bluuebunny/crossref_metadata_2025_split huggingface dataset:
Variation–iteration solution of the Hartree–Fock equations for atomic systems
Authors: Blakemore M. , Evans G. A. , Hyslop J. 
Published: December 2004

AbstractA recently proposed scheme based on the variation–iteration method is applied to the solution of the Hartree–Fock equations for atomic systems. The procedure depends on the repeated application of a Green's integral operator which involves a single numerical quadrature at each stage. The integrations are performed by means of a prescription described in a recent paper by the present authors and it is shown that the parallel philosophies of the self‐consistent field and the variation–iteration approaches combine quite naturally. Representative calculations are carried out on three‐and four‐electron systems and the flexibility of the proposed scheme is demonstrated by considering solutions of various forms of the SCF equations.
On the kashiwagi–sasaki generalization of the löwdin orthogonalization and the inverse vibrational problem
Authors: Stepanov N. F. 
Published: December 2004

AbstractBy use of the pseudo‐inverse matrix technique the generalization of the Löwdin orthogonalization, given by Kashiwagi and Sasaki, is shown to be valid in a case of singular metric matrices for two basis sets of functions. The application of the same idea to the inverse vibrational problem brings about new procedures for solving this problem in an iterative way.
Investigation of the convergence of an expansion of the screening‐charge density for impurity ions in semiconductors
Authors: Csavinszky P. 
Published: December 2004

AbstractIn theories of the potential of an impurity ion in semiconductors, the common starting point is an expansion of the Fermi–Dirac integral ℱ1/2 appearing in the screening‐charge density. In the present paper, a formal test of the convergence of the series for ℱ1/2 is carried out. The results show that, in the physically important limits of complete degeneracy and complete nondegeneracy, respectively, the series for ℱ1/2 do converge. This establishes the validity of nonlinear Poisson equations for the impurity‐ion potential, which can be solved approximately by making use of a variational principle.
Reduced first order density matrix for the Be ground state
Authors: Bunge Carlos F. 
Published: December 2004

AbstractA reduced first order density matrix for the Be ground state is computed from an extensive configuration interaction (CI) wave function. A sequence of increasingly accurate CI wave functions Φq converging towards the exact Ψ is used to assess the quality of the results which include approximate bounds for the overlaps 〈Φq|Ψ〉, electron–nuclear coalescence cusp data, Weinhold's overlap between density matrices, virial ratios, occupation number spectra, and some expectation values. The nuclear magnetic shielding constant and the molar diamagnetic susceptibility are determined with 2.0 and 1.5% of uncertainty, respectively.
Approximate exchange perturbation study of intermolecular interactions in molecular complexes
Authors: Sokalski W. Andrzej , Chojnacki Henryk 
Published: December 2004

AbstractIn an attempt to rationalize and improve an approximate exchange perturbation scheme related to the model of Murrell et al., more accurate approximations are introduced eliminating the use of empirical parameters. The total interaction energy has been evaluated as the sum of additive electrostatic, exchange, charge transfer, and dispersion contributions. It is proven that the assumption of intramolecular ZDO approximation is equivalent to the use of the Mulliken formula for intermolecular intergrals in the region of small overlap. It is also shown that the use of exact two‐center resonance integrals and atomic multipole expansion leads to a substantial improvement of results. The method has been applied to calculation of interaction energies of some model hydrogen‐bonded complexes of water, hydrogen fluoride, adenine, thymine, guanine, and cytosine using approximate all‐valence wave functions.
Electron correlation theories and their application to the study of simple reaction potential surfaces
Authors: Pople J. A. , Krishnan R. , Schlegel H. B. , Binkley J. S. 
Published: December 2004

AbstractThis paper serves two purposes. The first is to describe an implementation of the coupled cluster theory with double substitutions (CCD) previously developed by Cizek. The second is to apply this method and closely related fourth‐order perturbation methods to some simple molecules and reaction potential surfaces. These studies show that CCD theory gives results close to those of a Møller‐Plesset perturbation treatment to fourth order in the space of double and quadruple substitutions MP4(DQ). Addition of contributions from single substitutions at fourth order makes little change in predicted relative energies. Preliminary results on the potential surfaces for 1,2‐hydrogen shifts in C2H2, HCN, CH2O, and N2H2 are discussed and compared with previous studies.
Generalized‐molecular‐orbital theory: Simple multiconfiguration self‐consistent‐field method
Authors: Hall Michael B. 
Published: December 2004

