bluuebunny/crossref_metadata_2025_split huggingface dataset:
One‐Center coulomb, two‐center hybrid, and two‐center coulomb integrals over STP functions
Authors: England Walter 
Published: December 2004

AbstractDiatomic integral formulas involving Slater‐transform‐Preuss functions are obtained for the Coulomb and hybrid types. One‐center Coulomb integrals are also considered. All results have finite sums containing a one‐dimensional numerical integration.
Coupled Hartree‐Fock calculation of the uniform field quadrupole polarizabilities and shielding factors of S‐state ions
Authors: Bhattacharya A. K. , Mukherjee P. K. 
Published: December 2004

AbstractUniform field quadrupole polarizabilities and shielding factors of the first three ions of 2‐, 3‐, and 4‐electron sequences in their ground states have been calculated in the coupled Hartree‐Fock (CHF) scheme. For comparison the uncoupled Hartree‐Fock (UHF) results are also presented. It is observed that the uniform field quadrupole polarizabilities differ slightly in the two schemes and agree well with the UHF results of Lyons, Langhoff and Hurst. However the shielding factors differ appreciably in the two schemes and are completely different from those of Lyons et al. The cause of the difference in the shielding factor values is investigated.
Long‐range dispersion interactions involving excited atoms; The H(1s)–H(2s) interaction
Authors: Deal Walter J. , Young Ralph H. 
Published: December 2004

AbstractThe dispersion interaction between two nonoverlapping atoms (or molecules) is expressed in terms of single‐atom “polarizabilities.” The formulation is valid even if one atom (or both) is in an excited state. To illustrate the procedure, the dispersion interaction between a 1s and a 2s hydrogen atom is computed accurately through order R−10 (R = internuclear separation).
Validity of the three‐center, four‐electron model for stability of rare gas halides on the basis of exchange perturbation theory
Authors: Lombardi E. , Pirola L. , Tarantini G. , Jansen L. , Ritter R. 
Published: December 2004

AbstractIn a previous publication [1] we analyzed the stability of rare gas halides on the basis of exchange perturbation theory of the Rayleigh‐Schrödinger type, using a three‐center, four‐electron model. In this paper, the analysis is extended to a (n + 1)‐center, (n + 2)‐electron model for rare gas halides of composition RXn, where R is the rare gas atom and X the halogen atom, in order to investigate the validity of the three‐center, four‐electron model. The compounds analyzed are XeFn, XeCln, KrFn and KrCln with n = 3 and 4, in different geometric configurations and for different states of total spin S. As before, we use exchange perturbation theory in first and second orders. The results are in good agreement with those obtained in the previous analysis and with experiments. Specifically, it is found that chlorides of rare gas atoms are not stable, that XeF4 has the square‐planar configuration and that trifluorides cannot exist. The possible existence of KrF4 is discussed.
Correlation effects in the low–lying excited states of the PPP models of alternant hydrocarbons. I. Qualitative rules for the effect of limited configuration interaction
Authors: Čížek J. , Paldus J. , Hubač I. 
Published: December 2004

AbstractSimple rules for an estimate of the correlation effects in the low‐lying states of alternant hydrocarbons, as described by the Pariser–Parr–Pople Hamiltonian, are formulated. These rules are based on the alternancy and spin symmetry classification of states in both strongly and weakly correlated limits and on the valence bond characteristics of those states in the fully correlated limit. It is shown that the largest effect of the electron correlation will be found for the singlet “minus” states (using Pariser's classification of the alternancy symmetry species), a smaller effect for the triplet “plus” states, and a much smaller effect for the remaining states. These rules are exemplified by limited CI calculations including all monoexcited and all mono‐ and bi‐excited configurations, respectively, for a number of π‐electronic systems. In view of these rules the success of the PPP model in the monoexcited CI approximation may be understood.
Resolvent technique and Padé approximants in configuration interaction calculations
Authors: Røeggen I. 
Published: December 2004

