bluuebunny/crossref_metadata_2025_split huggingface dataset:
QSAR and Mechanistic studies on the genotoxic compounds including environmental effects
Authors: Miertuš S. , Frecer V. , Májeková M. 
Published: January 2005

AbstractThe effects of a proximate condensed environment as the solvent and cellular structured patterns (biopolymers, membranes, etc.) play an important role in determination of the courses of molecular processes in biology. We present here the background of methods developed for such an environmental effects estimation combining the continuum and discrete models. Their applications within theoretical studies into the mechanisms of carcinogenic action of alkylating N‐nitrosocompounds are shown. The results given cover four different areas, namely the quantitative structure‐activity relationship, mechanistic studies into their metabolic activation reactions, interactions of the ultimate carcinogens with DNA, and finally their genetic consequences.
Normally ordered Franck–Condon operator: A new approach for the calculation of Frank–Condon factors
Authors: Hong‐Yi Fan , Zaidi H. R. 
Published: January 2005

AbstractThe idea of a Franck–Condon (FC) operator is introduced, and its normally ordered form is obtained through the newly developed technique of “integration within an ordered product of operators (IWOP).” It is shown that the FC operator leads to a new approach for the calculation of FC factors. The results of existing theories are viewed, and the connection between the FC operator and the “squeeze‐operator” is pointed out.
Structure and electronic structure of polyacene
Authors: Kertesz Miklos , Lee Yong S. , Stewart James J. P. 
Published: January 2005

AbstractIt is shown that infinite long polyacene chains may have three energetically close but structurally distinct isomers (a symmetrical, sym, form and two lower symmetry forms: one with double bonds in a trans and another isomer with double bonds in a cis pattern). The energetics is based on solid state MNDO theory. We discuss that the symmetrical form has a substantial energy gap Eg in the Hartree–Fock approach owing to exact exchange terms, which are nonlocal. Broken symmetry Hartree–Fock (HF) solutions for polyacene are also described. An angularly distorted structure suggested earlier on Jahn–Teller grounds is found to be energetically not favorable.
Local energy as an indicator of the accuracy of wave functions and probability densities for helium
Authors: Deal Walter J. 
Published: January 2005

AbstractThe local energy is examined as an indicator of the accuracy of approximate wave functions for the ground state of helium. It is observed that at a given point (1) an inaccurate local energy may or may not correspond to an inaccurate value of the wave function or probability density, but (2) a value of the local energy within 0.1 a.u. of the ground‐state energy corresponds to a value of the approximate wave function or probability density within about 10% of that for the ground‐state wave function.
Scattering of fast electrons and X‐rays from molecules: CH4 and C2H2
Authors: Tripathi A. N. , Smith Vedene H. , Thakkar Ajit J. 
Published: January 2005

AbstractSelf‐consistent‐field (SCF) wave functions are used to calculate cross sections for the elastic and inelastic scattering of fast electrons and x‐rays from CH4 and C2H2 molecules. The effects of basis set choice and free rotation on these cross sections are investigated. The utility of an approximate scheme to correct SCF inelastic cross sections for the effects of electron correlation is examined. The probability density for the interelectronic distance, or radial intracule density, is obtained and discussed.
OH stretching force constant in associated methanol species and the cooperativity effect
Authors: Shivaglal M. C. , Singh Surjit 
Published: January 2005

AbstractThe effect of molecular interaction on the OH stretching force constant of methanol (MeOH) is reported for its associated species. The various electron donors (D) and acceptors (A) considered include organic molecules such as methanol, dimethylether, acetone, acetonitrile, dimethyl formamide, pyridine, and ions such as F−, Cl−, Li+, and H+. The variation in the OH stretching force constant of MeOH…︁D species on interaction with the electron acceptor such as in the species is explained on the basis of the cooperativity effect. (CE). The effect is discussed in terms of the relationship CE = (ΔF/F) × 100, where ΔF is the reduction is force constant of the hydrogen‐bonded OH stretching mode of the associated methanol species MeOH…︁D when the lone pair electrons on oxygen of the methanol molecule are involved in hydrogen bonding with A, and F is the hydrogen‐bonded OH stretching force constant of the species when the lone pair electrons are free. The cooperativity effect (CE) is found to increase with increasing electron acceptor and electron donor capacities of A and D. The calculated force constants are compared with the experimental results.
Finite chain approximation for linear and nonlinear polarizabilities of polydiacetylene and polybutatriene
Authors: Kirtman Bernard 
Published: January 2005

