bluuebunny/crossref_metadata_2025_split huggingface dataset:
Influence of madelung (interatomic Coulomb) energy on Wolfsberg–Helmholz calculations
Authors: Jørgensen Chr. Klixbüll , Horner Sally M. , Hatfield William E. , Tyree S. Y. 
Published: December 2004

AbstractA model is presented for the estimation of ionicities in molecules and complex ions. The model uses the minimization of total energy by the method of differential ionization energies. The effect of Madelung corrections to the energies is considered, and the model is refined by evaluating the covalent‐bond energies.Wolfsberg–Helmholz calculations have been applied to the same type of model, also incorporating Madelung corrections. The Madelung corrections make the metal ionization energy curves less steep, and the ligand ionization energies are nearly invariant with charge. This creates a situation which has previously been artificially imposed by selecting the ligand ionization energies to give desirable terms in the Wolfsberg–Helmholz secular determinant. The effect of Madelung energy is shown to be the primary influence in describing the ionicity and total energy of a chromophore; covalent bonding effects are shown to be secondary when the ligands and the central atom have fairly different electronegativities.
Localized impurity states in the Hartree‐Fock, LCAO approximation II. The F Center in KCI
Authors: Martino F. 
Published: December 2004

AbstractWe apply the techniques of a previous paper (I) to the F center in KCl. Our purpose is to place the application of Hartree‐Fock methods to the F center on a firm theoretical basis by calculating in a consistent manner the magnitude and effect of approximations commonly made in less complete treatments. It is shown that the familiar point‐ion approximations and crystal‐field approximations with partial consideration of exchange effects are special cases of our results. We compute wave functions and energies step by step for each of the various levels of approximation possible with our model. It is found that the functions resulting from the point‐ion model are not good approximations to the final wave functions. Our results show that exchange effects with at least the first two shells of nearest neighbors should be considered since they are of the same order of magnitude as terms in the point‐ion model. Overlaps of the F‐center function with ion functions out to sixth neighbors are considered. The absorption energy for the F center is calculated to be 0.1619 Ry as compared with the experimentally observed energy of 0.170 Ry. The magnetic hyperfine structure contact terms are calculated for the first two shells of nearest neighbor ions, using approximate orthogonalized functions, and found to be 29.7 Mc/h for the nearest neighbor K+ ions and 10.9 Mc/h for the next nearest neighbor Cl− ions. The experimentally observed values are 21.6 and 7.0, respectively. Given these differences and the excessively low values of the one‐electron energies, it is concluded that electronic and ionic polarization effects in the ionized crystal states must be considered to calculate accurate F‐center wave functions and absolute energy levels.
The first excited triplet states of the nucleotide bases calculated with different semiempirical SCF schemes
Authors: Packer J. C. , Avery J. S. , Ladik J. , Biczó G. 
Published: December 2004

AbstractThe π‐electron distributions, spin densities, and energies of the first triplets of the nucleotide bases, uracil, thymine, cytosine, adenine, and guanine, were investigated in various semiempirical approximations. Results are presented for calculations using the semiempirical form of the closed‐shell SCF configuration interaction method, of the different orbitals for different spins (unrestricted Hartree–Fock) treatment, with and without spin projection, and of the Roothaan's open‐shell procedure.
On the convergence properties of the Rayleigh–Schrödinger and the Hirschfelder–Silbey perturbation expansions for molecular interaction energies
Authors: Chałasiński Grzegorz , Jeziorski Bogumił , Szalewicz Krzysztof 
Published: December 2004

AbstractThe Rayleigh–Schrödinger polarization and the Hirschfelder–Silbey (HS) perturbation theories are applied, through the 38th order, to the interaction of a ground‐state hydrogen atom with a proton. The calculations were made with high precision using a large basis set of orbitals expressed in the confocal elliptic coordinates. The results obtained show that for small internuclear separations R the polarization series converges slowly in an oscillatory way to the energy of the ground 1sσg state of the H molecule. At large R, however, the polarization expansion reproduces only the Coulomb part of the interaction energy effectively. When the value of the Coulomb energy is reached, the rate of convergence deteriorates drastically so that the exchange energy is not reproduced practically. The HS method converges fast when applied to both the 1sσg and the 2pσu states, the rate of convergence being roughly independent of the internuclear distance. If the finite basis set employed to solve the perturbation equations is stable under the symmetry operations, the HS expansion is shown to converge to the energy obtained by minimizing the Rayleigh–Ritz functional within the space spanned by the functions used in the perturbation theory calculations.
Environmental effects on hydrogen bonded systems: A quantum chemical study of proton relay model systems
Authors: Tapia O. , Poulain E. 
Published: December 2004