AbstractA perturbation approach based on resolvent technique and Padé approximants is proposed. The eigenvalue of interest is part of a solution of two nonlinear algebraic equations. The nonlinear equations are arrived at by considering two different expression of the expectation value of the resolvent of an outer projection of the Hamiltonian. The first expression is based on the spectral resolution of the resolvent, and the second one is obtained by a power series expansion analogous to that applied in the derivation of the energy expression in the Brillouin–Wigner perturbation theory. The truncated power series is extrapolated by Padé approximants of type II. The method is tested on a CI calculation of the energy of the lowest 1Σ state of the B2 molecule.
Ab initio calculation of the electronic structure of TCNQ and its ions
Authors: Johansen Helge 
Published: December 2004

AbstractAb initio SCF‐LCAO‐MO calculations have been performed for TCNQ and its positive and negative ions in various electronic states. A basis set consisting of 412 primitive Gaussian type orbitals contracted to 180 was used in the investigation. The electron density distribution in TCNQ and the negative ions, and the redistribution upon ionization has been illustrated by plotting difference density contour maps. The quinone structure of the neutral TCNQ system undergoes a transformation to a benzenoid structure when electrons are added. Electronic transitions, ionization potentials and electron affinities have also been calculated.
Rearrangement correction to higher polarizabilities of two‐electron systems
Authors: Roy H. P. , Bhattacharya A. K. 
Published: December 2004

AbstractIn this paper we include the rearrangement correction (discussed in the preceding paper) in a coupled Hartree–Fock (CHF) calculation of atomic hyperpolarizabilities and other related properties. We have studied the effect of these corrections on properties like electric dipole hyperpolarizabilities, uniform electric field quadrupole polarizabilities and shielding factors in two‐electron ions and have noticed significant changes in the computed values over the CHF results.
Spin‐projected EHF method. III. Applications to π‐electron systems
Authors: Mayer I. , Kertész M. 
Published: December 2004

AbstractThe surely convergent procedure to obtain extended Hartree–Fock (EHF) solutions of the spin‐projected scheme, for which the equations are given in Part II, is applied to eleven π‐electronic systems with 2–10 electrons at the PPP level of integral approximations. The method takes into account a considerable part of the correlation energy. The symmetry properties of the EHF wave functions obtained are discussed together with some computational details. A simplified algorithm is also described.
Integral Hellmann‐Feynman computations on H3ABHn—H2ABHn+1 rearrangements
Authors: Trindle Carl , George John K. 
Published: December 2004

AbstractExtensively optimized Lewis orbital (Frost model) structures are reported for CH3N, CH3NH+, CH3CH, CH3CH, CH3BH2, CH2NH, and CH2NH (spiro and planar). Electronic energy differences between these isoelectronic species were estimated by the integral Hellmann‐Feynman (iHF) formula, with the hope that satisfaction of the Hellmann‐Feynman conditions would lead to accurate iHF values of energy changes. We observed a strongly nonlinear relation between the iHF error and the departure of the overlap of wave‐functions of the structures from unity. MO computations in common orbital (not determinantal) basis sets for CH3NH+CH2NH (planar), CH3NCH2NH, and CH3CHCH2CH2 produced greatly improved iHF estimates of energy changes, reducing errors by as much as 80 times. Certain features of the static optimum structures and the transition densities suggested that the syn path for rearrangement of methyl carbene to ethylene is a general feature of rearrangements in these systems.
Calculation of j and jm symbols for arbitrary compact groups. I. Methodology
Authors: Butler P. H. , Wybourne B. G. 
Published: December 2004

AbstractA systematic method is established for computing the coupling coefficients associated with arbitrary compact groups using only the general properties of the coefficients and the specific character theory of the relevant groups. The basic character theory is first outlined. Then a primitive set of 6j symbols is defined and their computation sketched with careful attention being given to matters of phase fixing. A recursive method is developed for calculating arbitrary 6j symbols using the primitive set. Finally, a primitive set of 3jm factors is set up and then the general 3jm factors computed recursively.
Calculation of j and jm symbols for arbitrary compact groups. II. An alternate procedure for angular momentum
Authors: Butler P. H. 
Published: December 2004

