bluuebunny/crossref_metadata_2025_split huggingface dataset:
Vibrationally inelastic scattering and relaxation times in gaseous HF
Authors: Gianturco F. A. , Lamanna U. T. , Battaglia F. 
Published: December 2004

AbstractA numerical fit to computed ab initio points of the HF–HF potential energy surface is here employed in its simplest spherical form to solve the corresponding close‐coupling equations that yield vibrationally inelastic collision cross sections for the V/T process in the gas phase. The existence of a deep potential well in the interaction is examined here for its effect on near‐threshold cross section behavior and evidence is found, through the present calculations, for the relative importance of several (orbiting) shape resonances that contribute to the Q1←0 cross sections at low collision energies (≤0.4 eV). The bearing of such lengthened interaction times on the corresponding low temperature relaxation times is shown in the ensuing calculation of pτ in the T range between 300 and 4000 K. In spite of the simplicity of the present model, quite good agreement is in fact found with experiments in the low‐ T region, while the lack of V, R/T coupling explains the departure of the computed pτ, above 1000 K, from the experimental findings.
Analysis of molecular orbital wave functions in terms of valence bond functions for molecular fragments. I. Theory
Authors: Hiberty P. C. 
Published: December 2004

AbstractA method of expansion of molecular orbital wave functions into valence bond (VB) functions is extended to molecular fragments. The wave function is projected onto a basis of mixed determinants, involving molecular orbitals as well as fragment atomic orbitals, and is further expressed as a linear combination of VB functions, characteristic of structural formulas of the fragment but whose remaining bonds are frozen. Structural weights for the fragment are deduced from this expression. Delocalized molecular orbitals are used as a startpoint, as they are after an ordinary SCF calculation. Wave functions of medium‐sized molecules may be analyzed with reasonable storage requirements in a computer.
Mapping between local potentials and ground state densities
Authors: Katriel J. , Appellof C. J. , Davidson E. R. 
Published: December 2004

AbstractThe mapping between the local one‐body potential and the ground state density is discussed. The role of degenerate ground states, which form a set of measure zero in potential space, is shown to be amplified by the unitary transformation between degenerate densities, forming a space filling set in density space. A numerical experiment demonstrating these features is carried out. No indication of non‐V representable densities is encountered.
New finite‐difference method for solving the pair radial equations of the many electron perturbation theory
Authors: Ischenko S. Ya. , Pavlov‐Verevkin V. B. 
Published: December 2004

AbstractA new convenient and efficient finite‐difference (FD) method is developed for solving the pair radial equations (PRE) of many electron perturbation theory (MPT). This method can be interpreted as the FD analog of the well‐known Fourier method. The method is used to solve PRE arising in the second order of MPT based on the hydrogenic and Hartree–Fock (HF) zero approximations. Being very simple programmed our method gives results comparable with the most accurate variational calculations.
Approximate exchange and electrostatic interaction energies of deformed ions
Authors: Gutowski Maciej , Kąkol Małgorzata , Andzelm Jan , Piela Lucjan 
Published: December 2004

AbstractComputation of exchange‐polarization and electrostatic‐polarization interaction energies between ions is the most expensive step in ab initio investigations of the properties of perfect and imperfect ionic crystals. In the present paper approximate formulas are proposed for these quantities. They save about 95% of the computation time and give these values with an error less than 0.2 kcal mol−1 as compared to ab initio results. The formulas for the exchange‐ and electrostatic‐polarization energies involve the generalized overlap integral between the one‐determinantal wave functions of the deformed ions. The approximations are tested in the calculations of the interactions of deformed ions in LiF and NaF crystals.
Generation of generalized branching diagram spin functions by Schmidt orthogonalization of spin‐paired functions
Authors: Ruttink Paul J. A. 
Published: December 2004