AbstractEven after completing a multiconfiguration self‐consistent‐field (MCSCF) calculation, one must often include additional configuration interaction (CI) to obtain quantitative or semiquantitative results. There is some question of whether the prior MCSCF calculation is worthwhile, if additional CI is needed later. We have developed a new MCSCF computational method, which, because of our assumptions about the nature of the configurations, yields one Fock‐like operator for all the “filled” orbitals (high occupation numbers) and a second Fock‐like operator for all the “virtual” orbitals (low occupation numbers). Since there are only two matrices to build, our method is considerably faster than other MCSCF approaches. Because of these similarities to standard molecular‐orbital (MO) calculations, we have termed our approach generalized‐molecular‐orbital (GMO) theory. However, the “virtual” orbitals, unlike those of standard MO theory, are optimized to correlate the “filled” ones and can he used in a subsequent CI calculation. Results are presented for the correlation energy of H2O, the spectroscopic constants of N2, the singlet–triplet energy separations in CH2, and the nature of the chromium–chromium quadruple bond. Although these results are at a very low level of CI, the GMO approach appears to correct for the gross deficiencies of the single‐determinant SCF procedure.
Theoretical approaches to intramolecular electron transfer processes
Authors: Ratner Mark A. 
Published: December 2004

AbstractTheoretical issues that must be considered for calculating rates of intramolecular electron transfer are discussed. The process itself is well defined only if the ground state is well approximated by a localized electronic function (Robin–Day class I or II). Under those conditions, the linear‐response formalism yields the rate as the Fourier transform of the particle–hole propagator. We discuss the validity of electron propagators as approximations to particle‐hole propagators, and find that they are valid, for simple models, within the stability range of the Hartree–Fock procedure. The vibronic nature of the transfer process is stressed, and formal schemes are given for calculating direct and through‐bridge transfer rates. Outstanding difficulties, and applications to biological systems, are very briefly noted.
Geometric approximation to nuclear spin–spin coupling constants in the water molecule
Authors: Lazzeretti P. 
Published: December 2004

AbstractThe geometric aproximation is used within the framework of triple perturbation theory to evaluate the contributions to nuclear spin–spin coupling constants in the water molecule provided by the Fermi contact, the spin–orbit, and the spin–dipolar interactions. The results, obtained with SCF wave functions expanded over Gaussian basis sets of increasing quality, are compared with corresponding coupled Hartree–Fock estimates. The limits of the geometric approximation to coupling constants are discussed.
Are intermediate states responsible for certain specific properties of biological macromolecules?
Authors: Biczó G. , Lukovits I. 
Published: December 2004

AbstractA new electron structural mechanism is proposed for interpretations in molecular biology, in addition to the already existing theories of Szent‐Györgyi (semiconductivity) and Fröhlich (long‐range coherence). The hypothetical “intermediate‐” or “zigzag states” (ZZS) of solids are investigated by the recursion (transfer matrix) method. The physical reality of the ZZS is discussed up to an SCF DODS‐type theory and the necessity of additional less approximate investigations is emphasized. The possible role of ZZS in the explanation of: (i) translation in protein synthesis, (ii) energy and charge transfer processes, as well as (iii) initiation of protein formation is outlined.
Electron pair correlation energies for ZN2+
Authors: Jankowski K. , Malinowski P. , Polasik M. 
Published: December 2004

AbstractPair energies contributing to the correlation energies of the outer‐shell electrons (n = 3) as well as for the 1s2 and 2s2 pairs are computed for the Zn2+ closed‐shell ion by means of the variational‐perturbation method starting with the sum of one‐electron Hartree–Fock operators as the zeroth‐order Hamiltonian. The results allow an understanding of the electron correlation for pairs of electrons of the p and d type. For 3p3d pairs it has been found that the correlation energy for the singlet pair of 1D symmetry is lower than for the triplet pair 3D. The 3l‐3l′ correlation energies are compared with the MBPT results of Kelly and Ron for Fe. The total correlation energy of the outer shell is −1.032 a. u.
Mechanisms of electron transfer through proteins
Authors: Petrov E. G. 
Published: December 2004