AbstractThe various orthogonality and sum rules which the 6j and 3jm symbols satisfy are sufficient to obtain the algebraic formulas for these symbols for SO3 ⊃ SO2. Character theory enters in that the j's and m's occurring in the various sums are given by the triangle rule

together with information on the symmetrized product and the branching, SO3 ⊃ SO2,

The resulting calculation is somewhat simpler, algebraically speaking, than previous calculations and has the pedagogical advantage that only the concept of an irreducible representation of a group is required, instead of the more elaborate concept of ladder operators.
Molecular symmetry and closed‐shell SCF calculations. I
Authors: Dupuis Michel , King Harry F. 
Published: December 2004

AbstractFor efficient integral evaluation, orbital basis functions are grouped into shells and integrals into blocks in the recently developed SCF program Hondo. This shell structure is ideally suited to the scheme of Dacre and Elder for using point group symmetry. An entire block of two‐electron integrals is eliminated if it is symmetrically equivalent to another block with a higher index (four label). Using the “petite list” of blocks of integrals, a skeleton Fock matrix is formed from which the true Fock matrix is generated by “symmetrization.” We prove two theorems which provide a clear and rigorous justification for this version of the Dacre–Elder procedure. We compare SCF calculations on the phosphorus molecule using Td symmetry with those using various subgroups of Td. The number of integrals computed is found to be approximately inversely proportional to the order of the group. Integral evaluation time and SCF iteration time are each linear functions of the number of integrals. The computer spends a negligible amount of time in executing symmetry‐related code, and the human effort involved is little more than picking the appropriate Schönflies symbol for the molecule.
Nonrigid molecule effects on the energy levels of XeF6
Authors: Trindle C. , Datta S. N. , Bouman T. D. 
Published: December 2004

AbstractThe geometric and electronic structure of XeF6 is not yet fully understood, even though most of the means of structure determination available to the chemist have been employed in an effort to describe the molecule. We suggest that many of the experimental anomalies and theoretical difficulties arise from the possibility of rapid polytopal rearrangement in this system, and put forward in this report an analysis which accommodates the experimental data. We show that easy rearrangement produces substantial splittings in rigid‐molecule energy levels, and with the aid of Longuet‐Higgins group theory developed expressly for flexible molecules, compute energy‐level schemes for a set of plausible rearrangement modes. By appropriate choice of parameters specifying the extent of splitting, we match the reported infrared and Raman spectra, and their temperature dependence. The puzzling “time lag” in the Raman (but not the infrared) spectrum is tentatively ascribed to selective relaxation of Raman‐active vibrational modes.
Completeness and linear independence of basis sets used in quantum chemistry
Authors: Klahn Bruno , Bingel Werner A. 
Published: December 2004

AbstractInfinite sets of functions in Hilbert space are characterized by their completeness properties and the extent of linear independence. Different measures of linear independence such as orthonormality, Gram's determinant, the special measure of linear independence, and the asymptotic dimension are related to each other and with the degrees of completeness such as overcompleteness, exact completeness, and incompleteness as far as possible.
AMOS and insulator‐to‐metal transitions
Authors: Dugay M. , Debarge G. 
Published: December 2004

AbstractWe describe an attempt to use the AMO method for the investigation of insulator metal transitions. One of the main features of the calculation is that it permits an extension of the AMO method to finite temperatures.The ability of the model to account for such transitions is emphasized for the special case of linear antiferromagnetic systems. We discuss possible generalizations by pointing out that there exists a connection with Peierls' model.
Molecular orbital studies of Al2F6 and Al2Cl6 using a minimal basis set
Authors: Curtiss L. A. 
Published: December 2004

AbstractAb initio minimal basis LCAO‐SCF molecular orbital (MO) calculations have been carried out on various structures of the AIF3 and AlCl3 dimers. A bridged D2h structure is found to be the most stable of the structures investigated for Al2F6. A similar structure was obtained for Al2Cl6 in agreement with the experimental evidence.
Formal theory of effective π‐electron hamiltonians
Authors: Brandow B. H. 
Published: December 2004

