bluuebunny/crossref_metadata_2025 huggingface dataset:
Recent advances in simple valence‐bond theory and the theory of hybrid bond orbitals
Authors: Herman Zelek S.
Published: December 2004

The history of important developments in the theory of hybrid bond orbitals and its application in valence‐bond theory is reviewed. One of the salient points, the bond strength of a hybrid orbital, is defined as the value of the angular part of the orbital along the bond direction. Characteristic bond angles corresponding to maxima in the bond strength are presented for various basis sets. In order to alleviate computational difficulties in determining the bond strength for complex systems, an approximation to it, the pair‐defect‐sum approximation, is described. The results of an exhaustive test of the validity of this approximation are presented. Applicaitions of these ideas to coordinate chemistry are provided. Finally, the case is made for the continued viability of valence‐bond theory in this age of omnipresent computer terminals.
Computer‐generated formulas for some three‐center molecular integrals over Slater‐type orbitals
Authors: Jones Herbert W.
Published: December 2004

Exact computer‐generated formulas can be produced for each term of an infinite series that gives the value of three‐center Coulomb integrals over Slater‐type orbitals of the s‐type with equal screening constants. As a specific example, the Coulomb energy of an equilateral triangular arrangement of all 1s orbitals is calculated using seven terms of an infinite expansion to obtain an answer comparable to earlier work in elliptic coordinates. Generalization to all three‐center cases of this implementation of the Löwdin α‐function method is straightforward.
Application of the unitary group approach to crystal field theory
Authors: Zhenyi Wen
Published: December 2004

An analytical treatment for a strong crystal field in an octahedral symmetry by using the unitary group approach is given. It shows that the convenience of the unitary group approach is comparable with that of the Racah method.
Cumulative BK approximation as a method to select configurations for CI calculations of transition energies
Authors: Nascimento Marco Antonio Chaer
Published: December 2004

A cumulative Bk approximation is examined as a method to select configurations for CI calculations of transition energies where all the matrix elements are computed (full CI). The results obtained by this approach indicate that the transition energies are comparable to the ones obtained at the full CI level. Even for truncation errors of 1 mhartree, the transition energies differ from the full CI ones by less than 0.1 eV.
Transmission mechanisms of spin–spin coupling constants within the CHF approximation: Their study using inner projections of the polarization propagator
Authors: Engelmann A. R. , Contreras R. H.
Published: December 2004

A new method for separating differently transmitted components of spin–spin coupling constants is introduced which is suitable when couplings are calculated using the coupled Hartree–Fock (CHF) approximation via the polarization propagator. Contributions transmitted through different electronic subsystems are found choosing different subsets of orbitals with their corresponding virtual spaces for their description. Examples for two different channels of transmission are given, which are very well known among NMR spectroscopists, namely, components trasmitted via π‐electron systems, and “through‐space” interactions. In the first case a set of canonical orbitals is chosen, while in the latter noncanonical ones are used. For both cases, numerical results are given within the INDO approximation. These results are compared with others obtained previously by means of the partially restricted molecular orbitals approach. The possibility of using this method of inner projections of the polarization propagator in decomposing other second orther properties is suggested.
Theoretical studies of molecular adsorption on metal surfaces
Authors: Lundqvist B. I. , Hellsing B. , Holmström S. , Nordlander P. , Persson M. , Nørskov J. K.
Published: December 2004

A theoretical program in surface physics aiming at a description of different aspects of adsorption of atoms and molecules on metal surfaces is shortly reviewed.
Hindered internal rotations in Van der Waals molecules and molecular crystals
Authors: Briels W. J. , Tennyson J. , Claessens M. , Van Der Lee Th. , Van Der Avoird A.
Published: December 2004

The quantum dynamical behavior of the Van der Waals molecule (N2)2 and that of the ordered α and γ phases of solid N2 have recently been calculated, starting from the same ab initio N2N2 potential. By interpreting the results of these calculations we try to improve our understanding of the libration/internal rotation motions of the N2 monomers and the orientational order–disorder (α–β) phase transition. Some new results are presented and further (mean‐field and libron‐model) calculations are proposed which assess explicitly the intermolecular pair correlation effects caused by the anisotropic interaction potential.
Electron localization in a finite one‐dimensional chain
Authors: Tanaka Kazuyoshi , Nagaoka Masataka , Yamabe Tokio
Published: December 2004

The localization characteristics of the electronic wave functions in a finite one‐dimensional chain with the diagonal or the off‐diagonal disorder of the potentials have been studied. It has been shown that the eigenfuction at the frontier level is relatively “strong” against the temptation to localize caused by the existence of the random potentials. It has also been pointed out that the spatial behavior of the total density reflects that of the diagonal random potentials, but that under the off‐diagonal random potentials the total density is spatially uniform (completely extended).
Importance of exchange effects in the deformation of interacting ions
Authors: Gutowski Maciej , Ka̧lkol Małgorzata , Piela Lucjan
Published: December 2004