AbstractThe analysis of modern physical mechanisms of electron transfer in proteins is given. The tunnel electron transfer and donor–acceptor electron transfer through conducting states of a protein chain are discussed in detail. The expressions for the values of the electron resonance interaction and the formulas for probabilities of electron transfer between vibronic levels of donor and acceptor states in the presence of “transverse” and “longitudinal” relaxation are given.
Carbon‐13 magnetic resonance study of lanthanide‐substituted muscle calcium binding parvalbumins
Authors: Nelson Donald J. , Theoharides Anthony D. , Nieburgs Andra C. , Murray Richard K. , Gonzalez‐Fernandez Federico , Brenner Daeg S. 
Published: December 2004

AbstractHigh resolution natural abundance carbon‐13 NMR has been used previously to examine molecular motions and conformational transitions in the muscle calcium binding parvalbumins isolated from mirror carp. [S. J. Opella, D. J. Nelson, and O. Jardetzky, J. Chem. Phys. 64, 2533 (1976) and D. J. Nelson, S. J. Opella, and O. Jardetzky, Biochemistry 15, 5552 (1976).] The carbon‐13 NMR spectrum of parvalbumin typically reveals the presence of a number of well‐resolved resonances from single‐carbon sites in the protein. Since accurate assignment of these resonances to specific carbon atoms in the protein is essential before these resonances can be employed as probes of local conformational events, an investigation of the origin of a number of single‐carbon resonances has been performed. Carbon‐13 NMR on Tb(III)‐ and Yb(III)‐substituted parvalbumin is the principal spectroscopic technique employed; however, results from terbium fluorescence spectroscopy, γ‐ray scintillation spectroscopy, and x‐ray difference Fourier analysis all contribute to single‐carbon site assignment. The principal conclusions drawn from the combined spectroscopic results are that: (1) the downfield carboxyl resonance at 184.6 ppm, previously attributed to Glu‐81, whose carboxyl group is involved in an internal ionic bond, originates rather from a carboxyl function coordinating the solvent exposed metal ion, (2) the carbonyl resonance at 168.9 ppm arises from Lys‐96, as was previously suggested; and (3) the 11.2 ppm resonance derives from the δ‐methyl carbon of Ile‐97, a resonance assignment that could not be made previously.
Quantum chemical and physicochemical influences on structure–activity relations and drug design
Authors: Kaufman Joyce J. 
Published: December 2004

AbstractQuantum chemical results will be presented on drugs, carcinogens, teratogens, and endogenous biomolecules using our new nonempirical ab initio MODPOT/VRDDO method, which incorporates as options to our ab initio LCAO‐MO‐SCF/CI programs ab initio effective core model potentials (MODPOT) permitting one to calculate only the valence electrons explicitly yet accurately and an integral prescreening technique (VRDDO, variable retention of diatomic differential overlap) especially effective for spatially extended molecules. For molecules of the size of those of interest the MODPOT/VRDDO calculations run an order‐of‐magnitude faster than with our own fast ab initio programs and still retain accuracy to the third decimal place for the valence electron properties. We have also just implemented a new efficient MERGE technique which allows us to reuse integrals from a common skeletal fragment and only to have to recalculate those for a new atom or group or a change in its position. Examples will be presented of the use of this technique on a carcinogenic polycyclic aromatic hydrocarbon and its metabolites. The pKa's, oil‐water partition, and drug distribution coefficients as a sensitive function of pH have been measured for a number of drugs as well as for relevant endogenous biomolecules. The pH dependence of the lipophilicities of such molecules has profound implication on appropriate use of such data in QSAR studies.
Role of entropy in formation of van der Waals complexes
Authors: Hobza P. , Čársky P. , Zahradník R. 
Published: December 2004

AbstractThe TΔS0 term has been estimated for various types of van der Waals molecules and its importance for the stability predictions has been demonstrated. With hydrogen‐bonded complexes the absolute values of ΔH0 and TΔS0 are comparable at 289°K. With true van der Waals molecules the TΔS0 term even represents a dominant contribution to ΔG0.
Charge transfer interactions between nucleic acid bases and strong acceptors
Authors: Sheina G. G. , Radchenko E. D. , Egupov S. A. , Blagoy Yu. P. , Orlov V. M. 
Published: December 2004

