bluuebunny/crossref_metadata_2025_split huggingface dataset:
Molecular calculations with the MODPOT, VRDDO, and MODPOT/VRDDO procedures. IV. Boron hydrides and carboranes
Authors: Popkie Herbert E. , Kaufman Joyce J. 
Published: December 2004

AbstractReference completely ab initio 6–3G and nonempirical 3G/MODPOT (ab initio effective core model potential) LCAO‐MO‐SCF calculations (using the same valence atomic orbital basis) were performed for a series of boron hydrides (B4H10, B5H9, B5H11, and B6H10) and a test 3G/MODPOT + VRDDO (variable retention of diatomic differential overlap for charge conserving integral prescreening) calculation were also performed for B5H9, B6H10, and B10H14. The agreement between the ab initio 6–3G and the corresponding 3G/MODPOT calculations was excellent for valence orbital energies, gross atomic populations, and dipole moments. The results also compared favorably to previous ab initio minimum STO basis results of Lipscomb and coworkers. The 3G/MODPOT + VRDDO calculations verified that for such spatially compact molecules (such as boron hydrides, which are fragments of polyhedra), the VRDDO procedure does not result in a noticeable savings in computer time for molecules of the size and shape of B5H9 and B6H10, in contrast to the savings previously realized for organic molecules of comparable atomic size. However, the agreement in calculational results between the 3G/MODPOT and the 3G/MODPOT+VRDDO results was still as extremely close as it had been for the organic molecules.3G/MODPOT calculations were also carried out for B8H12, B9H15, B10H14, B10H14−2, 1,2‐C2B4H6, and 1,6‐C2B4H6 and the results compared to the previous minimum STO basis results. For B10H14, the 3G/MODPOT+VRDDO method led to savings in computer time of 28% over the 3G/MODPOT method itself.The agreement of the 3G/MODPOT results with available experimental photoelectron spectral data for B5H9 and 1,6‐C2B4H6 was as good as that of the previous ab initio minimum STO basis calculations.
Electronic structure of TCNQ, studied with HAM/3
Authors: Åsbrink L. , Fridh C. , Lindholm E. 
Published: December 2004

AbstractThe electronic structure of tetracyanoquinodimethane (TCNQ) is calculated using the new semiempirical method HAM/3. The calculated photoelectron spectrum is in reasonable agreement with the measured spectrum. The excitation energies are obtained directly in HAM as the differences of the energies of the unoccupied and the occupied orbitals. The calculated UV spectrum is in good agreement with the measurements. The weak band at 5.3 eV, which earlier had been assumed to correspond to a forbidden transition, is allowed according to HAM. The electron affinity is also in reasonable agreement with the measured value. An explanation has been given for the experimental observation of several resonance states (negative electron affinities). p‐Quinodimethane has also been studied.
A comparative study of multiple scattering calculations on CrF63−
Authors: Kai Aldwyn Tang , Larsson Sven 
Published: December 2004

AbstractThe multiple scattering molecular orbital method, with different choices of sphere radii and local exchange, is applied to the cluster CrF63−. The orbital energies are almost always in the same order but uniformly moved up or down the energy scale depending on which parameters are used. Charge densities are only slightly dependent on the parameters. A method to avoid interaction with Rydberg‐like states is suggested.
Complex coordinate calculations of resonances in N‐electron atoms
Authors: Junker B. R. 
Published: December 2004