AbstractIn this paper an application of a reaction field theory of solvent effects has been made to study proton transfer mechanisms in hydrogen bonded systems coupled to an environment. The latter is simulated with reaction fields having variable strength and direction (defined with respect to the supermolecule's total dipole moment direction), together with superposed uniform external electric fields. Changes in proton potential curves and some other properties of a model water dimer and a water trimer are reported. The results are discussed in relation to relevant phenomena in biology and biochemistry, namely proton relay systems in enzymatic catalysis.
The MTXαR method. The determination and use of “molecular” α values in molecular multiple scattering Xα calculations
Authors: Rosenkrantz Marcy E. , Konowalow Daniel D. 
Published: December 2004

AbstractA semiempirical approach is used to fix the α value for use in the extraatomic regions in multiple‐scattering (MSXα) calculations which retain the muffin‐tin treatment of the potential. Such a “molecular” α value for an atom is determined by requiring the corresponding homonuclear diatomic molecule to have its minimum at the experimentally determined equilibrium separation; hence they are called αR. Molecular αR values are determined for the ground state Li2 and F2 molecules and are tested in a calculation of the ground state LiF potential curve. We find a binding energy at the calculated equilibrium separation to be within 1% of the experimental value. The LiF curve based entirely on the ordinary atomic α values is substantially inferior. The present MTXαR approach appears to be competitive with others which are intended to improve the muffin‐tin version of MSXα calculations.
Selected valence electron SCF MO calculations on alkali metal diatomics
Authors: Mohammad S. Noor 
Published: December 2004

AbstractThe selected valence electron closed‐shell MO theory of Pohl, Rein, and Appel has been modified with a view to accurately determining the potential energy curves and related spectroscopic quantities of alkali metal diatomics. Binding energies −De equilibrium internuclear distances re, vibrational force constants ke, and dipole moments μbond have been calculated by this method. For the determination of ke, however, use has been made of three different formulas. The calculated potential energy curves have been fitted to a semiempirical potential function constructed in terms of De, re, and ke and then following the Woolley and Dunham method, the harmonic force constants le, me, ne, and oe, the vibrational‐rotational coupling constants αe, the vibrational anharmonicity constants wexe, and two other second order spectroscopic constants, viz., βe and γe, have been calculated with the potential function. The computed results have been compared with experiments wherever the latter are available and it has been observed that these results are in very good agreement with experiment.
Breit‐type three‐electron equation in the Pauli approximation
Authors: Rai D. K. , Ladik J. 
Published: December 2004

AbstractWe have reduced the Breit‐type equation written down for 3 electrons to the 8 large components of the wave function in the (v/c)2 (Pauli) approximation. This procedure required appropriate handling of the 64 scalar equations which result in this instance. According to the results obtained within the accuracy of (v/c)2 all the terms are the same as those obtained by summing up the terms of the reduced 2‐electron Breit's equation to 3 electrons. This statement is no longer valid, however, if during the reduction procedure we would go beyond the (v/c)2 approximation. Finally there is a discussion about how to begin developing a relativistic many‐electron theory valid for the inner shell electrons of heavy atoms within the accuracy of the fine structure constant.
A comparative analysis of pair equations for the H2 molecule
Authors: Adamowicz Ludwik 
Published: December 2004

AbstractThe perturbation contributions to the correlation energy of the H2 molecule are analyzed. This analysis is based on the computer solutions of several two‐electron equations in the Gaussian geminal basis set. The pair equations differ by the choice of the two‐electron interaction potential which may result from distinct infinite summations of correlation diagrams in Rayleigh–Schrödinger perturbation theory.
On the use of symmetry in first‐order perturbed Hartree–Fock theory
Authors: Lazzeretti Paolo 
Published: December 2004