AbstractStatic longitudinal polarizabilities αzz and cubic hyperpolarizabilities γzzzz are obtained by the FPT‐INDO method for finite chain C4N+2H2N+4 (N = 1–15) models of polydiacetylene and polybutatriene. For the acetylenic structure the onset of saturation first becomes evident at about C14C18 for the linear polarizability and C22C26 for the cubic term. The values per C4H2 unit are accurately extrapolated to the infinite chain limit. In the case of the butatrienic structure, no saturation is observed through C54.
The hyperfine coupling constants of 19F: An ab initio MRD‐CI basis set study
Authors: Karna S. P. , Grein F. , Engels B. , Peyerimhoff S. D. 
Published: January 2005

AbstractThe isotropic (aiso) and dipolar (Adip) hyperfine coupling constants of 19F were obtained from MRD‐CI wave functions using a variety of basis sets. In series I, increasing numbers of d functions were added to a 5s4p contracted Huzinaga/Dunning basis. In series II, the 5s3p basis set was uncontracted in several steps until 9s5p was reached, to which were added from one to three d‐polarization functions. CI parameters (selection thresholds and the number of reference configurations) were also varied. A study of the R dependence of aiso and Adip was performed. The best values obtained at Re are 260 G for aiso and 308 G for Adip, compared with experimental values of about 280 G for aiso and 320 G for Adip.
Potentials of an alternative scheme for ab initio polymer band structure calculations. Illustration on the chain of hydrogen atoms
Authors: Fripiat J. G. , Delhalle J. , André J. M. , Calais J. L. 
Published: January 2005

AbstractAn alternative scheme for ab initio polymer band structure calculations based on a Filon‐type quadrature is proposed. This scheme avoids the explicit calculation and the storage of the “troublesome” Fourier transforms of the LCAO density matrix elements and is a first step towards a better control of the convergence of the different lattice sums appearing in the configuration space LCAO‐SCF‐CO method. The potential of the proposed technique is illustrated by a minimal basis set calculations on an infinite chain of H atoms.
Role of doubly excited states in calculations of optical activity
Authors: Volosov Andrey 
Published: January 2005

AbstractIt is shown that doubly excited states play an important role in calculations of the optical activity of molecules with well‐conjugated electron systems, such as the DNA bases. In some significant cases it is necessary to include a large number of excited states in the configuration interaction (CI) to obtain a reliable, converging result. A new version of the CNDO/OPTIC method, which includes doubly excited states in the CI, is proposed. As an application, the electric transition moments in different pyrimidines are considered. The calculated results agree with experimental data and results obtained from ab initio calculations and INDO calculations using doubly excited states in the CI.
Graphical techniques in the configuration interaction approach based on pure slater determinants
Authors: Wasilewski J. 
Published: January 2005

AbstractA graphical approach to the configuration interaction in the basis of pure Slater determinants is presented. The formulation based on the spin‐separated two‐slope graph (SSTSG), enabling the selection of determinants with the fixed Ms value, has a direct relation to the well‐known concept of the group‐function product. The commonly used excitation criterion and the spatial (Abelian) symmetry properties are analyzed in terms of the graph's internal structure. The Slater formulae for the Hamiltonian matrix elements between determinants, in the particle‐hole formalism and in the spin‐separated form, are related to different classes of loops within graphs. Some aspects of implementation within both the matrix‐element‐driven (ME) and integral driven (ID, direct) CI algorithms are discussed. The presented formulation, of a general complete active space (CAS) CI type, is a basis of the Graphical Determinantal Configuration Interaction (GDCI) computer program.
Ultraviolet photoelectron and absorption spectra of substituted phenols
Authors: LaFemina John P. 
Published: January 2005