AbstractWe present a discussion of the functional dependences of the wave functions for bound, resonant, and scattering states on the radial coordinate ρ and the rotation angle α in the complex coordinate method. We conclude that for bound states and resonances, ρ and α are constrained to appear in the wave functions only in the for ρ exp(iα). On the other hand, this constraint is not obtained for the scattering states since the energy of the scattering states depends on α. In addition we suggest a partitioning of the resonant wave function into two parts—a boundlike or “Q‐space” part and a scattering like or “P‐space” part. With these concepts one can incorporate physical insight into the choice of configurations as one does in other methods and can apply the complex coordinate method to many electron systems with an expected rate of convergence similar to other techniques. Its advantages are that a single calculation yields the position and width of the resonance, only square integrable functions are used, only a solution of a straightforward eigenvalue problem is required unlike some methods, arbitrarily accurate target states are easily incorporated, and polarization terms can easily be explicitly included. Variational calculations for the position and width of the lowest 2S resonance in the negative helium ion are reported using trial wave functions containing 39, 43, 55, 24, and 32 “P‐space” configurations, respectively. Values of 19.387 eV and 12.13 meV are obtained for the position and width, respectively, for the resonance over a range in the rotation angle of almost two orders of magnitude. One also finds that inclusion of free‐particle‐like basis functions improves the representation of the scattering states.
Closed‐shell SCF–CIMO treatment of all‐reactive electrons in complex heteroorganic molecules
Authors: Hilal Rifaat 
Published: December 2004

AbstractA semiempirical molecular orbital (MO) method is outlined that is designed for correlation and prediction of the spectra and other properties of complex molecules. Within the ZDO approximation, the reactive (π‐bonding and nonbonding) electrons are treated explicitly considering the σ electrons as an unreactive static potential. The electronic repulsion integrals are evaluated using the multipole expansion formulas, which allow the characterization of each orbital by specification of its quadrupole moment, from which all two‐center repulsion integrals involving it would be computed. A preliminary application of the all reactive electron self‐consistent field (ARE‐SCF‐CI) MO method is made to organic carbonyls as well as some selected cyanine dyes. The predicted n → π* and π → π* transition energies and intensities are in good agreement with experimental data. The calculated ionization potentials are 0.2–0.7 eV lower than the observed values. The present method, although it requires further possible refinements, bridges the gap between the simplicity of the PPP method and the required overall interpretation of the electronic properties.
Diatomics‐in‐molecules study of LiOH
Authors: Nemukhin A. V. , Stepanov N. F. 
Published: December 2004

AbstractThe diatomics‐in‐molecules method is applied to calculate the ground state 1A′ potential energy surface for LiOH. Three different sets of polyatomic basis functions are investigated. An adequate description of the surface is obtained only with the set including both neutral and ionic atomic states.
Atomic energy levels and oscillator strengths calculated with a screened potential
Authors: Wu Irene C. , Miller Kenneth J. 
Published: December 2004

AbstractA screened potential model (SPM) is formulated with Slater‐type functions to reproduce the density of the electron shells. The orbital exponent for the valence shell is optimized to calibrate the SPM against experimental valence and Rydberg s and p energy levels for the alkali metals (Li, Na, and K) rare gases (He, Ne, Ar, and Kr), second‐row atoms, and Cu, Zn, Mg, S, and Ga. The resulting one‐electron wave functions are used to calculate the dipole and velocity forms of the oscillator strengths and 〈1/r3〉 for spin‐orbit coupling. The excellent agreement with the averaged experimental results suggests that the SPM atomic orbitals are a good starting point for the evaluation of one‐electron properties.
Fourth‐Order terms in the diagrammatic perturbation expansion for the electronic energy of atoms and molecules
Authors: Wilson Stephen , Silver David M. 
Published: December 2004

AbstractThird‐order diagrammatic perturbation theory provides a simple and accurate description of the electronic structure of atoms and molecules beyond that afforded by independent electron models. The largest corrections to such treatments, the fourth‐order terms, are presented and discussed. All of the diagrams, which arise when the closed‐shell Hartree–Fock function is utilized as a reference function, are given through fourth order.
Perturbative configuration interaction using localized orbitals in the INDO hypothesis. I. Theory and applications to energetical problems
Authors: Douady J. , Barone V. , Ellinger Y. , Subra R. 
Published: December 2004

AbstractA complete development of the PCILO method is presented for the INDO approximation. The introduction of exchange terms in the theoretical treatment is discussed and a detailed analysis of the successive contributions to the total energy is given. The method is applied to various conformational problems (geometry optimization, rotation, and inversion barriers). Its ability to deal with biological systems is tested on the acetylcholine neurotransmitter. Inversion barriers are highly improved with respect to the corresponding CNDO algorithm. A better account of σ‐π exchange phenomena can be expected with the presented PCILO‐INDO development.
Quantum‐chemical investigation of the silicon and carbon coordination bond in their isostructural compounds
Authors: Vitkovskaya N. M. , Mantsivoda V. B. , Moskovskaya T. E. , Voronkov M. G. 
Published: December 2004