AbstractA method for the determination of the symmetry of first‐order vectors in Hartree–Fock perturbation theory is developed. This leads to the definition of symmetry‐adapted basis vectors to be employed at first order in the perturbation. It is shown that computer time can be saved, to some extent, in the calculation of second‐order properties, by exploiting molecular symmetry. Specific examples are given for methane, ammonia, and water.
London potential‐energy surfaces for symmetrical alkali trimers
Authors: Mohammad S. Noor 
Published: December 2004

AbstractSemiempirical valence bond calculations have been performed on the symmetrical alkali trimers in order to obtain their potential‐energy surface, binding energy, and equilibrium geometry. For these calculations original and generalized Rosen–Morse potentials for the ground state and two similar types of new potentials for the triplet state of alkali dimers have been used. At small internuclear distances the potential surfaces show a shallow well which extends into the entrance and exit valleys without an energy barrier. The trimers are found to be stable both in linear and bent configurations, but the most stable configuration in each case is linear and symmetric. The force constants corresponding to stretching and bending deformations of the trimers have been calculated for this configuration. It has been observed that unlike the stretching, the bending deformation does not sensitively affect the energy variation of the ground state of the trimers.
On the use of spatial symmetry in atomic‐integral calculations: An efficient permutational approach
Authors: Le Rouzo H. 
Published: December 2004

AbstractThe minimal number of independent nonzero atomic integrals that occur over arbitrarily oriented basis orbitals of the form ℛ(r) · Ylm(Ω) is theoretically derived. The corresponding method can be easily applied to any point group, including the molecular continuous groups C∞v and D∞h. On the basis of this (theoretical) lower bound, the efficiency of the permutational approach in generating sets of independent integrals is discussed. It is proved that lobe orbitals are always more efficient than the familiar Cartesian Gaussians, in the sense that GLOs provide the shortest integral lists. Moreover, it appears that the new axial GLOs often lead to a number of integrals, which is the theoretical lower bound previously defined. With AGLOs, the numbers of two‐electron integrals to be computed, stored, and processed are divided by factors 2.9 (NH3), 4.2 (C5H5), and 3.6 (C6H6) with reference to the corresponding CGTOs calculations. Remembering that in the permutational approach, atomic integrals are directly computed without any four‐indice transformation, it appears that its utilization in connection with AGLOs provides one of the most powerful tools for treating symmetrical species.
PCILO Calculations on “true” van der Waals complexes
Authors: Lochmann R. , Hobza P. 
Published: December 2004

AbstractThe PCILO method was applied to complexes where dispersion energy forms the dominant attractive contribution of the intermolecular interaction energy. With alkane dimers, especially with dimers of higher alkanes (C4H10)2, (C5H12)2, etc., and the ethylene dimer PCILO was successful as to stabilization energy, equilibrium distance, optimal dimer structure, and energy composition. The dispersion energy part is always underestimated, however. With (X2)2 dimers (X = N, F, Cl) PCILO failed, which might be explained by the presence of lone pairs in the subsystems.
Quantum chemistry by random walk: H4 square
Authors: Anderson James B. 
Published: December 2004

AbstractThe random‐walk method of solving the Schrödinger equation is applied to the 1B1g and 1B2g states of the H4 square with side length 2.4 a.u. The results provide an independent check of the accuracy of prior variational calculations for these states. With node structures for the 1B1g state taken from a variational calculation with a single‐zeta basis set and taken in the simplest form meeting symmetry requirements, the calculated energies are lower than the expectation value of the energy for the single‐zeta basis set but not as low as the expectation value for an optimized‐exponent double‐zeta basis set with polarization terms. Comparisons of results give no suggestion of a barrier height lower than ∼120 kcal/mol for passage through the square configuration in the exchange reaction H2 + D2 → 2HD. For the 1B2g state with node structure in the simplest form meeting symmetry requirements the calculated energy is ∼65 kcal/mol lower than the expectation value of the energy for variational calculations with a double‐zeta basis set.
Neon L‐shell photoabsorption cross sections from moment theory and finite basis pseudospectra
Authors: Faegri K. 
Published: December 2004