In calculations for liquids or solids one often considers a limited cluster of atoms or molecules in the electrostatic field of their environment. In this paper possibilities of improving this model by inclusion of the exchange deformation potential are shown. The reported numerical results for the LiF and NaF molecules as well as for the LiF and NaF perfect crystals indicate the importance of the exchange deformation effects.
Elastic forward scattering of high‐energy electrons and molecular electron density
Authors: Konaka Shigehiro
Published: December 2004

The elastic forward scattering of high‐energy electrons from molecules has been studied in the second Born approximation. An integral transformation has been adopted to evaluate the second Born integrals analytically without explicit use of molecular wave functions. In the high‐energy limit, the differential cross section for the forward scattering is expressed in terms of electric dipole and quadrupole moments, the second moment of charge distribution with respect to the molecular center, and transition dipole moments. All these quantities are shown to be computable from molecular electron densities in the ground state.
On some problems in quantum pharmacology I. The partition functions
Authors: Gomez‐Jeria J. S.
Published: December 2004

We performed an analysis of the drug‐receptor equilibrium constant, Ki = {QDiRQDi−1QR−1} exp (−Δεi/kT). It is shown that, for a group of nonrotating molecules we may consider the product of the partition functions as constant if log(mass) = constant for all the molecules.
Gradient expansion correction to the Dirac exchange term in statistical models for the Na atom with shell structure
Authors: Vinszky P. Csa , Vosman F.
Published: December 2004

In the study of the ground‐state binding energy of atoms, relatively more attention has been paid to the gradient expansion of the kinetic energy term of the Thomas–Fermi–Dirac model than to the gradient expansion of the exchange term of this model. Recently Shih and Shih, Murphy, and Wang have focused on this problem and calculated the first gradient expansion correction to the Dirac exchange term by making use of electron densities constructed from Hartree–Fock wave functions. In previous work, aimed to introduce the shell structure of atoms via a variational procedure using the energy density functional formalism, electron densities have been obtained for the Na atom within the Thomas–Fermi–Dirac model with and without the Weizsäcker and Hodges gradient expansion corrections to the kinetic energy term. In this paper we make use of the respective electron densities and calculate the first gradient expansion corrections to the Dirac exchange term. The results show that, for the Na atom, the magnitude of this correction is about 1% of the magnitude of the total binding energy.
The modified Hartree–Fock self‐consistent field equation
Authors: Deng Conghao
Published: December 2004

A one‐electron correlation operator is introduced into the Hartree–Fock self‐consistent field equation. The correlation operator is derived from the second‐order perturbation theory. Energies of atomic and molecular systems calculated from this modified Hartree–Fock equation are equal to that from second‐order perturbation of Hartree–Fock equation. The modified equation can also be solved self‐consistently by the LCAO approximation. We also presented the modified expressions for other operators.
Hydrogen‐like atomic orbitals: Addition and expansion theorems, integrals
Authors: Novosadov Boris K.
Published: December 2004

Analytical properties of hydrogen‐like atomic orbitals (HAO) that are used in the MOLCAO approach to the quantum theory of molecules have been studied. Addition and expansion theorems for HAO have been proved, both in coordinate and momentum representations. A close relation has been established between HAO and the reduced Bessel functions of half‐integer indices. New methods are suggested to calculate integrals for atomic and molecular form factors, and multicenter integrals, for the HAO basis in the MO LCAO theory.
Second‐ and higher‐order convergence in linear and nonlinear multiconfigurational Hartree–Fock theory
Authors: Olsen Jeppe , J⊘rgensen Poul , Yeager Danny L.
Published: December 2004

We discuss how the local convergence of Newton–Raphson and fixed Hessian MCSCF iterative models may be rationalized in terms of a total order of convergence in an error vector and a corresponding error term. We demonstrate that a sequence of N Newton–Raphson iterations has a total order of convergence of 2N and that a sequence of N fixed Hessian iterations has a total order of convergence of N + 1. We derive the error terms of a Newton–Raphson and a fixed Hessian sequence of iterations. We discuss the implementation of the fixed Hessian and the Newton–Raphson approaches both when linear and nonlinear transformations of the variables are carried out. Sample calculations show that insight into the structure of the local convergence of Newton–Raphson and fixed Hessian models can be based on an order of convergence and an error term analysis.
Electron–phonon coupling detection in infrared spectra of ion–radical salts based on dibenztetrathiofulvalen
Authors: Starodub V. A. , Drushlyak T. B. , Sleta L. A.
Published: December 2004

The electron–phonon coupling constants in a cation radical of dibenztetrathiofulvalen (DBTTF) have been calculated. To calculate the electronic structure of DBTTF in equilibrium and distorted structures use was made of the unrestricted Hartree–Fock method as π‐electron approximation for symmetrical vibrations of DBTTF. The results obtained are in good agreement with the experimental data on IR spectra of DBTTF‐based cation–radical salts.
Comments on energy minimization in subspaces of coordinates and on “chemical hysteresis”
Authors: Bálint I. , Bán M. I.
Published: December 2004

Using partial minimization of the potential energy function, the determination of the points of the reaction path does not often lead to unambiguous results. The reason is that the subspace of the coordinates where the minimization takes place cannot be arbitrarily chosen.
Symmetry breaking in the Hartree–Fock approximation for binuclear transition metal compounds—a theoretical investigation based on a variable model operator
Authors: Böhm Michael C.
Published: December 2004