AbstractThe generalized branching diagram (GBD) spin representation is defined as the method of sequentially coupling together a number of subsystem spin eigenfunctions using the general rules of angular momentum coupling. It is shown that any GBD representation may also be obtained by Schmidt orthogonalizing a set of cannonical spin–paired (SP) functions, provided the SP basis is suitably ordered. The ordering procedure used is well suited to computer implementation. This is a generalization of results known in the literature for the Yamanouchi–Kotani and for the Serber spin representations.
Improved CI formula evaluation: Elimination of the phase determination at the determinant level for arbitrarily coupled CSFs
Authors: Bauschlicher Charles W. , Lengsfield Byron H. 
Published: December 2004

AbstractThe amount of work needed to evaluate the matrix elements in a configuration interaction calculation can be very large. This is especially true if the phase associated with bringing determinants into maximum coincidence is evaluated at the determinant level. A method is presented which allows the phase to be determined at the occupation level for arbitrarily coupled configuration state functions.
Effective potential theory of atom–diatom scattering in the presence of “intermediate” reactive channels
Authors: Varracchio E. Ficocelli 
Published: December 2004

AbstractA field‐theoretical formalism is applied to the problem of atom–diatom scattering, in the presence of “intermediate” reactive channels. This leads to an integral equation formulation for elastic processes and to a numerical quadrature problem for inelastic vibro–rotational transitions. The key quantity of the theory is represented by an effective potential that contains full information on the dynamics of the process. A closed equation is obtained for this quantity, along with an approximation scheme of the second order in the system interaction potential. A diagrammatic technique is introduced and the physical interpretation of the approximate form of the effective potential considered.
Self‐Consistent APW–k · p method. I. Theory
Authors: Lima I. C. Da Cunha , Da Silva A. Ferreira , Parada N. J. 
Published: December 2004

AbstractA first principles method based on an APW–k · p band structure calculation is derived in order to obtain the self‐consistent eigenenergies, crystal potential, and electronic density. The simplicity and extensions of the method are discussed, including comparison with the results obtained by the use of the Chadi and Cohen special points technique.
Uniform quality gaussian basis sets for molecular calculations. III. Charge optimized basis sets
Authors: Poirier R. A. , Daudel R. , Csizmadia I. G. 
Published: December 2004

AbstractGradient optimized constrained (2s ≠ 2p) and unconstrained (2s ≠ 2p) Gaussian 3G basis sets are reported for the first‐row atoms and ions XO, for Q = −2 to +4. Analytic equations have been fitted to the logarithm of the exponents as a function of the nuclear charge Z and formal charge Q. Consequently only two parameters Z and Q have to be specified in order to completely define a basis set.
Importance of the concept of bond and binding in the physical chemistry of solids
Authors: Lafourcade L. 
Published: December 2004

AbstractThe nature of problems connected with binding which arise in the everyday work of experimental physicists and material scientists is illustrated by reference to two different experimental studies. (i) Sliding in crystals of group V elements is found to take place not only along planes cutting the quasimetallic bonds, but also along planes apparently cutting their “covalent” bonds, thus suggesting that a more elaborate picture of the two types of bonds and of their interrelation is necessary. (ii) The physisorption of linear pentatriacontane on alkali chlorides indicates alignment of the paraffin molecules along the [110] directions. This may be associated with polarization of methylene groups by ions in the substrate. This hypothesis must be substantiated by suitable quantum‐theoretical calculations, which in turn must be based on an adequate description of the adsorption bond.
From molecular crystals and organic polymers to chemisorption processes: Importance of local interactions
Authors: Seel M. 
Published: December 2004

AbstractSome aspects of studies on organic solids, polymers, and solid surfaces are discussed in connection with the suggestion that local interactions can constitute an important common feature of these quite different systems, which allows a common treatment of certain properties in cluster‐type calculations. The examples of molecular crystals, of the influence of different side groups bound to the backbone of a regular polypeptide, and of the interactions of fluorine and chlorine atoms with a silicon surface are treated in order to illustrate this thesis, with special emphasis on the reduction of a variety of bonding situations to local interactions rather than on details of computations.
Nonempirical analysis of unusual chemical bonds. II. AlH2BH4 and AlH2C3H5
Authors: Barone Vincenzo , Lelj Francesco , Russo Nino 
Published: December 2004

