bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the combination of two methods for the calculation of multicenter integrals using exponential–type orbitals
Authors: Perevozchikov V. I. , Maslov I. V. , Niukkanen A. W. , Homeier H. H. H. , Steinborn E. O. 
Published: January 2005

AbstractThe possible combination of two methods for the calculation of multicenter two‐electron integrals using STO and B function basis sets is discussed. The first method (Method I), which is of approximate nature, is based on a simplified version of the so‐called Σ‐factorization method [A. W. Niukkanen and L. A. Gribov, Theor. Chim. Acta 62, 443 (1983)], where the radial part of the two‐center one‐electron density ρab(r) is represented as a sum of two radial functions fa(r) and fb(r), placed on two different centers a and b. After such a transformation, the calculation of the two‐electron integrals boils down to the calculation of some type of Coulomb integrals. The second method (Method II), which calculates each integral separately to a given accuracy, is based on Mobius‐type quadrature used for a three‐dimensional integral representation for the two‐electron integral of B functions [E. O. Steinborn and H. H. H. Homeier, Int. J. Quantum Chem. Symp. 24, 349 (1990)]. In Method I, the choice of the radial functions placed on the different centers has an essential influence on the final value of the multicenter integrals. In the present study of Method I, a rather simple approximation of the radial part was made that reproduced the qualitative behavior of the molecular integral curves as function of the geometry rather well. At the present state of development, Method I produces fast order‐of‐magnitude estimates that are useful for screening purposes, i.e., to decide which integrals have to be evaluated more accurately by other methods like Method II. Method II reproduces data given in the literature [R. M. Pitzer and D. P. Merrifield, J. Chem. Phys. 52, 4782 (1970)] correctly. © 1992 John Wiley & Sons, Inc.
Biuret and its sulfur analogs: Structures and energies
Authors: LeszczyńSki Jerzy , Sullivan Richard H. 
Published: January 2005

AbstractThe calculations for biuret (B) and its two sulfur analogs, thiobiuret (TB) and dithiobiuret (DTB), were carried out using the ab initio LCAO–MO method at the Hartree–Fock level. The molecular structure of B was fully optimized using standard 3‐21G and 6‐31G* basis sets, while geometry optimizations of the TB's and DTB's were performed at the 3‐21G* and 6‐31G* levels. Only trans‐isomers were found to be minimum structures on the HF/3‐21G* potential‐energy surfaces, whereas cis‐forms are the first‐order transition structures. The relative energies of the cis‐conformers at MP 2/6‐31G**//HF/6‐31G* + 0.9 zero point energy (ZPE) level are equal to 47.4, 51.7, and 59.7 kJ mol−1 for TB, B, and DTB, respectively. Because of relatively large dipole moments, cis‐conformers are predicted to be stabilized in polar solvents.
Comparison of MNDO to tight‐binding, total‐energy methods for surface atomic structure determination: Aluminum phosphide (110)
Authors: Godin T. J. , LaFemina John P. 
Published: January 2005

AbstractTight‐binding, total‐energy (TBTE) models have been successfully used to calculate the equilibrium surface atomic structures of a variety of materials, but are difficult to apply to substances with complicated interatomic repulsions. In these cases, the modified neglect of diatomic overlap (MNDO) method, which specifically includes longer‐range interactions, may provide an effective alternative. We present new calculations of the surface atomic structure of the AlP (110) surface using both techniques. This surface, whose structure has been determined quantitatively using a low‐energy, electron‐diffraction intensity analysis, displays a well‐known relaxation characteristic of (110) surfaces of zincblende‐structure semiconductors. The TBTE model, parameterized to bulk AlP properties, provides a more accurate prediction of the relaxed surface atomic positions, although MNDO, parameterized to small molecules, produces acceptable results. Despite the greater demands placed by MNDO on computer resources, it may prove useful in the study of materials that are difficult to model within a TBTE framework. © 1992 John Wiley & Sons, Inc.
Chirality and spin density: Ab initio and density functional approaches
Authors: Fluekiger P. , Weber J. , Chiarelli R. , Rassat A. , Ellinger Y. 
Published: January 2005

