactions. It is shown that an essentially geometric model of the receptor site can explain the activity of most sweet molecules, both rigid and flexible. The relevant conformations of flexible molecules in solution are extracted from a combination of NMR data and of energy calculations. A possible experimental simulation of the receptor environment in solution is illustrated by the complex of a dipeptide sweetener with a crown‐ether.
Hartree‐Fock cluster study of interstitial transition metals in silicon
Authors: Broer R. , Aissing G. , Nieuwpoort W. C. , Feiner L. F. 
Published: December 2004

AbstractResults are presented of a Hartree‐Fock cluster study of interstitial Ti, V, Cr, and Mn impurities in silicon. A Si10 cluster models the nearest Si atoms around a tetrahedral interstitial site in crystalline Si. The dangling bonds of the Si atoms are saturated by hydrogens. The effect of the Si core electrons is represented by an effective potential. Characteristic for the electronic structure of the low‐lying states of the neutral, singly positive, and doubly positive ions in silicon is the presence of fairly delocalized but still predominantly transition‐metal (3d)‐like orbitals of t2 and e symmetry. For all ions the energy of the weighted average of the terms belonging to a configuration is lowest for the configuration with maximum occupation of the t2 orbitals. Ground states with maximum spin multiplicity are found for all ions, except Ti0.
Analytical fitting of potential energy surfaces for the Hxy systems
Authors: Last I. , Baer M. 
Published: December 2004

AbstractUse of the London‐Eyring‐Polanyi + 3‐Center + power‐series (LEP‐3C‐PS) analytical potential as a fit to potential energy surfaces (PES) which are known numerically only are suggested. This analytical fit was performed for the diatomics‐in‐molecules + 3 Center (DIM‐3C) PES of HCl2 and HI2 systems. The HCl2 analytical LEP‐3C‐PS potential was used for classical trajectory calculations of the Cl' + HCl → HCl' + Cl reaction. The rate constant obtained from these calculations for T = 358° K is 1.95 X 109 cm3/mol sec which is close to the experimental value of 2.5 109 cm3/mol sec.
Studies of unoccupied orbitals of BF3 and BCL3 by electron transmission spectroscopy and multiple scattering Xα calculations
Authors: Tossell J. A. , Moore J. H. , Olthoff J. K. 
Published: December 2004

AbstractElectron transmission spectroscopy (ETS) and bound‐state and continuum multiple‐scattering Xα (MS‐Xα) calculations are employed to characterize the unoccupied a′2, a′1 and e′ orbitals of BF3 and BCl3. The a′2 orbital of BF3, which produces a peak about 7 eV below threshold in the x‐ray absorption spectrum (XAS), generates a scattering resonance at 3.5 eV in ETS. Similarly, the e′ orbital that lies about 2 eV above threshold in XAS occurs about 13‒16 eV above threshold in ETS. Dissociation of F− from BF3 due to electron attachment is attributed to a core‐excited shape resonance involving an e″ → a″2 excitation and electron capture into the a′2 orbital. In BCl3 all the unoccupied orbitals lie at lower energy than in BF3 and are closely spaced, making definitive spectral assignments difficult. Both Hartree‐Fock (HF) and MS‐Xα methods apparently underestimate the stability of the unoccupied e′ orbital of BCl3. Vibronic coupling due to out‐of‐plane bending may significantly affect the spectral intensities. Feshbach resonances are observed for BCl3 at energies close to those observed in the vacum‐UV absorption spectrum. The lower energies of the unoccupied orbitals of BCl3 are consistent with their stronger bonding to nucleophiles.
The effect of correlation energy (MP2) on computed vibrational frequencies
Authors: Michalska D. , Andes Jr. B. Hess, , Schaad L. J. 
Published: December 2004

AbstractThe IR frequencies of cyclopropene and methylenecyclopropene were computed with inclusion of correlation (MP2/6‐31G*). These along with those of ethylene obtained earlier by Pople were compared with SCF results (DZ + P). It was found that the MP2 frequencies agreed better with experiment, but that the order of the bands remained the same as those computed without correlation.
Some new aspects of bonding in cyclopropane
Authors: Pan Dao‐Kai , Gao Jian‐Nan , Lin Hai‐Lun , Huang Ming‐Bao , Eugen Schwarz W. H. 
Published: December 2004

AbstractThe peculiar properties of the three‐membered ring in cyclopropane are partially due to characteristic relaxation effects of the carbon orbitals in the molecule. AO contraction in the two strongly CC bonding MO's of A type is essential for the stability of the C3 ring, whereas AO expansion in the E'‐type HOMO contributes to the π character of the peripheral CC bonds.
Atomic partitioning of two‐center potentials for slater basis
Authors: Rico Jaime Fernáandez , Lóapez Rafael , Paniagua Miguel , Fernáandez‐Alonso Jose Ignacio 
Published: December 2004