AbstractAb initio computations have been performed on the compounds AlH2BH4 and AlH2C3H5 in order to achieve a better understanding of the peculiar characteristics of the metal to ligand bond. Our results suggest a bidentate coordination for the tetrahydroborate complex and an η3 coordination for the allyl complex. Due to the nonrigidity of the BH and allyl complexes, possible interconversion paths between different coordination modes have also been analyzed.
Convergence problem in ab initio crystal orbital calculations
Authors: Karpfen Alfred 
Published: December 2004

AbstractThe various computational procedures for treating the problem of lattice summations in ab initio crystal orbital calculations are briefly reviewed and the cutoff problem is discussed. Several numerical examples are presented for demonstrating the fast convergence of the total energy per unit cell with respect to the inclusion of further neighbors' interactions, in case reasonable cutoff procedures are applied. The importance of electrostatic considerations for the interpretation of long range effects is stressed.
Some thermodynamic parameters for hydroxy amino acids: Bicine and tricine
Authors: Soni M. L. , Kapoor R. C. 
Published: December 2004

AbstractpK values of N,N‐dihydroxyethylglycine (bicine) and N‐[tris(hydroxymethyl)methyl]‐glycine (tricine) have been determined by the Irving‐Rossotti method in an aqueous medium at 25, 30, 35, 40, 45, and 50°C and at different ionic strengths (I = 0.1, 0.5, and 1.0). Plots between pKa(NH) and 1/T for various ionic strengths have been obtained and the values of slopes have been used to calculate the ΔH, ΔS, and ΔG for the dissociation reactions of bicine and tricine. The ΔH, ΔS, and ΔG values for bicine were found to be 10.6 ± 0.6 kcal mol−1, −1.9 ± 1.8 e.u., and 11.1 ± 0.06 kcal mol−1, respectively, and for tricine 11.2 ± 0.6 kcal mol−1, 1.6 ± 1.6 e.u., and 10.7 ± 0.06 kcal mol−1, respectively. The pKa(NH) values decrease with rise in temperature but the influence of ionic strength is not significant.
Theoretical studies of photo‐oxidative cleavage reactions of nitrogen‐activated CC double bonds of enamines, indoles, and tryptamines
Authors: Yamaguchi Kizashi 
Published: December 2004

AbstractThe ab initio MO and semiempirical multireference (MR) CI calculations were carried out to elucidate the energy differences between peroxy 1,4‐diradical (DR) and 1,4‐zwitterionic (ZWI) intermediates, which play crucial roles in photo‐oxygenation reactions of nitrogen‐activated CC double bonds of enamines, indoles, and tryptamines. It is found that ZWI is more stable than DR in the case of electron‐rich enamines. Protic solvents such as methanol are highly effective for the stabilization of ZWI by hydrogen bondings. From the present theoretical results, it is concluded that tryptophane undergoes the photooxidative cleavage reaction to give formylkynurenine under a specific condition, where polar solvents or catalysts sufficiently stabilize the ZWI state for the species.
Interaction of the mutagenic base analogs O6‐methylguanine and N4‐hydroxycytosine with potentially complementary bases
Authors: Psoda Anna , Kierdaszuk B. , Pohorille A. , Geller M. , Kusmierek J. T. , Shugar D. 
Published: December 2004

AbstractInfrared spectroscopy has been employed to study possible base pair interactions, in nonpolar media, between O6‐methylguanine and uracil, cytosine and adenine; and between derivatives of N4‐hydroxycytosine and adenine. The association constants of O6‐methylguanine with uracil, cytosine, and adenine are well below 1M–1, whereas those for interaction of 1‐methyl‐N4‐methoxycytosine and its 5‐methyl derivative with adenine are identical, K = 14M–1. The significance of the latter finding is discussed in relation to the conformation of the exocyclic N4‐methoxy group. Quantum chemical calculations, with the aid of the perturbation method, were carried out for the interaction of O6‐methylguanine with uracil, cytosine, and adenine, to establish the most energetically favoured configurations for interactions of the free bases, and of the same base pairs in the B form of DNA. The role of the conformation of the exocyclic —OCH3 group in O6‐methylguanine is discussed. The relevance of both the experimental and theoretical results to mutagenesis by O6‐methylguanine and N4‐hydroxycytosine is examined.
Construction of hydration shells for the tautomeric forms of heterocycles based on electrostatic potentials
Authors: Hofmann Hans‐Jörg , Peinel Gustav , Krebs Christine , Weiss Cornelius 
Published: December 2004

