bluuebunny/crossref_metadata_2025_split huggingface dataset:
Alkylation of DNA bases by carcinogenic N‐nitrosamine metabolites: A theoretical study
Authors: Duncan Richard H. , Davies G. S. 
Published: January 2005

AbstractN‐Nitrosamines are believed to act as carcinogens by alkylating DNA in their ultimate carcinogenic forms which are produced by metabolic activation. Alkylation at certain oxygen sites in DNA, described as “promutagenic,” appears to be of particular significance for mutagenesis and cancer, as indicated by experimental findings. This theoretical study deals with two factors involved in the alkylation of these promutagenic oxygen sites by N‐nitrosamine ultimate carcinogens. The first is the competition between alkylation at the promutagenic O6‐guanine and O4‐thymine sites and that at the nonmutagenic N7‐guanine site, which is here related to the degree of participation of cationic ultimate carcinogens as compared with neutral ultimate carcinogens. Parent dialkylnitrosamines are classified structurally according to their degree of cationic ultimate carcinogen participation and preference for promutagenic alkylation. The second factor is the thermodynamic facileness of alkylation at the O6‐guanine and O4‐thymine sites. Heats of alkylation by candidate ultimate carcinogens are calculated here for numerous parent dialkylnitrosamines. Finally, these two factors are jointly considered in an attempt to correlate them with experimental carcinogenic potency of the parent nitrosamines. Out of the patterns of correlation observed, light is shed on mechanistic factors likely to be involved in the modulation of parent carcinogenic potency.
Graphical unitary group approach to spin–spin interaction
Authors: Kent R. D. , Schlesinger M. , Shavitt I. 
Published: January 2005

AbstractA detailed exposition of spin–spin operator matrix elements is presented in the context of the graphical unitary group approach (GUGA) to atomic and molecular physics and quantum chemistry. A compendium of subgraph types and formulae is given. Aspects of computer implementation within the structure of the Columbus CI programs is discussed.
Eigenvector and eigenvalues of some special graphs. IV. Multilevel circulants
Authors: Lee Shyi‐Long , Luo Yeung‐Long , Sagan Bruce E. , Yeh Yeong‐Nan 
Published: January 2005

AbstractA multilevel circulant is defined as a graph whose adjacency matrix has a certain block decomposition into circulant matrices. A general algebraic method for finding the eigenvectors and the eigenvalues of multilevel circulants is given. Several classes of graphs, including regular polyhedra, suns, and cylinders can be analyzed using this scheme.
Valence bond approach exploiting Clifford algebra realization of Rumer–Weyl basis
Authors: Li Xiangzhu , Paldus Josef 
Published: January 2005

AbstractA detailed algorithm is described that enables an implementation of a general valence bond (VB) method using the Clifford algebra unitary group approach (CAUGA). In particular, a convenient scheme for the generation and labeling of classical Rumer–Weyl basis (up to a phase) is formulated, and simple rules are given for the evaluation of matrix elements of unitary group generators, and thus of any spin‐independent operator, in this basis. The case of both orthogonal and nonrothogonal atomic orbital bases is considered, so that the proposed algorithm can also be exploited in molecular orbital configuration interaction calculations, if desired, enabling a greater flexibility for N‐electron basis‐set truncation than is possible with the standard Gel'fand–Tsetlin basis. Finally, an exploitation of this formalism for the VB method, based on semiempirical Pariser–Parr–Pople (PPP)‐type Hamiltonian and nonorthogonal overlap‐enhanced atomic orbital basis, and its computer implementation, enabling us to carry out arbitrarily truncated or full VB calculations, is described in detail.
Eigenvalues of single‐cycle class‐sums in the symmetric group
Authors: Pauncz Ruben , Katriel Jacob 
Published: January 2005

AbstractExplicit expressions for the eigenvalues of the class sums [(p)(1)n−p]n, p = 2, 3,…,14, of the symmetric group Sn are presented. Partial results are given for the eigenvalues corresponding to arbitrary p.
The measure cone: Irreversibility as a geometrical phenomenon
Authors: Busch Paul , Ruch Ernst 
Published: January 2005

AbstractThe statistical state space is discussed in terms of the geometry of normed real vector spaces with particular reference to the novel concept of direction distance. Specialization to the geometry of the measure cone and the correspondingly specialized concept of mixing distance suggest strong mutual relationship as is shown subsequently in physically required generality. Thereby, the phenomenon of irreversibility attains an interpretation that seems to be the canonical mathematical background for classical and nonclassical statistical physics.
On rotations as special cases of unitary transformations with some applications to the theory of spin
Authors: Löwdin Per‐Olov 
Published: January 2005