AbstractAb initio calculations within the minimal basis set of STO‐4LGTO have been carried out on molecules of type H3MX (M = Si, C; X = H, NH2, OH, F). The influence of the MH3‐group inversion on the electronic structure of these compounds has been investigated and illustrated by MO electron density maps. The ability of the central atom to form an additional bond has been estimated with the help of calculations on the complexes of these molecules with the hydrogen negative ion. The complexes of type [H4SiX]− have been found to be more stable than their unbonded components. The [H4CX]− complex formation has not proved to be advantageous.
Further analysis of the iterative hypervirial‐scaling method
Authors: Fernández Francisco M. , Castro Eduardo A. 
Published: December 2004

AbstractA previously given iterative procedure to improve wave functions is analyzed. Its relationship with other well‐known approximation methods is investigated. Hypervirial operators depending on a real parameter are proposed and their connection with the employment of an infinite number of hypervirial relations is analyzed. A way to use formulas that are valid for exact eigenfunctions in the case of approximate functions is presented. Formal results are applied to the harmonic oscillator and hydrogen atom models in order to show their practical utilization.
Ab initiomolecular orbital study of the hydrogen‐bonded pyrrole ⃛acetonitrile complex
Authors: Lumbroso Henri , Pappalardo Giuseppe Concetto 
Published: December 2004

AbstractThe total dipole moments, molecular energies, and π‐electron densities for the linear and orthogonal pyrrole ⃛acetonitrile hydrogen‐bonded complexes were studied in theab initiovalence bond framework using the minimalSTO‐3G basis set. That the orthogonal conformation, although slightly less stable than the other, is predominant as observed in carbon tetrachloride, can be explained by its relatively high symmetry number.
Potential‐energy surfaces in the lower excited states of benzene⇌Dewar–benzene isomerization process
Authors: Tsuda Minoru , Oikawa Setsuko , Kimura Katsuhiko 
Published: December 2004

AbstractAdiabatic potential‐energy surfaces in the lower excited states following the benzene⇌Dewar–benzene isomerization process were calculated by the INDO/S method postulating the concerted reaction mechanism which is proved to be valid in the ground state by the calculation of the intrinsic reaction coordinate. It was concluded that the benzene molecule in the 1B1u and 1E2u excited states readily isomerizes to Dewar–benzene in the condensed phase although the quantum yield may not be large. Bryce‐Smith's proposal, that the isomerization to Dewar–benzene occurs only after the intersystem crossing to the 3B1u state in benzene molecule, will not be probable; for the 3B1u route is not favorable to the isomerization in comparison with these singlet routes. However, the rearomatization of Dewar–benzene in the ground state may produce the 3B1u benzene in small yield with higher yield of the ground‐state benzene. The activation energy in the rearomatization is calculated to be 19.03 kcal/mol. These conclusions are consistent with the experimental facts. Molecular orbital correlations in the isomerization and the ionization potentials of Dewar–benzene were also discussed.
Parametrization of the equations‐of‐motion method for conjugated hydrocarbons
Authors: Shibuya Tai‐Ichi , Funada Tadashi 
Published: December 2004

AbstractThe equations‐of‐motion (EOM) method has been parametrized for the evaluation of the “π → π*” transition energies and moments of conjugated hydrocarbons. In this new semiempirical scheme, the effect of the dynamical screening by σ electrons is explicitly included.
Binding energies at different momenta for the valence orbitals of HCl by the binary (e, 2e) method
Authors: Suzuki I. H. , Brion C. E. , Weigold E. , Williams G. R. J. 
Published: December 2004

AbstractThe binding energy spectra for the valence orbitals of hydrogen chloride have been obtained using the binary (e,2e) method at 1200 eV. The strength of the innermost valence orbital (4σ) is severely split among several ion states in the energy range 25 to 41 eV. The measured cross sections are compared with results of calculations using contracted Gaussian basis sets of double‐zeta quality, and with a one‐particle Green's function calculation.
Comparison between intervalence charge transfer and molecular tautomerism
Authors: Chu San‐Yan , Lee Shyi‐Long 
Published: December 2004