AbstractA formalism for the determination of the most favored sites of monohydration and the orientation of water molecules around a solute by means of electrostatic potentials is suggested. The results of this method may serve as starting point for explicit supermolecule calculations. The hydration schemes determined in this way for the tautomeric forms of 2‐ and 4‐hydroxypyridine can be used as a basis for the evaluation of the displacement of the lactam–lactim equilibrium under the influence of the water environment.
Molecular electrostatic potentials from PCILO wave functions
Authors: Lochmann R. , Peinel G. , Weller T. 
Published: December 2004

AbstractThe molecular electrostatic potentials for several small molecules were investigated using PCILO wave functions at different stages of the perturbation expansion. The utility of the localized picture within the PCILO framework can be shown, even at zeroth order only. For simplicity the approximation of fixed standard bond polarities could be introduced in order to reproduce shape and values of the potential with a sufficient degree of accuracy.
A new look at correlations in atomic and molecular systems. I. Application of fermion monte carlo variational method
Authors: Moskowitz Jules W. , Kalos M. H. 
Published: December 2004

AbstractWe are engaged in research directed toward the development of compact and accurate correlation functions for many‐electron systems. Our computational tool is the variational method in which the many‐electron integrals are calculated by Monte Carlo using the fermion Metropolis sampling algorithm. That is, a many‐fermion system is simulated by sampling the square of a correlated antisymmetric wave function. The principal advantage of the method is that interelectronic distance rij may be included directly in the wave function without adding significant computational complexity. In addition, other quantities of physical and theoretical interest such as electron correlation functions and representations of Coulomb and Fermi “holes” are very easily obtained. Preliminary results are reported for He, H2, and Li2.
MSXα method and transition‐state procedure
Authors: Murilo Oliveira P. , Maffeo B. 
Published: December 2004

AbstractTheoretical aspects of the MS Xα method are analyzed. It is shown that real field self‐consistency is not achieved in an MS Xα calculation and some consequences of this fact are discussed. The approximations involved in the use of the transition‐state procedure to evaluate ionization and excitation energies from a MS Xα calculation are presented in some detail and commented.
A posteriori symmetry analysis of polymer electronic band structures
Authors: Božović I. B. , Delhalle J. , Damnjanović M. 
Published: December 2004

AbstractIn an attempt to illustrate how to apply abstract line group theoretical results to electronic structure polymer calculations, an a posteriori symmetry analysis is carried out on the extended Hückel energy band structures of two model chains of H2 molecules, whose symmetries are respectively characterized by the 𝕃;312 and 𝕃;63/mcm line groups. The symmetry quantum numbers have been assigned to the bands, and the compatibility relations and selection rules for direct optical absorption are derived.
Connections between perturbation theory and the unitary transformation methods of deriving effective Hamiltonians
Authors: Westhaus Paul 
Published: December 2004

AbstractThe connections between open shell Brillouin–Wigner perturbation theory and the Van Vleck unitary transformation formalisms for generating effective Hamiltonians are explored. An explicit expression is obtained relating the generator Ŝ of the unitary transformation eiŝ with the amplitudes to be found from perturbation theory. The “renormalization effects” needed to produce the explicit “orthogonal‐Hermitian” form of the effective Hamiltonian in perturbation theory are related directly to the generator of the unitary transformation. The conclusions reached previously by Jørgensen and Brandow regarding the identity of the effective Hamiltonians of the formalisms are explicitly verified for the case that the generator Ŝ satisfied the Kemble condition. The procedure suggests how the powerful techniques of perturbation theory can be used within the unitary transformation framework to guarantee properly renormalized wave functions.
Cluster expansion of the wave function. Electron correlations in singlet and triplet excited states, ionized states, and electron attached states by SAC and SAC–CI theories
Authors: Nakatsuji Hiroshi , Hirao Kimihiko 
Published: December 2004