AbstractThe regularities of the donor‐acceptor interaction of nucleic acid bases and some of their derivatives with the strong acceptor 7,7,8,8‐tetracyanoquinodimethane (TCNQ) in thin amorphous films at 77 K were studied using UV and visible spectroscopy. Purine and pyrimidine bases are shown to form charge transfer TCNQ complexes whose band energies hνct = Ip − EA − (H* − H0) are linearly dependent of the donor ionization potential Ip. Calculated differences between the interaction energies of uncharged molecules and ion radicals, ΔE ≈ 3.0 eV, are in good agreement with experimental variations of the complex dissociation energies in the ground and excited states, (H* − H0) = 2.73 eV. Annealing of the films of complexes containing cytosine, 1‐methylcytosine, uracil, and caffeine leads to ordering of their structure and the formation of ion radical salts.
Some peculiarities of the electronic transfer in redox enzymes
Authors: Likhtenstein G. I. , Kotelnikov A. I. , Kulikov A. W. , Syrtsova L. A. , Bogatyrenko V. R. , Melnikov A. I. , Frolov E. N. , Berg A. I. 
Published: December 2004

AbstractThe new properties of clusters—the polynuclear Fe, Cu, Mo—containing metal‐protein complexes have been discussed. The properties arose as a result of strong electron interaction and of the multiorbitals system existence in which the orbitals energetically fall close together. The clusters are characterized by (i) a high electronic capacity, (ii) an ability to multielectronic transfer without essential rearrangement of nuclei configuration, (iii) a high degree of donor‐acceptor energetical levels fitting at tunnel transfer, (iv) high possibilities for avoiding of reaction pathways being quantum mechanically forbidded, and (v) an ability to provide smooth reaction energetic relief in coordinated sphere. The analysis of data on spin exchange between paramagnetic centers (binuclear transition‐metal complexes, nitroxile biradicals, triplet exited chromophores) showed that in the range of spinexchanged constants Kex = 1014−1 sec −1 of the distances between the centers r = 3−17 Å the approximate relation Kex =1017 exp (−2.3r)sec−1 takes place. This relation may be considered as a criterion of nonspecific electron density transfer through nonconducting medium. The quantitation of exchange triplet‐triplet energy migration permits one to estimate the degree of quantum‐mechanical electron density penetration through biological matrix. By means of measurement of spin‐lattice relaxation rate for oxidized primary donor in bacterial photosynthetic system—bacteriochlorophyll cation (BChl+ )—it is shown that the distance between BChl+ and primary acceptor (complex FeQ) is about 34 Å. The proposed two‐step photoelectron transfer model explains the effective charge separation by relatively slow tunnel recombination of the charges BChl+ FeQ−. As spin and Mössbauer labeling experiments showed the conformation mobility of surrounding protein and membrane matrix with frequency more than 107 sec−1 is required for photoelectron output from primary photosynthetic cell in chromotophores and reaction center to secondary acceptor.
Theory of the excitation spectrum of nondiagonal disordered systems
Authors: Onipko A. I. 
Published: December 2004

AbstractThe spectrum of a two‐component solid solution with a nondiagonal disorder is studied in the framework of the average T matrix method. For a one‐dimensional system in the nearest‐neighbor approximation the criteria for the system parameters are given such that at an in‐band resonance, one or two “impurity bands” may be realized, and the corresponding model calculation is performed. In the single‐site approximation an expression of the self‐energy part of a nondiagonal disordered system Green's function is found taking into account multiple occupancy corrections. The possibility of using it to describe a disordered system excitation spectrum and the calculation of state density moments are discussed.
Tautomerism of pyrimidine bases—uracil, cytosine, isocytosine: Theoretical study with complete optimization of geometry
Authors: Czermiński R. , Lesyng B. , Pohorille A. 
Published: December 2004