AbstractThe charge exchange process in the intervalence (mixed‐valence) system is interpreted in terms of the nonstationary state mechanism which is a well‐known approach for rationalizing the molecular tautomerism process. The two kinds of processes are viewed in a close analogy in the sense that both involve a pair of near‐degenerate levels in a double‐well potential. However, a comparison is made between the two cases to demonstrate that an electron moves not only faster but also farther in distance than a nucleus does.
Electronic structure of transition‐metal complexes. I. Parametrization for CNDO/2 method
Authors: Kai Eiko , Nishimoto Kichisuke 
Published: December 2004

AbstractA new parameterization for the first transition metal has been proposed in the framework of CNDO/2 method. We carried out CNDO/2 calculation of hexamine complexes [M(NH3)6]2+ and hexa‐aquo complexes [M(OH2)6]2+ in the high spin state where M = Mn, Fe, Co, Ni, and Cu, using new parameters. It is shown that the calculated order of binding energy is MnL < FeL < CoL < NiL ≈ CuL (where L means the ligand), and is in good agreement with experiment. We discussed how the orbital nodes affect the nature of bonding between metal and ligand.
EHF Description of superexchange: Relations to Anderson's model
Authors: Mayer I. , Angelov S. A. 
Published: December 2004

AbstractIt is shown that the treatment of the superexchange in insulators, based on the spin projected extended Hartree–Fock (EHF) method may be considered as a direct generalization of Anderson's ligand field (or kinetic exchange) theory: the EHF methods permits to construct explicitly the effective orbitals for the magnetic electrons and to determine all the parameters occurring in Anderson's theory from only one variational problem.
Electronic spectra of nucleic acid bases: Variable electronegativity calculations and an analysis of existing results
Authors: Srivastava S. K. , Mishra P. C. 
Published: December 2004

AbstractVariable electronegativity Pariser–Parr–Pople (VE‐PPP) calculations have been carried out on the bases of nucleic acids and 5‐fluorouracil. Directions of polarization for the various electronic transitions in these molecules have been calculated using the wave functions obtained by this and the CNDO/S‐CI methods. The calculated excitation energies, oscillator strengths, and directions of polarization have been compared with the PPP results due to Pullman and co‐workers and Bailey as well as the experimental ones. It has been found that a comparative study of this type is much more useful for understanding the transitions in the molecules than the individual calculations. The VE‐PPP method is found to generally give the best results. The directions of polarization have been used to identify the transitions in the molecules and to compare the excitation energies obtained by different methods. It is shown that the transitions in these molecules do not have the characteristics of those of benzene derivatives. All the observed peaks in the molecules have been explained as π*→π.
Configuration interaction matrix elements. III. Spin functions relating the unitary and symmetric group approaches
Authors: Wormer P. E. S. , Paldus J. 
Published: December 2004

AbstractSpin functions that are compatible with orbital ordering and geminal antisymmetry conditions are investigated. It is shown that two widely used classes of spin functions, namely, the spin‐bonded functions and Yamanouchi–Kotani (or, equivalently, Gelfand–Tsetlin) functions possess these properties. The relationship of the latter with Young–Yamanouchi spin functions is also outlined using graphical techniques of spin algebras. These techniques are also used to rederive the Hamiltonian matrix elements between spin‐bonded functions and to show the relationship among the various schemes used in this case.
Nonconservation of excitons as a mechanism of light‐energy capturing
Authors: Hadžiselimović E. A. , Marinković M. M. , Tošić B. S. 
Published: December 2004

AbstractThe analysis of nonstationary processes, taking place in exciton systems in consequence of the nonconservation of the number of elementary excitations, is presented and it is pointed out to a new possibility of light‐energy capturing which exists in ecological systems due to the nonconservation of excitons. It is shown that, due to the nonconservation, the one‐dimensional molecular chain is able to capture permanently the light energy of the order of 50 keV to 5 MeV. A part of the captured energy is expected to have a role in internal bioprocesses. It is also shown that the system is a seat where the processes of continuous creations and annihilations of pairs of excitons having opposite momenta take place with a probability of the order of 10−4 to 10−2. Further investigations would allow to identify biological phenomena caused by those processes.
Thermal dissociation of 1,2‐dioxethane. III. Localized molecular‐orbital study
Authors: Hilal Rifaat 
Published: December 2004