AbstractThe symmetry‐adapted‐cluster (SAC) and SAC–CI theories reported previously have been applied to the study of electron correlations in ground state, singlet and triplet excited states, ionized state, and electron attached state. Formulas for calculations of one‐electron properties and transition properties from the SAC and SAC–CI wave functions are given. Calculations were carried out for the ground and Rydberg excited states of water and its positive and negative ions, with the use of the simpler computational scheme than the previous one. The results compare well with experiments.
Internal stresses and chemical binding in the H molecule
Authors: Bamzai Anjuli S. , Deb B. M. 
Published: December 2004

AbstractFollowing an interpretive formalism presented earlier, chemical binding in the H molecule has been studied in terms of the variations, with respect to R, of electrostatic field and stress components at five selected points on the interuclear axis. At three points phenomena analogous to those recorded earlier for H2 have been observed. In particular, the existence of extremal relationships for the difference density field and the total field, as well as for the corresponding stresses, at R ≃ Req for certain specific points on the internuclear axis has been confirmed. As in the case of H2, chemical binding in H occurs due to local variations of electrostatic pressure from point to point in such a manner as to cause the vanishing of either the total electrostatic force density or the difference force density at certain points on the internuclear axis.
Large order perturbation theory in the context of atomic and molecular physics—interdisciplinary aspects
Authors: Čížek Jiří , Vrscay Edward R. 
Published: December 2004

AbstractThe paper is introduced by a description of the role of divergent series in mathematics and their particular use in quantum mechanics. It is emphasized that there is a formal similarity between divergent series in nonrelativistic quantum mechanics and those series occurring in quantum field theory. It is indicated that studies of divergent series in nonrelativistic quantum mechanics can be helpful in the verification of suminability techniques in quantum field theory. The general problems of the application of perturbation theory to the study of the hydrogen atom in external fields are discussed. It is shown how the problem of the continuous spectrum can be bypassed. Further it is shown that algebraic techniques are very useful for a transparent formulation of perturbation theory. The theory is applied to the study of the hydrogen atom in a magnetic field and in an electric field and also to the study of the hydrogen molecule ion. The results for the hydrogen atom in a magnetic field and the hydrogen molecule ion are discussed in detail and the large order behavior of perturbation terms is compared with analytic formulas. Finally the questions of the summability for the series corresponding to the quartic anharmonic oscillator and the hydrogen atom in a magnetic field are discussed. It is seen that large order perturbation theory can be built into a continued fraction representation of a divergent perturbation series. Numerical results are presented for the two perturbation series mentioned above.
Size consistency corrections for configurational interaction calculations
Authors: Brändas Erkki J. , Combs Leon L. , Correia Nestor S. 
Published: December 2004

AbstractA size consistency error formula, correcting for the erratic dependence on the number of particles in a doubly substituted (or otherwise restricted) configurational interaction (CI) treatment, is derived. The formula is expressed in terms of the particle pair eneriges ϵk and the normalization integrals Δk of the corrections to the unperturbed normalized wave function. The theory as well as the results for the 1A1 ground state of H2O and the 2B1 state of H2O+ show close agreement with the coupled electron pair approach (CEPA).
Spectra of chemical trees
Authors: Balasubramanian K. 
Published: December 2004

AbstractA method is developed for obtaining the spectra of trees of NMR and chemical interests. The characteristic polynomials of branched trees can be obtained in terms of the characteristic polynomials of unbranched trees and branches by pruning the tree at the joints. The unbranched trees can also be broken down further till we obtain a tree containing just two vertices. This effectively reduces the order of the secular determinant of the tree we started with to determinants of orders atmost equal to the number of vertices in the branch containing the largest number of vertices. An illustrative example of a NMR graph is given for which the 22 × 22 secular determinant is reduced to determinants of orders atmost 4 × 4 in just the second step of the algorithm. The tree pruning algorithm can be applied even to trees with no symmetry elements and such a factoring can be achieved. Methods developed here can be elegantly used to find if two trees are cospectral and to construct cospectral trees.