bluuebunny/crossref_metadata_2025_split huggingface dataset:
Analysis of the inadequacies of some semi‐empirical MO methods as theories of structure and reactivity
Authors: de Bruijn Steef 
Published: December 2004

AbstractEssential 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

AbstractThe 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

AbstractWe 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

AbstractThe 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

AbstractAb 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

AbstractA 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

AbstractFor 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

AbstractPrevious 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

AbstractTheoretical 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

AbstractSimple 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

AbstractThe 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

AbstractThe 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.
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

AbstractOur 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

AbstractAn 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

AbstractHartree‐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

AbstractThe 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

AbstractHartree‐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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractAn 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

AbstractThis 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.