AbstractThe localized MO's (LMO's) of 1,2‐dioxethane in its ground state and along the dissociation reaction path (to formaldehyde products) are generated using Boys' criteria for localization. The total charge density in each LMO is partitioned into atomic and overlap densities and the binding or antibinding character of each LMO is discussed in terms of the forces exerted on the nuclei by these densities. The driving force for the dissociation reaction is shown to arise essentially from the atomic dipole forces exerted on the oxygen nuclei by their lone‐pair LMO's. The characterization of a saddle point on the potential energy surface has been discussed in terms of the electrostatic equilibrium between forces exerted by the electron clouds “incomplete following” and “preceding.” The differences between the LMO's obtained from the two Hartree–Fock solutions to which the SCF procedure converges have been discussed.
Theory of the potential of a donor ion in silicon
Authors: Csavinszky P. , Morrow R. A. 
Published: December 2004

AbstractIn this paper a variational principle has been formulated for obtaining an approximate solution of Poisson's equation for the potential of a donor ion embedded in Si. A comparison of a two parameter approximate analytical potential with the exact numerical potential shows excellent agreement for several electron concentrations. It is also found that the present theory, using the concept of a “spatially variable dielectric constant,” leads to a donor‐ion potential which deviates significantly from the potential of Dingle, which has been the traditional starting point of calculations of electron mobility limited by ionized‐impurity scattering.
Dynamics and conformation of polypeptides
Authors: Gupta V. D. 
Published: December 2004

AbstractThe dynamics of simple polypeptides is studied both from theoretical and experimental viewpoints. Theoretically, dispersion curves and frequency distributions are obtained from Wilson's GF matrix method as modified by Higgs for an infinite helical system. The systems studied are polyglycine I and polyglycine II, polyalanine in α and α forms, poly‐L‐proline I and poly‐L‐proline II, and poly‐L‐hydroxyproline. Conformation sensitive modes and their dispersion in the BZ are reported. In several of these, the results are compared with the measurements from inelastic neutron scattering. Since the neutrons have a large incoherent scattering cross section for hydrogen, it enables us to examine a normal mode via the motion of protons involved. In addition, neutron scattering is not controlled by symmetry dependent “selection rules.” Conformational studies of oligomers of glycine, alanine, and proline in relation to their polymeric forms as deduced from the dispersion curves and spectroscopic studies are presented. Recent measurements on the specific heat of polyglycine and polyalanine by Finegold et al. are also reported.
Protein backbone interaction with nucleic acid base pairs: A theoretical investigation
Authors: Hosur Ramakrishna V. , Pohorille A. 
Published: December 2004

AbstractA novel model of interaction between peptide backbone and nucleic acid base pairs, proposed earlier on the basis of model building studies has been theoretically investigated. Interaction energies for two different schemes of H‐bond interaction between peptide backbone and base pairs AU and GC have been calculated for different conformations of peptide backbone. The different H‐bond parameters have been calculated. The importance of such an interaction in specific recognition has been discussed in light of the results of theoretical calculations.
Theoretical model for exploration of catalytic activity of enzymes and design of new catalysts: CO2 hydration reaction
Authors: Sokalski W. Andrzej 
Published: December 2004

AbstractThe quantum chemical model for predicting the optimal molecular environment exerting the highest catalytic activity on transition complex has been proposed. It was applied for the CO2 hydration reaction at the nonempirical LCAO MO SCF level. The possible use of the resulting optimal charge distribution of environment for design of new synthetic catalysts and explanation of the carbonic anhydrase action was discussed. In addition, a simplified approach based on the difference electrostatic molecular potentials evaluated within the many center multipole expansion is presented. It may enable us to perform corresponding estimations even by means of a programmable calculator.