AbstractPseudospectra from Gaussian basis set calculations within a frozen‐core approximation have been used in a moment analysis to obtain Tchebychev profiles for the photoabsorption process in the valence shell of Ne. The profiles show good agreement with cross sections obtained in equivalent calculations using numerical atomic wave functions and continuum orbitals, particularly when the dipole‐velocity form is employed. Variation of the basis sets shows that it is possible to obtain meaningful photoabsorption profiles using 13–15 virtual orbitals to describe the outgoing electron.
On the use of symmetry in first‐order perturbed HF theory. II
Authors: Lazzeretti Paolo , Zanasi Riccardo 
Published: December 2004

AbstractA method is described whereby molecular symmetry is employed to reduce the number of two‐electron integrals in perturbed Hartree–Fock calculations of second‐order properties. The method is a generalization of the Dacre–Elder procedure. First‐ and second‐rank perturbing tensor operators are examined in the coupled HF approach to electric dipole polarizabilities, magnetic susceptibilities, quadrupole polarizabilities, and spin‐dopolar contributions to spin–spin coupling constants. The procedure sketched here permits a large saving of computer efforts, which is shown by some illustrative examples.
Power moments of hydrogenic Green's functions and Green's functions of the second kind
Authors: Laurenzi Bernard J. 
Published: December 2004

AbstractPower moments for all of the reduced hydrogenic Green's functions have been computed. These moments have the same form as the corresponding moments of the free‐particle Green's functions. Green's functions of the second kind are defined, and uses for these objects in model potential theory and the theory of many‐body Green's functions are pointed out. In the case of the ground state of the hydrogenic atom, the Green's function of the second kind has been given.
Study of the muffin‐tin approximation in the multiple‐scattering method
Authors: Ferreira Luiz G. , Fazzio Adalberto , Closs Huberto , Brescansin Luiz M. 
Published: December 2004

AbstractWe consider the effects of a non‐muffin‐tin charge density in the multiple‐scattering method. The study is carried out by means of a parametrized electronic density, which is used in a variational expression for the energy of the molecular system. We show that, at least in two situations, the parametrized density reproduces the true density very well. In a numerical study of the electronic configuration of the CH4 molecule, we show that the non‐muffin‐tin density may shift the one‐electron energies by as much as 1 eV and the total energy by as much as 1 Ry.
Toward a microscopic description of molecular electronic systems
Authors: Pucci Renato , Baldo Marcello , Grassi Antonio , Tomasello Pasquale 
Published: December 2004

AbstractThe many‐body perturbation theory is reformulated within the dielectric function method presented in the preceding paper [12]. The self‐energy effects are explicitly considered. These corrections turn out to be important and are calculated to the same level of approximation as the corresponding local‐field terms. The method is at the moment applicable to π‐electron systems within the zero differential overlap approximation. Explicit calculations of the excitation energies for the benzene molecule, using different parametrizations, are presented. Comparison is made with the results obtained in the random phase approximation (RPA) and other schemes.
Perturbation calculation of transition moments in the ArH+ system
Authors: Chambaud G. , Levy B. , Millié Ph. 
Published: December 2004

AbstractThe matrix elements of the transition dipolar momentum IR have been computed between the low‐lying states associated to the Lyman–α transition. The orthogonalization of the involved electronic states (obtained by the CIPSI method) has been performed and led to a better value for the asymptotic behavior of the {Ar(3p5, 4s) + H+} configuration. Position of the satellites in the red and blue wing of the line are deduced from the potential‐energy curves in the frame of the quasistatic theory.
Spin–orbit coupling for the motion of a particle in a ring‐shaped potential
Authors: Hartmann Hermann , Schuch Dieter 
Published: December 2004