AbstractIn a previous paper we have shown that the long‐range potential generated by the two‐center distributions can be written as a sum of two multipolar expansions centered at the terminal atoms and defined so that the two series of multipoles be minimal. In this paper we show that both charge distributions and short‐range potentials can also be separated in atomic contributions in a way consistent with the above mentioned partition of the long‐range potentials.
Calculation of the minimum energy conformation of biomolecules by using a global optimization technique. V. Preferred conformations of the thyrotropin‐releasing hormone
Authors: Rao Gita Subba , Tyagi R. S. , Mishra R. K. 
Published: December 2004

AbstractThe preferred conformations of the thyrotropin‐releasing hormone (TRH) have been calculated by the global optimization method proposed earlier by us. [G. Subba Rao, R. S. Tyagi, and R. K. Mishra, J. Theor. Biol. 90, 377 (1981)]. The potential function used comprises the electrostatic, nonbonded, torsional and hydrogen‐bonding terms. The results are in good agreement with the crystal structures of TRH. No intramolecular hydrogen bonding is found to occur.
Some theoretical studies on hydrogen molecule capture by platinum atoms
Authors: Poulain E. , Garcia‐Prieto J. , Ruiz M. E. , Novaro O. 
Published: December 2004

AbstractPseudopotential SCF‐LCAO‐MO and variational and perturbative Cl calculations were carried out for H2 molecule capture by a single Pt atom with C2v symmetry. A pseudopotential for the platinum atom including relativistic effects was used. Singlet and triplet states of the Pt‐H2 interaction having different representations of the mentioned C2v symmetry were studied. The triplet ground state of Pt leads to two A1 and B2 states in which the metal atom cannot capture H2; i.e., both have repulsive interaction energies. The electronic state responsible for the capture of H2 is the closed‐shell, singlet A1 excited state. The equilibrium geometry of the system is reached with a broken HH bond at a HPtH angle of about 100°. Additionally another shallower minimum for a singlet A1 linear structure is observed. Specific predictions for the thermal and photochemical Pt + H2 reactions that can be carried out under matrix isolation conditions are made.
An ab initio study of intermolecular hydrogen bonding between small peptide fragments
Authors: Sapse A. M. , Fugler L. M. , Cowburn D. 
Published: December 2004

AbstractSeveral small peptide fragments are investigated with ab initio (Hartree‐Fock) calculations, using Gaussian basis sets. Complexation energies, net atomic charges, and optimum geometries are obtained. The geometries predicted by the STO‐6G, and 6–31G* basis sets are quite similar, whereas the binding energies obtaiend by the 6–31G calculations are higher than those obtained with STO‐6G and 6–31G* basis sets.
An ab initio molecular orbital study on the stacking interaction between nucleic acid bases: Dependence on the sequence and relation to the conformation
Authors: Aida Misako , Nagata Chikayoshi 
Published: December 2004

AbstractThe stacking interaction energies between nucleic acid bases in A DNA and B DNA are calculated by means of the ab initio molecular orbital method. The calculated values agree well with the experimental values of stacking enthalpy changes. The stacking interaction energy is shown to be highly sequence dependent, particularly when the sequence includes guanine or cytosine. The possibility is shown that the conformation of a DNA double helix fragment is determined by the constituent stacking interaction. Electrostatic energy is the cause of the sequence dependency of the stacking energy, while charge transfer and dispersion energies contribute to the overall stability.
Scattering of high‐energy electrons and x‐rays from molecules: The 10‐electon series Ne, HF, H2O, NH3, and Ch4
Authors: Kumar Mukesh , Tripathi A. N. , Smith Vedene H. 
Published: December 2004

AbstractThe elastic, inelastic, and total intensities for x‐ray and high‐energy electron scattering from the 10‐electron systems Ne, HF, H2O, NH3, and CH4 have been calculated by using SCF‐MO wave functions obtained with double‐zeta quality bases of Gaussian contractd wave functions. The effect of molecular binding and various other trends and systematics in the intensities have been examined with the help of difference functions computed between the present scattering intensities and those for the indpendent atom model (IAM).
Theoretical study of several Fe(H2o)n2+ clusters at different temperatures
Authors: Gonzáalez‐Lafont A. , Lluch J. M. , Oliva A. , Bertrágn J. 
Published: December 2004

AbstractSeveral Fe(H2O)n2+ clusters, with n up to 20, have been studied, both by energy minimization in the pairwise approximation and by Monte Carlo simulation. In the last case the calculations have been carried out at three different temperatures in order to investigate the effect of thermal agitation. The most interesting result which can be deduced from this work is the existence of eight water molecules in the first hydration shell of the iron (II) ion. A microscopic analysis has shown that the minimum energy structure of the Fe(H2O)82+ cluster presents a D4d symmetry. This structure is slightly distorted as far as the temperature is increased. The validity of these theoretical predictions is discussed.