AbstractWe argue that the goal of developing a satisfactory general formalism for the justification of effective π‐electron Hamiltonians, as well as for ab initio calculation of their parameters (α, β, and γ), has now been achieved. The need for a fully linked many‐body formalism is emphasized; this feature requires a Rayleigh–Schrödinger (RS) type of degenerate perturbation theory. A number of apparently different degenerate RS perturbation formalisms are reviewed. Most of these formalisms are actually identical term‐by‐term, when their RS expansions are worked out explicitly; the formal relations that prove their complete equivalence are presented and discussed. One of these formalisms, a version developed by the author for related open‐shell problems in nuclear physics, is shown to be most convenient for many‐body applications. This is owing to the relatively simple and transparent nature of its general algebraic structure, which facilitates partial summation to infinite order. A simple and concise derivation is presented for the algebraic features of this preferred formalism, and its many‐body (linked cluster) aspects are briefly discussed. The recent development of a nonperturbative (coupled‐cluster) analog of this formalism is also described. Some practical issues are examined, including the choice of orbital basis. Illustrative numerical results are presented, based on the calculations of Iwata and Freed. Several remaining problems are described; these are both qualitative and quantitative in nature, and their resolution will require some detailed calculations.
Quantum‐Chemical and semiclassical calculations of intermolecular interactions of phospholipids
Authors: Frischleder H. , Lochmann R. 
Published: December 2004

AbstractBy use of empirical 0–1–6–12 atom–atom potential functions and the PCILOCC method intra‐ and intermolecular interactions of glycero–phosphoryl–ethanolamine model head groups in a planar layer crystal were calculated. Starting from investigations of the two‐dimensional energy‐contour diagrams the minima of energy as a function of all head group torsion angles were calculated using a gradient procedure. Within an interval of 15 kcal/mol above the energy of the global minimum we obtained about 30 local minima. These results demonstrate a high flexibility of the investigated phosphorylethanolamine head group in agreement with experiment. The ethanolamine moiety exists in enantiomeric conformations. With the torsion angles of the 0–1–6–12 energy minimization procedure PCILOCC calculations were carried out. These calculations yield the x‐ray conformation as the most stable one (unit‐cell stabilization energy = −36.3 kcal/mol). The PCILOCC as well as the potential function calculations show that the conformation of phospholipid head groups in layer crystals is determined by intramolecular as well as by intermolecular interactions with neighboring phospholipid molecules.
Conformational properties of biomacromolecules (proteins and nucleic acids) at low temperatures and the problem of interaction of biopolymers with water molecules and ions of dissolved salts
Authors: Andronikashvili E. L. , Mrevlishvili G. M. , Japaridze G. Sh. , Sokhadze V. M. 
Published: December 2004

AbstractMethods for determining the thermal measurements of biological objects have found ever increasing application in biology. This paper analyzes the temperature dependence of the heat capacity of biopolymer solutions between 4–400°K. Four distinct temperature regions were discovered, and experiments on these temperature intervals are given.
Quantitative approach to conformations of proteins
Authors: Popov E. M. 
Published: December 2004

AbstractA quantitative conformational theory of proteins is developed that enables one to predict the native structure of a protein from its amino acid sequence. The theory is based on the following principles: (1) the spatial structure and conformational properties of a protein are predetermined by its amino acid sequence; (2) the native conformation of a protein corresponds to the free energy minimum; (3) all interactions within a protein molecule are specified as short‐, mediumy‐, and long‐range types, interactions of different types being consistent with each other. The role of the short‐, medium‐, and long‐range interactions in the spatial organization of a protein globule is discussed, and a step‐by‐step analysis of amino acid sequences with gradually increasing lengths is presented. The proposed theory is based on a semiempirical computational method that involves quantitative evaluation of all pairwise atomic interactions within a protein molecule in an aqueous medium. Examples illustrating the suggested approach are presented.
Determination of the secondary structures of proteins by circular dichroism spectra. Calculation of the protein basic circular dichroism spectra for antiparallel and parallel β‐structures and β‐bends
Authors: Bolotina I. A. , Chekhov V. O. , Lugauskas V. Yu. 
Published: December 2004