The validity of the Hartree–Fock (HF) approximation in bis(π‐pentadienyl)dinickel (1) and in cyclopentadienyl‐allyl‐cyclobutadiene‐dinickel (2) has been investigated by means of the Thouless instability conditions in the computational framework of a variable model Hamiltonian. Singlet, nonsinglet (triplet), and nonreal instabilities in 1 and 2 have been studied as a function of the one‐electron resonance integral β and as a function of the one‐ and two‐center elements of the electron–electron interaction. The one‐center integrals of Coulomb (γ) and exchange‐type (K) have been modified by a multiplicative factor; the two‐center integrals (γ) have been calculated by means of an exponential interpolation formula with a variable decay amplitude. Additionally the Thouless conditions have been studied for nuclear deformations. The stability domain of the HF solution in the model space spanned by the variable INDO Hamiltonian has been analyzed. The nature of the many‐body interactions in the unstable region depends strongly on the parametrization of the model operator. HF instabilities in the high‐density region (long‐range forces) of 1 have their origin in individual particle–hole fluctuations while negative roots for short‐range forces (low‐density region) are similar to collective excitations in many‐body systems (strong off‐diagonal coupling). The opposite behavior is encountered in the Ni complex 2. The physical origin of these different types of correlation processes are analyzed in a simple two‐electron two‐orbital model. The nature of the HF fluctuations in 1 and 2, the importance of spatial and spin correlation, and the coupling of symmetry breaking of the electronic wave function with nuclear deformations are compared with the nature of phase transitions in solid‐state systems.
Stability properties of closed‐shell restricted Hartree–Fock solutions for electronic systems in the framework of the projected Hartree–Fock method and their utilization
Authors: Karadakov Peter , Castan̄o Obis
Published: December 2004

This paper contains a derivation of stability conditions for closed‐shell restricted Hartree–Fock (RHF) solutions in the framework of the projected Hartree–Fock (PHF) method. It is shown that for any projection operator whose choice is consistent with the symmetry of the RHF solution, one obtains four stability subproblems (one “singlet” and three “nonsinglet”). The stability conditions are specialized to several particular choices of the projection operator, including Löwdin's spin projection operator for the singlet case (this choice corresponds to the celebrated spin‐PHF method). The stability of closed‐shell RHF solutions for alternat π‐electronic systems in the framework of the alternant molecular orbital (AMO) method is also discussed. The theoretical considerations are illustrated by PPP calculations for several closed‐shell π‐electronic models. The results for even alternant conjugated hydrocarbons show that the AMO construction is suggested by the structure of the eigenvector pointing into the direction of steepest descent from the pertinent RHF solution in the framework of the spin‐PHF method. The minimization of the spin‐PHF energy expectation value in this direction in a number of cases allows that value to approach rather closely the energy values obtained in a fully variational spin‐PHF treatment and thus provides either a one‐parameter variational method which is shown to be superior to the one‐parameter AMO method or a convenient initial approximation to the solution of the spin‐PHF equations.
Groups of simple graphs
Authors: Flurry R. L.
Published: December 2004

The group structure of simple graphs can be found by factoring the adjacency matrix into cyclic blocks. The blocks correspond to permutational subgroups of the graph. The overall group structure is a product of the independent subgroups.
Momentum density interpretation of the H(1s)–H+ long‐range force
Authors: Koga Toshikatsu
Published: December 2004

The recently proposed method of using momentum densities for interatomic interactions is applied to the long‐range force between the ground‐state hydrogen atom and the proton, and the results are compared with those from using the position density based on the electrostatic Hellmann–Feynaman theorem. A new physical interpretation of the long‐range force is obtained, which is complementary to that in position space. It is found that some perturbative changes in the position density do not accompany changes in the momentum density.
Analysis of the inadequacies of some semi‐empirical MO methods as theories of structure and reactivity
Authors: de Bruijn Steef
Published: December 2004

Essential defects of present‐day semiempirical methods (CNDO/2, CNDO/S, MINDO/3, MNDO) can be identified on two levels. First, the formalism shared by nearly all these methods treats parametric expressions, supposed to refer to Löwdin orthogonalized orbitals, as transferable. At best, this idea is approximately valid for the β functions for strong bonds. Second, the expressions used for these approximately transferable functions are based on inadequate arguments. As a result, the errors in many terms of the energy expression amount to several electron volts. The mechanism of error compensation through the parametrization is investigated in detail and shown to be surprisingly flexible, at least for some combinations of errors. Some cases of systematic absence of compensation are identified, and for weak interactions, e.g., those responsible for the water dimer, the semiemperical expressions appear to be so irrelevant that a detailed analysis of the causes of their failure is no longer possible. Minimal requirements for a correct semiempirical approach are given in the MORBIT rules. The two‐center one‐electron integral β is to be replaced by a function introduced by Mulliken; an efficient approximation for this expression is proposed.
Bond length alternation in cyclic polyenes. IV. Finite‐order purturbation theory approach
Authors: Paldus J. , Takahashi M.
Published: December 2004