AbstractThe spin distribution in a stable nitroxide biradical that shows ferromagnetic interactions in the solid phase has been studied at three levels of theory: First, at the UHF level; then, including correlation effects in UMP2 calculations; and finally, the results are compared with the spin density obtained using the local density functional (LDF) approximation. It is shown that LDF spin densities are closer to UMP2 than to UHF predictions; the difference between the UHF and the (UMP2, LDF) results points to a redistribution of the spin repartition between N and O due to electronic correlation. For planar conformations of the NO group, there is symmetric distribution (D2d) of the spin density on the adamantane skeleton. For nonplanar nitroxides, the molecule is chiral (C2), which results in a breakdown of the spin transmission on part of the adamantane cage. © 1993 John Wiley & Sons, Inc.
Semi‐empirical molecular orbital studies of porphine and phthalocyanine derivatives, to simulate their intermolecular interactions
Authors: Gantchev Tsvetan G. , Beaudry Francis , Van Lier Johan E. , Michel André G. 
Published: January 2005

AbstractGound‐state electronic properties of porphine, phthalocyanine, and their derivatives, photosensitizers with potential use in the photodynamic therapy of tumors, were studied by semiempirical (MNDO) molecular orbital calculations. Geometry, bond orders, electron populations, and net atomic charges were analyzed. The obtained models were applied to study the intermolecular interactions of these molecules utilizing a classical mechanics approach. The dimerization characteristics of different derivatives were obtained by a separate evaluation of the contribution of electrostatic and steric factors. © 1993 John Wiley & Sons, Inc.
Effect of positive background on the ground‐state energy of a Wigner lattice
Authors: Sebasdiyar I. , Iyakutti K. , Mahendran M. 
Published: January 2005

AbstractAn attempt has been made to study the effect of a nonuniform positive background on the ground‐state energy of a nonmagnetic Wigner lattice. The background is modified with a variable “ripple” parameter in the background charge, represented by a Yukawa‐type charge distribution. Our calculation shows that a Yukawa‐type background leads to a lower ground‐state energy value compared to a Gaussian‐type and constant positive background. © 1993 John Wiley & Sons, Inc.
Nonlocal exchange and kinetic energy density functionals with correct asymptotic behavior for electronic systems
Authors: Glossman M. D. , Balbás L. C. , Rubio A. , Alonso J. A. 
Published: January 2005

AbstractThe search for a density functional theory (DFT) free of one‐electron orbitals needs the development of good kinetic energy functionals beyond the venerable Thomas–Fermi and gradient expansion functionals. With this goal in mind, we previously solved for several atoms and positive ions the Euler equation associated with the exchange and kinetic energy functionals of the nonlocal weighted density approximation (WDA). In this work, we improve the theory by enforcing the correct asymptotic behavior of the exchange potential following a prescription of Przybilsky and Borstel (PB). We have calculated electronic densities, density moments 〈rn〉 (n= −1,1,2) and the density at the nucleus ρ(0), as well as total and exchange energies and chemical potentials for neutral and charged atoms. All those quantities show a better agreement with Hartree–Fock results than those obtained using gradient‐corrected Thomas–Fermi functionals. The radial densities, 4πr2(r), show two local maxima at positions close to those of the first two maxima of Hartree–Fock radial densities. This effect, which is absent in the self‐consistent densities from the usual gradient‐corrected Thomas–Fermi functions, can be interpreted, in our view as an incipient emergence of the shell effect. © 1994 John Wiley & Sons, Inc.
Classification of substituents by σ and π electron energy separation
Authors: Jug Karl , Matuschewski Marta 
Published: January 2005

AbstractThe previously introduced σ and π energy separation is extended to subsystems. σ and π electron donor and acceptor properties of the substituents are tested with a Löwdin population analysis. The influence of the substituents on the σ and π electron energies is calculated for substituted benzene, cyclobutadiene, butadiene, and borazine. A linear relationship is found which shows that an increase of σ electron energy is accompanied by a decrease in π electron energy and vice versa. This allows a classification of substituent groups in terms of reactivity. © 1994 John Wiley & Sons, Inc.
Density functionals for exchange and correlation energies: Exact conditions and comparison of approximations
Authors: Levy Mel , Perdew John P. 
Published: January 2005