AbstractAs is known, the Schrödinger equation for a particle in the ring‐shaped potential V(r,v) = ησ2(2a0/r−a/r2 sin2v)ε0, defined in the whole space, has been solved exactly. Here the eigenfunctions are represented in a form which is advantageous for concrete evaluations. The spin–orbit interaction energy ELS in quasirelativistic approximation is determined analytically, for the first time with a nonspherically symmetric potential. The influence of spin–orbit interaction on the eigenvalues of the spin‐free problem and on the selection rules for electrical dipole transitions are investigated as well as the dependence of ELS on the position and depth of the potential minimum. The model can be useful for investigations of axial symmetric subjects like the benzene molecule or related problems and may be easily extended to a many‐electron theory.
Molecular calculations with the nonempirical ab initioMODPOT, VRDDO, and MODPOT/VRDDO procedures. XI. Theoretical study of the [C6H5OH ⃛OC6H5]− molecular complex: Ab initioMODPOT/VRDDO calculations and electrostatic molecular potential contour maps
Authors: Sokalski W. A. , Hariharan P. C. , Popkie Herbert E. , Kaufman Joyce J. , Petrongolo Carlo 
Published: December 2004

AbstractThe transport of C6H5O− (or similarly charged moieties) through a lipoidal membrane may possibly be facilitated by forming complexes with the neutral compound. Thus, theoretical studies were performed on the model [C6H5OH ⃛OC6H5]− molecular complex to obtain some information concerning the possible molecular and electronic structure of such complexes. Ab initio MODPOT/VRDDO SCF calculations were carried out on the neutral‐anion dimer [C6H5OH ⃛OC6H5] to optimize the equilibrium geometry. Electrostatic molecular potential contour maps have been generated from the ab initio MODPOT/VRDDO results in the molecular plane and in the plane perpendicular to the molecular plane and intersecting the hydrogen bond O ⃛HO. Difference maps have also been generated showing the change of potential on complex formation. There is a decrease of electrostatic interactions of the phenoxide anion upon complex formation with the neutral phenol. Counterpoise corrections for basis set size could not be made since calculation of the phenoxide anions in the basis set of the phenol plus the phenoxide anion led to an excited state for the phenoxide anion. This behavior is somewhat similar to that occurring in the stabilization method for excited states of negative ions as the size of the basis set is increased.
Evaluation of multicenter integrals over Slater‐type atomic orbitals by expansion in terms of complete sets
Authors: Steinborn E. Otto , Filter Eckhard 
Published: December 2004

AbstractMulticenter integrals over Slater‐type orbitals can be expanded in series with known coefficients if they are considered as elements of appropriate Hilbert spaces. Some test calculations for the three‐center nuclear attraction integrals are given.
Calculations of XPS spectra for oxyanions and related compounds by the discrete variational‐Xα method
Authors: Sasaki Teikichi , Adachi Hirohiko 
Published: December 2004

AbstractCalculations of molecular‐orbital energies and x‐ray photoelectron spectra have been carried out for the third‐row oxyanions, transition‐metal oxyanions, SiO2 and TiO2, by the discrete variational‐Xα cluster method. The calculated orbital energies are consistent with those determined from the XPS experiments. Theoretical XPS line shapes with Gaussian are generally in good agreement with the observed spectra. However, underestimation has been found for the photopeak intensities in the low‐binding‐energy region of TiO2. The discrepancy is partially attributed to the use of inaccurate photoionization cross‐section for the Ti3d orbital.
Laser‐induced fluorescence of Cs2 and role of kinetic energy in the Franck–Condon principle
Authors: Katô Hajime 
Published: December 2004

AbstractQuantum‐mechanical calculation of spectral intensity is done for the transitions from a particular vibrational level of the excited state E 1∏u to the discrete vibrational levels and dissociative continuum of the ground state X1Σ of the Cs2 molecule. The results are compared with the observed spectra by referring to the conservation of nuclear kinetic energy during the electronic transition.
Excimer and monomer emission of pyrene crystals associated with the phase transition
Authors: Uchida Kenji , Takahashi Yoshihiro 
Published: December 2004

AbstractEmission spectra and decay times of the fluorescence under the excitation of various energies were measured in pyrene crystals. At liquid nitrogen temperature, a new excimer emission was found under 390 nm excitation, and its peak shifts a little to the high energy in comparison with that of the familiar excimer. The decay time of the former emission abruptly changes around 125°K, which seems to be due to the phase transition in pyrene crystals. Also, when the crystals were rapidly cooled below 125°K, the monomer emission in the crystals was observed, and its decay time was about 600 nsec.