The problem of bond length alternation in linear extended systems with conjugated double bonds is examined on a simple cyclic polyene model using finite‐order many‐body perturbation theory. Three different partitionings of the model Hamiltonian are employed, namely the Hückel, Møller–Plesset, and Epstein–Nesbet partitionings. The dependence of correlation energy on bond length alternation is examined in each case, showing an almost constant behavior of Møller–Plesset and Epstein–Nesbet perturbation energies in contrast to a strong dependence on distortion, favoring undistorted structures, for the Hückel perturbation and UHF correlation energies. The origin of an unphysical character of the Hückel perturbation theory and the inappropriateness of the UHF approach for the problem considered are pointed out. The second‐ and third‐order Møller–Plesset and also the second‐order Epstein–Nesbet perturbation theories yield results which are similar to those obtained with the RHF method and which clearly favor the bond length alternating structures, leading to the bond length distortion of about 0.045 Å and to the stabilization energy per site (relative to the equidistant geometry) of about 0.03 eV.
Some reactive properties of chlorooxirane, a likely carcinogenic metabolite of vinyl chloride
Authors: Laurence Patricia R. , Politzer Peter
Published: December 2004

We have carried out a computational study of the reactive properties of chlorooxirane, the metabolically produced epoxide of vinyl chloride that is believed to be a direct‐acting carcinogenic form of this molecule. An ab initio SCF‐MO procedure (GAUSSIAN 70) was used to compute the energy requirements for stretching the CCl and both CO bonds (SN1 reactivity) and to determine the course of the epoxide's possible SN2 reactions with ammonia, taken as a model for nucleophilic sites on DNA. The epoxide was assumed to be protonated; both the oxygen‐ and chloro‐protonated forms were considered. At each step along the various reaction pathways, the structure of the system was reoptimized. For the oxygen‐protonated epoxide, the C1O bond has a significantly lower energy barrier to stretching than does the C2O. (The carbon bearing the chlorine is designated C1.) However, both are very much higher than that of the CCl bond in the chloro‐protonated form, confirming our earlier finding of the relative weakness of this bond. In the SN2 processes involving ammonia, intermediate complexes are formed with both carbons of the oxygen‐protonated epoxide, the C2‐complex being the more stable. However, the most stable ammonia complex occurs at C1 of the chloro‐protonated epoxide. Our calculated results, both the energies and also the geometry changes, allow us to propose two possible mechanisms for the formation of the 7‐N‐(2‐oxoethyl) derivative of guanine that has been observed to be the major in vivo DNA alkylation product of vinyl chloride and has been suggested as possibly being responsible for its carcinogenicity. One of these mechanisms is SN1 and starts with the chloro‐protonated epoxide; the other is SN2 and involves the oxygen‐protonated form.
Theoretical calculation of vertical ionization potentials of B2H6 by means of ΔESCF procedures
Authors: Pan Dai‐Kai , Zhang Bang‐Rong
Published: December 2004

The vertical ionization potentials (IPS) of B2H6 are calculated by means of the ΔESCF procedure, within the scheme of ab initio LCAO‐MO‐HF‐SCF. The basis set used is LEMAO‐3G. The scaling factors of the various atomic orbitals for the ground state and for the various hole states are optimized independently. The iteration procedure is specially designed to avoid the changes of the symmetry of the remaining occupied orbitals. The 1 ag (B1s) hole is found to be localized. The vertical IP of the 1 ag electron is calculated to be 196.5 eV, in fair agreement with experimental value. The D2h symmetry is thereby broken and reduced to C2V symmetry. The valence holes are found to be delocalized. The calculated vertical IPS are: 21.781, 16.974, 14.842, 14.389, 13.599, and 12.380 eV for the 2ag, 2b1u, 1b3u, 1b2u, 3ag, and 1b3g electrons, respectively. The agreement with experimental values is much better than the Koopmans' values. All these results are in favor of the concept that the nature of the convelent bond should be considered as a result of the mutual interactions and mutual conditioning between the wave nature of the electronic motion on the one side and the various attractive and repulsive factors on the other side.
Ab initio and pseudopotential calculations on the singlet and triplet states of the disilyne isomers
Authors: Kalcher Josef , Sax Alexander , Olbrich Gottfried
Published: December 2004

Ab initio SCF as well as pseudopotential calculations were performed for determining equilibrium structures and relative stabilities of several disilyne isomers. For the singlet state there are only two structures, the bridged and the silavinylidene carbene, which correspond to minima on the energy hypersurface. The most stable of the six isomeric structures investigated is the bridged conformer in the 1A1 electronic state, followed by the silavinylidene carbene in the 1A1 and 3A2 electronic states. Inclusion of electron correlation by MRD‐CI calculations has no qualitative influence on the relative stabilities found in the SCF calculations.
Statistical representation of atomic systems structure. I. Total binding energy as a function of the relative information content
Authors: Fraczak Marek
Published: December 2004