AbstractA set of exact conditions is compiled for the purpose of developing and testing approximations for the exchange‐correlation energy as a functional of the electron density. Special emphasis is placed upon recently developed density‐scaling relationships. Commonly used generalized gradient approximations are compared against several of these conditions. A direct tabular comparison of these functionals (not of calculated properties) with one another is also made. © 1994 John Wiley & Sons, Inc.
Localized Hartree product orbitals in correlated studies of molecules
Authors: Perera S. Ajith , Bernholdt David E. , Bartlett Rodney J. 
Published: January 2005

AbstractUse of orthogonalized Hartree product (OHP) orbitals as the reference orbitals in coupled‐cluster (cc) calculations is presented. Since such orbitals are determined without exchange, they provide as “classical” a description as possible. The OHP orbitals were generated by implementing Harris's formulation of the original Hartree method. Some computational considerations of the formulation are discussed. A critical evaluation of the OHP method as an orbital localization scheme is presented. The OHP orbitals were used as the reference in CCD and CCSD calculations and compared with corresponding Hartree‐Fock (HF) reference CC results. The average variation of localized Hartree product (LHP) reference CCSD energy from that of the HF reference is 0.83 kcal/mol, whereas for CCD, the average variation is 234 kcal/mol, indicating the importance of single‐excitation effects in CC calculations with non‐HF references. © 1994 John Wiley & Sons, Inc.
Does antiaromaticity imply net destabilization?
Authors: Murray Jane S. , Seminario Jorge M. , Politzer Peter 
Published: January 2005

AbstractAn analysis is presented of the results of earlier ab initio computational studies of cyclobutadiene, cyclooctatetraene, and 1,4‐dihydropyrazine. The first and third of these are normally categorized as antiaromatic. All three molecules are polyenes, even when the last two are forced into planar conformations. There is no driving force for extensive π delocalization, even when it would appear to have been facilitated. Calculated isodesmic energies show a net destabilization only in the case of cyclobutadiene, which we attribute to strain and repulsion between the π electrons of the CC double bonds. The other two molecules have negative isodesmic energies, indicative of net stabilizing effects. We conclude that the concept of antiaromaticity is useful for identifying molecules that resist the apparent opportunity for extensive © delocalization, but that it does not intrinsically imply net destabilization. © 1994 John Wiley & Sons, Inc.
The orthogonal and the natural representation for symmetric groups
Authors: Wu Wei , Zhang Qianer 
Published: January 2005

AbstractIn the present article, improved algorithms for evaluating the irreducible representations of the symmetric group associated with an arbitrary partition, such as the orthogonal, the natural, and the seminormal representation, are introduced and the relations among them are discussed. With the new algorithms, a representation matrix for the orthogonal or the seminormal representation is expressed as the product of three matrices, where two of them are the triangular intrinsic matrices depending on the irreducible representation of the symmetric group; the other relating to the permutation can be given explicitly. Furthermore, we give a concise description for the irreducible representations of the symmetric group and reach an interesting conclusion that the conjugation transformation matrix between the orthogonal and the natural representation is the intrinsic matrix of the symmetric group. © 1994 John Wiley & Sons, Inc.
Eclipsed and staggered conformations of (SIH3)2F+: An ab initio study
Authors: Palafox M. Alcolea 
Published: January 2005

AbstractAb initio calculations at 6–31G**, 6–31++G**, and MP2/6–31G** levels were performed on disilyl–fluoronium, (SiH3)2F+, with the SiH3 group eclipsed or staggered. Optimized geometries, total energies, dipole moments, atomic charges, electronic density, and vibrational frequencies were computed. The results were compared with calculated structural parameters and vibrational frequencies of H3SiF, H2SiF+, H2SiF−, and H4SiF+ ions. The basis‐set effects were studied. Several thermochemistry parameters—ZPE, thermal energy, rotational constants, and entropies—were also calculated. © 1994 John Wiley & Sons, Inc.
Ab initio study of the inversion barrier in NF3+
Authors: Wetzel T. L. , Shorrosh R. , Borkman R. F. 
Published: January 2005