AbstractThe spectroscopically parametrized CNDO/S3 model is used to provide a quantitative description of the valence electron photoemission and ultraviolet absorption spectra of a series of substituted phenols and anisoles. The CNDO/S3 density of valence states was used to determine the molecular conformation of these molecules via analysis of gas‐phase photoemission spectra. These conformations, in turn, were used to quantitatively determine the ultraviolet absorption spectra.
A representation of the McClelland method of graph splitting. Stack graphs
Authors: Shen Mingzuo 
Published: January 2005

AbstractMcClelland's rules on graph splitting can be represented using the generalized graph notation. Generalized graphs are edge‐ and vertex‐weighted graphs, which are becoming important to chemical problems. By this the McClelland method of graph splitting has a wider range of applications. “Stack graphs” are constructed from identical “base graphs” by connecting corresponding vertices from one base to another. Their eigenvalues are related to the eigenvalues of the base graph. Two‐ and even three‐layered graphs may be used as a simple model for the inter‐ring interaction in a cyclophane.
The PPP‐CI scheme as a fundamental physical model
Authors: Del Re Giuseppe 
Published: January 2005

AbstractParametrized computational procedures and simplified physical models of molecular reality are often related to one another but are by no means the same thing. With special reference to work by Koutecky and by Cizek, Paldus et al., it is shown here that the PPP model rests on a simple but physically sound electrostatic picture of a molecular π system, and its intrinsic difference from an independent particle model is emphasized. As an illustration of the heuristic power which makes physical models powerful tools of theoretical research, ground state charge transfer GSCT, in situ molecular electronegativity, and the principle of electronegativity equalization are derived within the PPP‐CI scheme, no use being made of molecular orbitals, and the assumptions on which the CT theory rests are made explicit.
Localization in aromatic and conjugated hydrocarbons. Shape studies on canonical PPP one‐electron eigenfunctions
Authors: Pipek J. , Varga I. , Nagy T. 
Published: January 2005

AbstractThe localization properties of the π‐electron system of some aromatic and conjugated hydrocarbon molecules have been investigated. The π orbitals have been obtained using a standard PPP‐SCF procedure. The shape analysis was carried out using the novel concept of the relation between the localization measure and structural entropy. According to our investigations the π‐electron system of these aromatic molecules is more localized than expected. The eigenstates seem to show one‐dimensional characteristics with a decay rate generally faster than exponential. Linear chains, on the other hand, have rather delocalized orbitals. As a byproduct we have found that in certain systems some states exhibit step‐function–like behavior, similar to bond‐centered charge density waves.
Modification of DEWAR‐PI to include ring strain
Authors: Dewar Michael J. S. , Dennington, Roy D. 
Published: January 2005

AbstractPrevious work has shown that the heats of formation of unstrained conjugated molecules can be reproduced with surpring accuracy by a semiempirical SCF MO treatment (DEWAR‐PI) based on the Pariser‐Parr‐Pople (PPP) π SCF MO approximation. The original version failed to allow for ring strain. This deficiency has now been remedied in a new version (DEWAR‐PI2).
The half‐projected Hartree‐Fock model for computing thermally “forbidden” pericyclic reactions and biradical processes. I. Formulation and results for some singlet biradical species
Authors: Olivella Santiago , Salvador Jose 
Published: January 2005

AbstractThe possible use of a half‐projected Hartree‐Fock (HPHF) wave function correlating a single pair of electrons, being defined as a linear combination of two unrestricted Slater determinants which are identical except that all the α and β spin functions are interchanged, is investigated in relation to the computation of the potential energy surfaces involving singlet biradical or biradical‐like species. The results of calculations, within the framework of the MINDO/3 semiempirical Hamiltonian, for a number of prototypical biradical molecules indicate that, in general, the HPHF model provides a geometrical and energetical description that is equivalent to that obtained from a conventional two‐configuration self‐consistent‐field wave function.
How to find a saddle point
Authors: Smith Colin M. 
Published: January 2005