A nonorbital representation of the many‐electron atomic systems is proposed. It is obtained by considering a certain equivalence class of mappings ƒ: ϵ → Π from the set ϵ of N electrons into the set Π of Z protons. Total binding energy of the systems (Z = 3,4,…, 18; Z − N = k = 0, 1,…, 8) arranged according to the Periodic Table criterion, turns out to be the linear function of ZIr, where Ir is an information functional related to our representation.
Method of recurrent construction of Löwdin spin‐adapted wave functions. IV. Matrix element evaluation problem
Authors: Panin A. I.
Published: December 2004

For the overlap integrals of arbitrary Löwdin functions, both analytical and recurrence formulas are obtained. Matrix elements of the unitary group generators and their products are analyzed in detail. A strategy for employing the Löwdin functions for evaluating matrix elements of arbitrary products of the fermion creation–annihilation operators is discussed.
Effective valence bond hamiltonian for the cations of conjugated hydrocarbons
Authors: Gadea Xavier , Maynau Daniel , Malrieu Jean‐Paul
Published: December 2004

Previous papers have developed magnetic effective valence bond (EVB) Hamiltonians for the neutral states of π systems; following the same philosophy and using either the quasidegenerate perturbation theory or wave operator formalism, other EVB+ Hamiltonians may be derived for the lowest states of positively charged conjugated hydrocarbons. Their effective operators introduce both charge delocalization (which plays a much more direct role than for the neutral molecules) and effective spin ordering effects. The transferability and efficiency of this EVB+ Hamiltonian are tested by comparison with the exact solutions of the full PPP Hamiltonians. The resulting spectrum is much more dense than in the Koopmans' description; non‐Koopmans' states are identified for a few molecules. The applicability of the model to the d bands of metals is discussed in terms of the β/U ratio.
An ab initioLCAO‐MO study of the substituent effect in benzenoid systems: Meta‐ and para‐substituted benzoic acids
Authors: Böhm Stanislav , Kuthan Josef
Published: December 2004

Theoretical substituent effects upon the acidity of the title compounds were investigated using STO‐3G basis sets augmented by diffuse functions for all oxygen atomic centers of the probe groups. The results are discussed in connection with the available gas‐phase data within a novel conception of attractive and repulsive substituent effects.
Relation between total energy, electronic potential at the nucleus, and chemical potential of positive ions
Authors: Balbás L. C. , Zorita M. L. , Alonso J. A.
Published: December 2004

Simple density functional theory gives the following relation between the energy EZ, N of an ion of nuclear charge Z and N electrons, the potential V(0) created at the nucleus by the electronic cloud, and the chemical potential μ

Using Hartree—Fock values for V(0) and μ, this equation has been tested in several isoelectronic series with 3 ≤ N ≤ 28. The importance of the term 3Nμ/7 increases as the degree of ionization increases.
Clebsch–Gordan coefficients for the group chains O ⊃ T ⊃ D2 and Td ⊃ T ⊃D2 by the method of projective representations
Authors: Herzig P.
Published: December 2004

The spinor representations for the groups forming the chains O ⊃ T ⊃ D2 and Td ⊃T ⊃ D2 are constructed as projective representations. The Clebsch–Gordan coefficients are then calculated using a standard method. Projective factor systems, irreducible representations, canonical bases, and tables of Clebsch–Gordan coefficients are given. The subduction from O to D3 is discussed.
General harmonic oscillator integrals by the operator method
Authors: Özkan İlker
Published: December 2004

The operator technique with a minimum of commutator algebra is employed to calculate matrix elements of any number of operators between distorted, displaced harmonic oscillator wavefunctions. The results are valid for multidimensional integrals, and regardless of the extent of the Duschinsky effect. General recursion relations useful in machine calculations are given. The formalism is illustrated for the well‐known one‐dimensional Franck–Condon integrals.
N2‐time‐dependent SCF scheme
Authors: Adamowicz Ludwik , McCullough E. A.
Published: December 2004

A new procedure for the Fock matrix operator construction is proposed. Its application for RHF calculations on diatomic molecules using Slater orbital basis sets shows that the computation time for the new SCF procedure is proportional to the square of the basis set size.
Reactive properties of trans‐dichlorooxirane in relation to the contrasting carcinogenicities of vinyl chloride and trans‐dichloroethylen
Authors: Laurence Patricia R. , Proctor Timothy R. , Politzer Peter
Published: December 2004