AbstractAb initio Hartree–Fock, Møller–Plesset perturbation theory (MP2), and quadratic configuration interaction, using single and double substitutions (QCISD), calculations were carried out for the NF3+ ion. Optimized structures were examined at the various levels of theory. Calculation of the inversion barrier height shows the importance of optimizing the geometry at the post‐Hartree–Fock level and the inclusion of polarization functions. The best calculated inversion barrier was 13.3 kcal/mol, compared to an experimental value of 17.3 kcal/mol. The dissociation transition state was computed to determine the well depth of the NF3+ ion and its stability toward dissociation. The computed well depth was 28 and 48 kcal/mol at the SCF and MP2 levels, respectively. © 1994 John Wiley & Sons, Inc.
Quantum chemical study of catalysts based on oxides of transition metals
Authors: Shcheka Oleg L. , Dobrodey Nickolay V. , Emelina Tatyna B. 
Published: January 2005

AbstractCatalysts based on oxides of transition metals were studied by Xα–DV calculations. The chemical composition and electronic structure of surface layers for platinum(IV) oxide catalysts modified under percompound electrosynthesis were determined by X‐ray photoelectron spectroscopic, quantum chemical, and electrochemical data. The main regularities in electronic structure change under the transition from solid pure oxide PtO2 to its, in part, N‐substituted PtO2–xNx were analyzed. Then, we looked for perspective catalysts, calculating the electronic structure for analogous compounds of Ir(III), Ir(IV), Rh(III), and Pd(II). We found that the changes in electronic structure of rhodium oxide under O—N‐substitution allowed us to predict the excellent properties of its compound as a catalyst for percompound electrosynthesis reactions. © 1994 John Wiley & Sons, Inc.
Orbital binding effect in molecular orbital theory
Authors: Wang Xiang‐Yang 
Published: January 2005

AbstractBased on the virial theorem and the virial function, a description of the orbital binding effect and the definition of orbital bond order are developed within the framework of molecular orbital theory for both diatomic and polyatomic molecules. Orbital binding effect calculations for N2, CH4, C2H4, C2H2, and C4H4 molecules show that the definition is reasonable. © 1994 John Wiley & Sons, Inc.
A comparison of semiempirical and ab initio transition states for HF elimination in unimolecular decompositions
Authors: Mulholland Adrian J. , Richards W. Graham 
Published: January 2005

AbstractThe transition states for unimolecular HF elimination from a series of methylene halides and vinyl halides have been located and properly characterized at the AM1, MNDO, PM3, RHF/6‐311G (d, p), and MP2/6‐311G (d, p) levels. Whereas the semiempirical MO methods deal well with the structures of the stable molecules, the structural differences between the ab initio and semiempirical transition states are considerably larger. The AM1 and PM3 activation energies appear to be relatively more accurate. © 1994 John Wiley & Sons, Inc.
A density‐functional method for calculating atomic polarizabilities: Application to negative ions
Authors: Harbola Manoj K. 
Published: January 2005

AbstractWe recently proposed a variational method for determining static polarizabilities that uses the ground‐state density, rather than the orbitals, of the system for calculations. Thus, the method is different from, and numerically easier than, the perturbation theory approach in which changes in each of the single‐particle orbitals have to be obtained self‐consistently. For neutral atoms and cations, it leads to results that are comparable to the results obtained by the perturbation theory. In this article, we apply this method, employing Hartree–Fock densities, to obtain polarizabilities for negative ions and show that for negative ions also the results are quite accurate. Thus, the method may prove useful in making quick and accurate estimates of the polarizabilities of more complex systems where orbital based self‐consistent calculations become quite complicated. © 1994 John Wiley & Sons, Inc.
Theoretical study of structure and basicity of some alkali metal oxides, hydroxides, and amides
Authors: Burk Peeter , Koppel Ilmar 
Published: January 2005

AbstractAb initio (TZV*, SBK*, and 3–21G* or 6–31G* basis sets) calculations were performed to predict the geometries and gas‐phase proton affinities of Li2O, LiOH, LiNH2, Na2O, NaOH, NaNH2, K2O, KOH, and KNH2. © 1994 John Wiley & Sons, Inc.
Continuum wave functions by least‐squares scheme in a B‐spline basis: Multicenter and multielectron formulations
Authors: Decleva P. , Brosolo M. , Lisini A. , Venuti M. 
Published: January 2005