AbstractThis paper explains a method for finding saddle points on a multidimensional surface and shows how it may be used to define saddle‐point seeking curves that have properties similar to well‐known orthogonal trajectories. It is shown that a gradient extremal is a special case of one of these curves, and its chemical significance as the path, defined by local criteria, which starts from a stable structure and leads to a transition state, is discussed briefly in relation to the intrinsic reaction coordinate. It is emphasized that this theory gives a natural method for locating points that have Hessians of similar structure to those of transition states.
Analytical expressions and recursion relations of two‐center harmonic oscillator matrix elements of arbitrary functions
Authors: Drallos P. J. , Wadehra J. M. 
Published: January 2005

AbstractThe matrix elements of various analytical functions f(X), X being the internuclear separation, are required for the description of transition probabilities and other molecular properties. These matrix elements can be conveniently estimated by assuming vibrational wave functions of two relatively dispalced linear harmonic oscillators of arbitrary frequencies to represent the vibrational levels of two electronic states of a molecule. Using this assumption, analytical expressions for the matrix elements of an arbitrary analytical function f(X) are obtained. Useful recursion relations among these matrix elements are derived and an elegant graphical representation of the recursion relations is obtained. These graphical representations are utilized to obtain new more general recursion relations among matrix elements of the arbitrary function f(X).
Optimum structures and vibrational frequencies of (SIC)2 clusters
Authors: Fitzgerald G. B. , Bartlett Rodney J. 
Published: January 2005

Abstract(SiC)2 clusters occupy an intermediate place between the well‐studied clusters Cn and Sin. Of the five closed‐shell isomers investigated in this paper, the rhombic form is found to be the lowest in energy at the MBPT(4) level. Analytic SCF and MBPT(2) second derivatives are used to predict vibrational frequencies for the isomers.
Triplet‐singlet splitting of the alkali metal clusters: Example of the Li6 clusters
Authors: Koutecký J. , Boustani I. , Bonačića‐Koutecký V. 
Published: January 2005

AbstractThe influence of the geometry on the specifics of the electronic structure of Li6 clusters is studied in detail, since planar and three‐dimensional lithium hexamers are found to be of comparable stability. The ab initio CI investigation of the Li6 isomers yields almost degenerate lowest singlet and triplet states for certain cluster geometries. Simple criteria for the energy gap between the lowest triplet and singlet state are derived, and their applicability is demonstrated.
Ab initio calculations of nuclear quadrupole coupling constants of low‐lying rovibrational levels in the X1Σ+ and a1Σ+ states of all isotopic species of LiH
Authors: Paidarová I. , Vojtík J. , Češpiva L. , Š;vrda J. , Špirko V. 
Published: January 2005

AbstractThe 2H, 6Li, and 7Li quadrupole coupling constants of the low‐lying rovibrational levels in the X1∑+ and A1∑+ electronic states of 7Li2H, 6Li2H, 7Li1H, and 6Li1H are calculated from molecular wave functions which explicitly describe nuclear motion. Except for the lithium coupling constants in the A1∑+ state, the vibrational dependence of the nuclear quadrupole coupling is found to be significant for all the studied isotopic species. On the other hand, the rotational dependence appears to be important only for the deuteron coupling constant of 7Li2H and 6Li2H in the A1∑+ state. Special attention is paid to relative magnitudes of the Li and D quadrupole coupling constants in the 7Li2H and 6Li2H isotopic species. The information about the relative magnitudes and their changes with vibrational excitation is used to outline a theoretical approach to the interpretation of the hyperfine structure of the spectra of these species.
Transfer of electronic excitation energy in partially disordered polymer chain
Authors: Pálszegi Tibor , Kyseľ Ondrej 
Published: January 2005

AbstractA new form of the generalized master equation for electronic excitation transfer in a partially disordered polymer chain was derived. Within this equation, in order to differentiate the transfer in ordered parts of the partially disordered polymer chain, we decomposed the memory function of electronic excitation into several parts. These parts are memory functions associated with various types of subdynamics of electronic excitation. The partially disordered polymer chain where the disorder is caused by conformational defects is discussed.
Molecular integrals in the generalized hylleraas–CI method
Authors: Lu Yannan , Huang Zuqia 
Published: January 2005