Our objective in this work is to gain insight into the contrasting carcinogenic activities of vinyl chloride (definitely carcinogenic) and trans‐dichloroethylene (apparently inactive). The initial metabolic step for each molecule is believed to be epoxidation of the double bond, and there is evidence indicating that for vinyl chloride, this epoxide (chlorooxirane) is its ultimate (direct‐acting) carcinogenic form. This article presents the findings of a computational study of the reactive properties of trans‐dichlorooxirane (the epoxide of trans‐dichloroethylene). An ab initio SCF‐MO procedure was used to determine the energy requirements for stretching the CO and CCl bonds (SN1 reactivity) and to study the epoxide's SN2 interactions with ammonia, taken as a model nucleophile. The starting points were the oxygen‐ and chlorine‐protonated forms of the epoxide. The structure of the system was reoptimized at each step along the various reaction pathways. The results of this work are compared to an analogous earlier study of the reactive properties of chlorooxirane. The chlorineprotonated CCl bonds are found to have much lower energy barriers to stretching than do the oxygen‐protonated CO bonds. In the SN2 processes, intermediate complexes are formed with ammonia by both the oxygen‐ and the chlorine‐protonated epoxides; the latter complexes are the more stable. Based on our results, we propose two mechanisms (one SN1 and the other SN2) whereby trans‐dichlorooxirane can interact with N7 of guanine to produce an adduct analogous to one formed by chlorooxirane, which has been found to be the primary in vivo DNA alkylation product of vinyl chloride and to which has been attributed the carcinogenicity of the latter. Overall, trans‐dichlorooxirane is found to be chemically more reactive than chlorooxirane; this may help to account for the much lesser carcinogenic and mutagenic activities of trans‐dichloroethylene, since the epoxide may be reacting with other cellular nucleophiles before it reaches the key site(s) at which the carcinogenic or mutagenic interaction would occur. We also offer some speculations concerning other possible factors related to the differing carcinogenicities of vinyl chloride and trans‐dichloroethylene, such as ease of epoxide formation and the likelihood of oxygen protonation.
Statistical computer‐aided calculation of molecular integrals
Authors: Fabbri M. , Perondi L. F.
Published: December 2004

An improved Monte Carlo procedure is described for the evaluation of molecular integrals, which is particularly suitable for multicenter and/or two‐electron calculations. The method is almost independent of the complexity of the atomic orbitals involved, and the results can be obtained with an uncertainty which is fairly adequate for most applications and with very moderate waste of computer resources. In the version presented here there is only a restriction concerned with the positive value of some functions involved, as described in the text, but this possibility does not arise in much of the practical work. An example with a two‐dimensional exchange integral is worked out in detail.
Convergence properties of Hartree–Fock SCF molecular calculations
Authors: Natiello Mario A. , Scuseria Gustavo E.
Published: December 2004

Hartree‐Fock equations are viewed as nonlinear algebraic equations that can be solved iteratively. Provided we assume the existence of a solution, valuable properties of convergence may be assessed. The close connection between convergence of the SCF procedure and stability properties of the solution is shown from a nonapproximate standpoint. The convergence features of level‐shifting convergence‐forcing techniques are analyzed. The connection between this nonlinear algebraic approach and the related gap equation is displayed and the example of the restricted Hartree‐Fock hydrogen molecule is discussed.
EOM/Propagator ionization potentials: Effect of an extended operator manifold on the predicted valence‐shell ionization spectra of ethane, ethylene, and acetylene
Authors: Baker Jon
Published: December 2004

The orthogonalized direct diagonalization (ODD) method is used to predict the valence‐shell ionization spectra for the series of hydrocarbons ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) within an EOM/propagator formalism. Both (a) third‐order and (b) higher‐order (with h5 terms in the excitation operator manifold) calculations are presented in order to illustrate the effect of the extended operator manifold on the predicted valence‐shell spectra. It is shown that the major effect of the h5 manifold is concentrated in its “occupied” part. Along with a general shifting of the main lines for outer valence ionizations to lower values (together with a slight reduction in pole strength), it is found that the effect of the h5 manifold in the inner valence region is dependent on the system being studied.
Hartree–Fock calculations on quartet s, p, d, and f states of boron iii
Authors: Lunell Sten , Öster Per
Published: December 2004

Hartree‐Fock energies are reported for about 20 low‐lying quartet states of B III. A comparison with experimental transition wavelengths shows that Hartree‐Fock energies can be helpful to rule out erroneous assignments but that, in general, their accuracy is insufficient for definite confirmation of proposed assignments.
Compliant fields for molecular interactions: Water dimer and formic acid dimer
Authors: Murthy A. S. N. , Ranganathan Shoba
Published: December 2004

The redundancy‐free internal valence compliance constants of open‐chain water dimer and formic acid cyclic dimer have been determined by the combined use of the CNDO/Force method and the compliance constant formalism. The final compliant fields of these dimers have been refined with the help of experimental frequency data.
One‐Parameter double‐zeta atomic functions for the hartree–fock energies of helium isoelectronic sequence
Authors: László I.
Published: December 2004

The double‐zeta atomic functions are characterized by the nuclear charge z of the two‐electron atomic system. The Hartree–Fock total energies and the corresponding orbital energies are calculated using various atomic wave functions for the helium isoelectronic sequence. The expectation values rn of various wave functions are also examined. It is found that the accuracy of our one‐parameter double‐zeta functions corresponds to the accuracy of the usual five‐parameter double‐zeta functions.
Relativistic total energy of heavy atomic ions: Dimensionality dependent scaling
Authors: March N. H.
Published: December 2004

The scaling property of the relativistic total energy E (Z, N) established by Marconi and the writer for heavy positive ions with N electrons and atomic number Z is demonstrated, by setting up the self‐consistent relativistic Thomas–Fermi equation in d dimensions, to be a special case of the scaling property