AbstractNew “reference” circular dichroism spectra of α helix, β‐structure (both parallel and antiparallel), β‐bends, and the unordered form are obtained from circular dichroism spectra and x‐ray data for six proteins (myoglobin, lysozyme, lactate dehydrogenase, papain, ribonuclease, and subtilisin BPN′). Circular dichroism spectra for α‐helix and antiparallel β‐structure are similar to those for poly(Llysine). The circular dichroism spectrum of the parallel β‐structure is qualitatively similar to that theoretically calculated by Madison and Schellman. The circular dichroism spectrum of β‐bends is qualitatively similar to that theoretically calculated by Woody. The spectrum of the unordered form is close to that of the denaturated proteins. These “reference” circular dichroism spectra used for the analysis of the secondary structure of ten globular proteins (besides the six reference proteins D‐glyceraldehyde 3‐phosphate dehydrogenase, concanavalin A, cytochrome c, and insulin).
Polarization Green's function with multiconfiguration self‐consistent‐field reference states
Authors: Banerjee Ajit , Kenney John W. , Simons Jack 
Published: December 2004

AbstractThe polarization Green's‐function formalism in the superoperator notation of Goscinski and Lukman is re‐derived using a multiconfiguration self‐consistent‐field (MC‐SCF) reference state to establish the superoperator metric. The potential advantages of employing this more general reference state in Green's‐function theories and certain inherent weaknesses associated with the traditional Hartree–Fock or Rayleigh–Schrödinger perturbation theory reference state choices are briefly discussed. The Hermiticity of the superoperators is analyzed within the framework of the MC–SCF reference state. Using a concept of order appropriate for this reference state choice, explicit formulas and computational procedures for the implementation of this Green's‐function theory are presented and specialized to include terms consistent through second order.
Collinear collision of an atom with a homonuclear diatomic molecule
Authors: van Dijk W. , Razavy M. 
Published: December 2004

AbstractThe problem of collinear scattering of an atom from a homonuclear diatomic molecule is formulated in terms of a first‐order nonlinear matrix differential equation for the variable coefficient of reflection. For a homonuclear molecule when the target Hamiltonian is invariant under the parity transformation, only transitions between even states or odd states are possible. This selection rule reduces the number of open or closed channels that contribute to the reflection and transmission coefficients. But for numerical calculation, under the conditions of the problem, one can approximate the target Hamiltonian by the Hamiltonian of a displaced harmonic oscillator. In this approximation, the reflectional symmetry of the Hamiltonian is not preserved and transitions between any two levels of the target are possible. To simplify the problem further, the interaction between the projectile and the target is assumed to be a sum of two Gaussian terms. For this combination of the potentials the many‐channel interaction can be expressed analytically. By fitting the Lennard–Jones potential with a sum of two Gaussian potentials and solving the matrix differential equation, transition probabilities are obtained for the HeH2 collision. The numerical results are compared with the results found by Secrest and Johnson, and by Clark and Dickinson.
Exact‐exchange Hartree–Fock calculations for periodic systems. I. Illustration of the method
Authors: Pisani C. , Dovesi R. 
Published: December 2004

AbstractA scheme is presented for performing linear‐combination‐of‐atomic‐orbitals (LCAO) self‐consistent‐field (SCF) ab initio Hartree–Fock calculations of the electronic structure of periodic systems. The main aspects which characterize the present method are (i) a thorough discussion of both translational and local symmetry properties and the derivation of general formulas for the transformation of all the relevant monoelectronic and bielectronic terms under symmetry operators. (ii) The use of general yet powerful criteria for the truncation of infinite sums; in particular, the Coulomb electron–electron interactions are subdivided into terms corresponding to intersecting or nonintersecting charge distributions; the latter are grouped into shell contributions and the interaction is evaluated by multipolar expansions; the exchange interaction may be evaluated with great precision by retaining a relatively small number of two‐electron integrals according to a truncation criterion which fully preserves its nonlocal character. (iii) The use of a procedure for performing integrals over k, as needed in the evaluation of the Fermi energy and in the reconstruction of the Fock matrix, which is particularly simple because it employs partially intersecting small spheres as integration subdomains where linear extrapolation is admitted. A comparison is finally made of our fundamental equations in the critical SCF stage with those obtainable by a recent proposal which uses Fourier transforms to express Coulomb and exchange integrals.