AbstractApplication of a purely algebraic least‐squares approach in a spline basis to the determination of continuum orbitals in the single‐electron molecular case illustrates the excellent numerical properties of the method. Initial work based on a single‐center expansion is extended to an LCAO formulation within a single‐particle Hamiltonian and to the full multielectron problem in the one‐center scheme. Preliminary results on H and He photoionization are reported. © 1994 John Wiley & Sons, Inc.
Computation of expectation values with Dirichlet one‐dimensional wave functions
Authors: Núñez Marco A. 
Published: January 2005

AbstractRecently, it was proven that the wave functions ψ of one‐dimensional Schrödinger operators can be approximated by those ψn corresponding to the Dirichlet eigenproblem in a finite box sufficiently large. In this article, sufficient conditions to compute expectation values are used to prove that the expectation value 〈ψ, Sψ〉 can be approximated by 〈ψn, Sψn〉 as n → ∞, for a wide class of unbounded symmetric operators S. © 1995 John Wiley & Sons, Inc.
Electronic structures of large, extended, nonperiodic systems by using the elongation method: Model calculations for the cluster series of a polymer and the molecular stacking on a surface
Authors: Mitani Masaki , Aoki Yuriko , Imamura Akira 
Published: January 2005

AbstractThe elongation method based on the molecular orbital (MO) theory, which enables us to extend a polymer with any molecular fragments theoretically, has recently been developed by our group. As the next step, we introduced an approach based on the crystal orbital (CO) theory into above treatment. In the present work, the elongation method was developed at the Hartree–Fock level with CNDO/2 parameters and applied to model systems composed of the cluster series of a polymer and the molecular stacking on a surface. In the cluster‐series calculations, the hydrogen molecule [(H2)n], hydrogen fluoride [(HF)n], polyethylene, and polyacetylene were created successively to approximate their one‐dimensional periodic polymers by using the MO‐based elongation method. In the molecular‐stacking models, we described the hypothetical surface of crystal as periodically arranged hydrogen molecules by the COs, and several hydrogen molecules were stacked up on the surface one after another with the elongation procedure. Furthermore, the lattice defect on surface in which a part of stacked layer is lacking was dealt with by our approach. We also treated carbon monoxide chemisorption on a periodic magnesium chain as a more realistic model. Results for these systems support the applicability of our method for nonperiodic interactions in one‐ and two‐dimensional large systems. © 1995 John Wiley & Sons, Inc.
Spectroscopic constants of SiH2, GeH2, SnH2, and their cations and anions from density functional computations
Authors: Mineva Tzonka , Russo Nino , Sicilia Emilia , Toscano Marirosa 
Published: December 2004

AbstractLocal (LSD) and nonlocal (NLSD) spin density calculations using different exchangecorrelation functionals have been performed to determine equilibrium geometries, harmonic vibrational frequencies (ωe), ionization potentials (IP), electron affinities (EA), dipole moments (μ), and singlet‐triplet energy gaps (Δ EST) of SiH2, GeH2, and SnH2. Geometrical structures as well as vibrational frequencies are in agreement with the available experimental data and compare favorably with the most sophisticated postHartree‐Fock computations performed until now. Both computed IPS (9.15 and 9.25 eV for SiH2 and GeH2, respectively) and EA of SiH2 (1.17 eV) compare favorably with experimental data (9.17, 9.21, and 1.2 eV). Accurate values are obtained also for singlet‐triplet energy gaps. We report for the first time the electron affinities of all neutral systems and the spectroscopic constants of the cations and anions. © 1995 John Wiley & Sons, Inc.
Ground‐state energies of atomic chains in superstrong magnetic fields
Authors: Relovsky B. M. , Ruder H. 
Published: December 2004

AbstractThe ground‐state energies of hydrogenic and helium chains in superstrong magnetic fields (B ≈︁ 107−109 Tesla) have been calculated within the single‐particle scheme of a heuristic density functional method. Applying the local density approximation (LDA), the simple Dirac exchange functional was used neglecting correlation effects. The equations were solved iteratively using a basis set of Landau functions in the transverse directions and directly integrating along the longitudinal direction. We obtained binding lengths and, by comparing the binding energies of the chains with the ground‐state energies of single atoms, we found condensation energies. © 1995 John Wiley & Sons, Inc.