α being the fine structure constant.
He2+2: A comparison between Hartree–Fock and density functional techniques
Authors: Ventura Oscar N. , Bartolucci J. Paolo , Sosa Ramón M.
Published: December 2004

A comparison of Roothaan‐Hartree–Fock methods (both restricted and unrestricted) with density functional ones (LCAO‐Xα and cellular MS‐Xαβ) is made using as test case the He22 + molecular ion. It is shown the analogy that exists between RHF and symmetry‐adapted LCAO‐Xα potential energy curves, as well as between UHF and symmetry‐unconstrained LCAO‐Xα ones. The influence of symmetry adaptation on the overall behavior of the potential energy curve is also discussed. Finally, the difference in the behavior of the LCAO‐Xα and cellular MS‐Xαβ calculations is explained as an artifact of the space partitioning in the latter technique. It is concluded that LCAO‐Xα method is superior to cellular MS‐Xαβ because it requires less effort to reach the same results and that the general behavior is similar to UHF, although the former affords a better equilibrium bond distance and a worse energy barrier than the latter.
Limiting behavior of partition functions of models with quadratically dependent energy spectra and consequences regarding theoretical study of chemical reactivity
Authors: Slanina Zdeně;k
Published: December 2004

The conventional partition functions of models with quadratically dependent energy spectra (e.g., translation in a box or free internal rotation), which are widely used in literature, exhibit an incorrect limit behavior in the region of low temperatures and/or small values of the characterizing molecular parameter. In particular, this leads to the absurd value —∞ for the contribution of these motions to the entropy term at these limit conditions. This incorrect behavior appears to be due to the replacement of rigorous summation by integration in conventional derivation of these partition functions. The derived improved practical formulas with the correct limit behavior indicate that it will be necessary to carry out quite substantial numerical corrections in the case of many reactivity characteristics reported in the literature. Particularly, the improved formulas should be applied with (small) molecular complexes at the typical temperatures of their observation, e.g., van der Waals molecules with free internal rotation. Simultaneously discussed is the possibility of application of the functional relation valid for a rigorous partition function of the considered type of energy spectrum. The reasoning in the case of the translational partition function indicates that, in principle, it can depend on the vessel shape. Possible consequences for thermodynamics of the ideal gas are analyzed.
Theory and mechanism of electron transfer in a condensed medium at high temperatures with allowance for change in vibrational frequencies
Authors: Zasukha V. A. , Volkov S. V.
Published: December 2004

An expression for electron transfer rate has been obtained through the solution of a time wave equation by the variational method by defining the wave function as a linear combination of functions corresponding to electron localization on the donor and on the acceptor. A dependence of electron transfer on temperature, on the electronic and vibrational characteristics of the system has been derived. An activation energy temperature‐variation effect has been obtained. It has been proved that many‐electron transfers are impossible.
Variational cellular method for polyatomic molecules: SF6
Authors: Kinoshita Dina L. , Ferreira Luiz G. , De Siqueira Manoel L.
Published: December 2004

This is the third paper on the cellular method for polyatomic systems. We show how to deal with nonspherical Coulomb potentials. We also show how to modify the variational expression for the energy eigenvalues so as to obtain a faster convergence in the angular momentum series for the wavefunctions. We apply both techniques to the self‐consistent calculation of SF6. Contrary to what we obtained in CH4 and SiH4, the cellular method cannot yield the correct equilibrium interatomic distance in the present case. The calculated ionization potentials are in the correct order but are all shifted by 2–3 eV. This shift is attributed to the wrong expression for exchange correlation.
Measures of stochasticity for nonstationary initial preparations of coupled oscillators
Authors: Brickmann Jürgen , Hofmann Paul , Schmidt Peter C.
Published: December 2004

The contributions of different eigenstates of a nonlinearly coupled oscillator system to the expansion of a local wave packet are analyzed from an information theoretical point of view. Such a wave packet can be considered as a nonstationary vibrational state of an electronically excited manifold of a molecule after Franck–Condon type initial preparation. The distributions of these contributions are compared to their individual stochastic ideals using Ruch's concept of “mixing distances.” The stochastic ideals are constructed via a probability diffusion process between neighboring states of the original distributions, representing an initial preparation corresponding to a Hamiltonian with only irregular eigenstates. Gaussian minimum uncertainty wave packets as initial states in a two‐dimensional nonlinear oscillator system with classically regular and chaotic energy ranges are studied numerically. It is found that distance measures, partly reflecting the “mixing distance” of a distribution from its stochastic ideal, show a large fluctuation in the classical regular energy range and a small fluctuation in the range where most of the classical trajectories move chaotically. This indicates that for this type of initial preparation process the actual location of the initial state in space plays the dominant role for the dynamics in the low‐energy range while for wave packets starting near the dissociation energy of the model system this location becomes unimportant.
Ab initio quantum‐mechanical calculations on trifluoromethylamine
Authors: Heaton M. M. , Mills D.
Published: December 2004