AbstractThe semiempirical MINDO/3 method with complete optimization of geometry is employed to calculate the electronic ground‐state properties (dipole moments and ionization potentials) and the energies of various tautomeric forms of uracil, uracil monoanion, cytosine, and isocytosine. The results are shown to be consistent with most of the experimental data. Accuracy of various quantum–mechanical methods is discussed. Particular attention is paid to the influence of the geometry optimization on energy differences between various tautomers. Some qualitative conclusions of biological importance are drawn from these calculations.
Quantum‐mechanical study of the interaction of glyoxal with arginine
Authors: Demoulin D. , Armbruster A‐M. , Pullman B. 
Published: December 2004

AbstractAb initio SCF computations indicate that the formation of an adduct of glyoxal to the guanidinium ion occurs in two steps. The first addition should occur on an NH2 group, rather than on NHCH3, and the formation of an unsymmetrical adduct IV is competitive and may be even favored over that of the symmetrical adduct III suggested by Takahashi. The barrier for that reaction is higher than for a similar reaction with guanine. The formation of a Schiff base between glyoxal and the guanidinium ion is disfavored because of the large endothermicity calculated for the process, much larger than that predicted for Schiff base formation with simple neutral or protonated amines.
Theory of electron transfer in donor–acceptor pairs
Authors: Zasukha V. A. , Volkov S. V. 
Published: December 2004

AbstractThe wave functions of donor–acceptor pairs before and after electron transfer are written as a product of the electron‐vibrational wave functions of the donor and acceptor with allowance for the change in the number of electrons on these particles by one after transition. In this approximation, the energy of the initial state is represented as a sum of the electron‐vibrational levels of the donor and acceptor and that of the final state as a sum of donor cation and acceptor anion levels. Formulas for the electron transfer probability of symmetrical and nonsymmetrical donor–acceptor pairs have been derived that express the dependence of this process on the ionization potential difference of the donor and the electron affinity of the acceptor, on the vibrational frequencies of these particles, and on temperature.
Electron‐conformational interactions and functioning of enzyme molecules
Authors: Volkenstein M. V. , Golovanov I. B. , Sobolev V. M. 
Published: December 2004

AbstractElectron‐configuration interaction is one of the most general physicochemical approaches used to explain an enzymes functioning. Studying enzymatic reactions requires investigation of the motions of nuclei on a certain PES. In order to obtain the shape of the PES numerous calculations of the total energy of the enzyme–substrate complex were calculated, and are presented in this paper.
Estimation of some parameters of the exchange mechanism of luminescence decay
Authors: Bazhin N. M. , Yakobson B. I. , Burshtein A. I. 
Published: December 2004

AbstractThe rates of variations of the relative yield (αη) and decay time (ατ) with the acceptor concentration (CA) have been calculated in the vicinity of CA = 0. On the basis of the equations obtained, the ratio αη/ατ has been found and a simple scheme of experimental data analysis proposed.
Transferable integrals in a deformation‐density approach to crystal orbital calculations. II
Authors: Avery John , Berg Erik 
Published: December 2004

AbstractA new method for calculating crystal orbitals in the Hartree‐Fock‐Slater approximation is proposed. The method makes use of x‐ray crystallographic measurements of the deformation density, and uses transferable integrals to treat the neutral–atom potentials. Methods for evaluating matrix elements of neutral‐atom potentials are discussed in detail, and in this connection, expansions of displaced Slater‐type orbitals in terms of modified spherical Bessel functions and Legendre polynomials are presented. Tables of transferable integrals (moments of the neutral‐atom potentials) are given for all the elements up to Z = 36, and tables of Fourier transforms of the neutral‐atom potentials are also presented.
Semigroups and the density matrix formulation of quantum mechanics
Authors: Green H. S. 
Published: December 2004

AbstractA formulation of quantum mechanics is presented based on the theory of semigroups and the associated enveloping algebras of functions defined on countable subsemigroups. The existence of a unique *‐involution is not assumed. The fundamental elements of a semigroup are identified with experimental precedures for the separation of subensembles from a given ensemble of experimental systems. Observables are represented as elements of enveloping algebras, and ensembles as density matrices within an enveloping algebra. The statistical properties of ensembles are expressed in terms of traces defined on the semigroup and its enveloping algebras. The elements and generators of the Poincaré group can be defined and interpreted in the usual way. A variety of applications is described, in which the theory of the density matrix plays an essential or effective role. Advantages associated with the resulting freedom from the limitations of Hilbert space are illustrated.