AbstractIn the generalized Hylleraas–CI method, the original correlation factor rvij is multiplied by a Gaussian geminal. Using the approach of generating functions, the general formulas of molecular integrals in this method are derived over Cartesian Gaussian orbitals. From differentiations of the generating functions, the expanding length in the incomplete Gamma functions is reduced, and some cancellations presented in other approaches are avoided. Preliminary calculations for H2 and H2—H2 systems are carried out over STO‐3G basis. The results are encouraging.
A population analysis based on hybrid orbitals
Authors: Ming Li 
Published: January 2005

AbstractBy a way of partitioning of Mulliken overlap populations, the hybrid orbitals of central atoms and ligands are obtained by the CNDO method. The dipole moments of some hybrids are calculated using the hybrid orbitals obtained, and they are mostly in accordance with experimental values. In addition, overlap integrals may be decomposed into components, and then σ, π, and δ hybrid orbitals can be obtained.
Applicability of multi‐reference many‐body perturbation theory to the determination of potential energy surfaces: A model study
Authors: Zarrabian S. , Paldus J. 
Published: January 2005

AbstractThe recently written CI‐based multi‐reference many‐body perturbation theory (MR‐MBPT) program package is exploited to study a simple ab initio minimum basis set model involving four hydrogen atoms in a rectangular configuration. This model was examined earlier by several authors using both coupled cluster (CC) and finite‐order MBPT approaches. Here we present the MR‐MBPT results up to the 50th order and examine the effect of various shifting techniques on the convergence behavior of this approach. It is shown that in contrast with CC methods, both single and MR finite‐order MBPT potential energy calculations are plagued with convergency and intruder state problems, which can be particularly severe when the latter approach is employed for non‐degenerate situations.
Approximating quasi‐particle density functional calculations of small active clusters: Strong electron correlation effects
Authors: Beznosjuk S. A. , Minaev B. F. , Dajanov R. D. , Muldakhmetov Z. M. 
Published: January 2005

AbstractA new constructive approach for deriving a quantum field chemistry (QFC) is proposed. As a matter of fact, the approach is a direct application of the concept of spontaneously broken symmetry of a free‐electron‐field vacuum to the exact definition of a condensed‐state chemical microstructure. The main idea is to identify the properly modified ground states of the vacuum with ground states of some compact quasi‐molecular systems condensed in a set of “kink”‐bounded molecular {vα} subspaces. Phase transitions of the electron vacuum are as usually expressed in terms of quantum order parameter set {ϕα1(x) = [ρ(x)]1/2 exp(iλα)}, which defines a single electron densities ρα1(x)=nα(x)/Nα (here Nα=∫nα(x) dx) in exact molecule ground states ρα1(x)=|α1(x)|2. The order parameters are obtained by the self‐consistent procedure of minimizing the ground‐state energy Eα=Nαϵα−1(N) for each open molecular “compacton” with respect to the number of electrons Nα and an average single‐electron energy ϵα−1(Nα). Account is taken of topological definitions of the various molecular constituents: atoms, atomic functional groups, molecules, and their clusters. The stability of diverse clusters is investigated by the method of approximating quasi‐particle density functional (AQDF). It gives particular attention to the description of peculiar intermolecular clusters (Mn) composed with single‐atom molecules (M1=A). Such clusters may be used to simulate some active centers that bear the responsibility for strong effects of nonlinearity and dissipation in condensed states. Some results of AQDF‐calculation of small active Rh‐clusters (n=2, 3, 4) are taken under consideration to illustrate that such clusters resemble neither fragments of bulk solids nor molecules in a gas.
The virial theorem scaling model for estimating the charge sensitivities of hydrogens in molecules
Authors: Streszewski Marcin , Nalewajski Roman F. 
Published: January 2005

AbstractA simple variational model of hydrogens in molecules is presented, using the virial theorem (Fock) scaling of the wave function to account for the orbital relaxation due to a change in the number of electrons. The resulting interpolative formulas for the energy allow realistic predictions of the spin‐nonpolarized or spin‐polarized hardness (electron repulsion) parameters and other sensitivities of the hydrogen systems in molecules, including the realistic bare nuclei limit (N → 0) data.