Extensive ab initio molecular‐orbital calculations were carried out on trifluoromethylamine (TFM) to elucidate changes in geometry and electronic structure upon fluorination. The calculations show that the decomposition of CF3NH2 is slightly endoenergetic, and the heats of atomization of CF3NH2 and CH3NH2 show decreased stability of the species upon fluorination. Characteristic of CF3NH2 is a highly polar, strong, short CN bond. More limited calculations were carried out on CF3OH and CH3OH, and the electronic structure of CF3OH is found to be generally similar to that of CF3NH2. The reduced basicity of the fluorinated amine cannot be ascribed to the inductive effect; the enhanced acidity of the fluorinated alcohol reflects the weakening of the OH bond. No evidence leads to a confirmation of the existence of nitrogen–fluorine hyperconjugation in the fluorinated amine.
An MC‐SCF procedure using an orthogonal transformation represented by a direct sum of two‐by‐two rotations
Authors: Igawa Akira
Published: December 2004

N molecular orbitals are transformed to K(≦N) molecular orbitals, which are appropriate for an orbital optimization by two‐by‐two orthogonal rotations among themselves. This transformation is determined by using the second‐order structure of the variational energy surface. The optimum K/2 rotation angles are exactly determined. This helps to ensure the reliability of convergence. K/2 two‐by‐two rotations are repeated among these K transformed orbitals. Then, two‐electron molecular integrals are calculated by transformations among only 16 elements and by a permutation among K4/8 elements. Calculational time is considerably reduced compared with that of more usual transformations. Test calculations have been carried out on the ground state of CO. It shows that this procedure brings the calculation to the local region rapidly and reliably.
Usage de l'Algèbre de Lie su (n) dans l'Etude des Systèmes Quantiques à n Etats. V. Application aux Systèmes composés
Authors: Van Groenendael Augustin , Tillieu Jacques
Published: December 2004

L'usage de l'algèbre de Lie su (n), proposé dans les articles précédents, est adapté a l'étude d'un système formé de deux sous‐systèmes, respectivement à nA et nB états. On montre comment diverses notions (opérateurs‐densités réduits, problèmes de corrélation, équations d'évolution globales et réduites) peuvent être précisées grâce au formalisme introduit.
First derivatives of correlated wave functions by a matrix‐oriented method: Preliminary application to molecular dipole and quadrupole moments
Authors: Jasien Paul G. , Dykstra Clifford E.
Published: December 2004

A direct, matrix‐oriented procedure in the context of the self‐consistent electron pairs (SCEP) method has been implemented for the evaluation of first derivatives of the energy of a variational configuration interaction wavefunction. This has been applied in calculating dipole and quadrupole moments of the correlated charge distributions of several small molecules. As already established, the analytical differentiation being carried out yields properties slightly different than those calculated by integration with the one‐electron density. The matrix‐oriented analytical differentiation is computationally competitive with construction of the one‐electron density.
Quantum‐chemical study of the relation between electronic structure and pA2 in a series of 5‐substituted tryptamines
Authors: Gomez‐Jeria J. S. , Morales‐Lagos D. , Rodriguez‐Gatica J. I. , Saavedra‐Aguilar J. C.
Published: December 2004

We have analyzed the dependence of the serotonin receptor binding affinity on the electronic and steric reactivity indexes for a group of 5‐substituted tryptamines. The approaches employed are a new nonempirical Quantitative Structure—Activity relationship approach and multiple regression analyses. The results suggest that the variation of the receptor binding affinity in 5‐substituted tryptamines is related to the variation of the net charge of two atoms and to the steric bulk of the N‐substituent. A receptor model is proposed.
Bond length alternation in cyclic polyenes. VI. Coupled cluster approach with wannier orbital basis
Authors: Takahashi M. , Paldus J.
Published: December 2004

The role of electron correlation effects on the bond‐length alternation in linear metalliclike systems, as modeled by cyclic polyenes CNHN, N = 2n = 4v + 2, v = 1,2,…, is examined using the coupled cluster approach in the localized Wannier basis formalism. A recently developed approximate coupled pair approach which accounts for connected quadruply excited clusters is employed together with various truncation schemes for the localized doubly‐excited cluster components. It is found that for the physical value of the coupling constant, the electron correlation has only a very slight effect on the potential energy curves, yielding almost the same values for both the magnitude of the bond‐length alternation and for the stabilization energy relative to the symmetric equidistant structures as the restricted Hartree‐Fock theory. This is in contrast to a strongly correlated region where the correlation effects stabilize the undistorted non‐alternating structures. Different mechanisms of the bond‐length alternation or Peierl's distortion as implied by a simple Hückel Hamiltonian and by the Pariser‐Parr‐Pople Hamiltonian models are also pointed out.
Symmetry aspects of diatomics‐in‐molecules (DIM) calculations: Construction of spin‐adapted bases
Authors: Vojtík Jan
Published: December 2004

A new procedure for construction of spin‐adapted DIM bases is described. The procedure is based on the direct diagonalization approach, is quite general, and seems to be especially suited for computer implementation. The structure of the bases obtained by the procedure appears to be favorable for performing symmetry operations necessary in setting up the Hamiltonian matrices in semiempirical applications of the DIM method.