AbstractSome problems in elementary geometry are approached from the point of view of linear algebra and generalized to the theory of linear spaces of finite or infinite dimensions having a positive definite binary product. The angle ω between two elements of the linear space is defined from the concept of length by means of the cosine‐theorem. A rotation is then defined as a special case of a unitary transformation moving all elements the same angle ω, except that under certain circumstances, some elements may stay invariant. In the former case, one speaks of a rotation around an “external axis,” and in the latter case, of a rotation around an “internal axis” defined by the invariant elements. It is shown that the finite rotations U of both types may be expressed in the simple exponential form U = exp(iωm), where the “generator” m in the former case is an operator satisfying the relation m2 = 1, and in the latter case, m3 = m. The structure of the group of finite rotations in the former case is clarified in some detail. As an illustration of the theory, some applications to the three‐ and two‐dimensional spaces as well as to the theory of spin are given. The coupling between the ordinary three‐dimensional rotations and the spinor transformations is considered in somewhat greater detail.
The Fix–Heiberger procedure for solving the generalized Ill‐conditioned symmetric eigenvalue problem
Authors: Jungen Martin , Kaufmann Karl 
Published: January 2005

AbstractWe review a formalism introduced by Fix and Heiberger in 1972 for solving the generalized (or nonorthogonal) eigenvalue problem for ill‐conditioned symmetric matrices and we discuss its application in quantum chemistry. A few examples dealing with the calculation of high Rydberg orbitals are presented.
Quasi‐Relativistic MS‐Xα approach to the electronic structures of protactinium halide ions of [PaX8]4− type
Authors: Nour Safouh , Mehadji Cherifa , Chermette Henry , Krupa Jean‐Claude 
Published: January 2005

AbstractThe recently developed inclusion of the spin‐orbit interaction into the quasi‐relativistic version of the MSXα method has been applied to the ThX4:Pa4+ (X ≡ Cl, Br) doped system. A new interpretation of the optical spectrum is given.
An optimized orbital transformation able to reproduce partially prefixed overlaps
Authors: Gianinetti Ermanno , Vandoni Ida 
Published: January 2005

AbstractGiven a function space spanned by a basis {ƒi}, we are interested in finding another basis {gi} for which the overlaps (gi | gi) assume arbitrarily prefixed values in a subset ℬ︁ of the full set of the pairs of indices (i, j). The other overlaps are let free. We show how it is possible to perform this linear transformation ƒi → gi minimizing the “distortion” J = Σi(gi – ƒi | gi – ƒi).
Molecular dynamic structure and dimerization of polyacetylene
Authors: Lü Tianxiong , Tachibana Akitomo , Yamabe Tokio 
Published: January 2005

AbstractThe vibronic interaction in an (A)2N chain (e.g., polyacetylene) has been studied within the Hückel framework. A Hückel framework scheme for calculating the parameters of molecular dynamic structure, i.e., the linear and quadratic orbital vibronic constants (OVCs), has been presented. Selection rules for the OVCs in this scheme have been obtained and discussed by using graph theory and group theory, under various boundary conditions. A Hückel noncoupling rule has been concluded and discussed. The dimerization of polyacetylene has then been discussed based on the molecular dynamic structure. It has been shown that for a finite undoped (A)2N chain the occupied orbital energy εn at the unified configuration has a negative slope in direction of Q2n. As a result, there exists a net Hellmann–Feynman force leading to dimerization. When the chain length goes to infinity, the slopes and forces tend to zero. However, a significant negative curvature in potential surface may occur in the direction of Q2n, due to the two‐phonon coupling of the π‐electrons, which could also induce the dimerization. These can be interpreted as the results of the hidden C4N+2 symmetry and the imaginary degeneracy in an (A)2N chain, according to the graph theory for molecular orbitals. Thus, the dimerization of an (A)2N chain actually destroys its hidden symmetry of C4N+2 and reduces its imaginary degeneracy.
Density of states due to donor‐pair molecules in three‐ and two‐dimensional semiconductor systems
Authors: Costa E. A. Guimarães , De Brito Mota F. , Ferreira Da Silva A. 
Published: January 2005

AbstractThe density of states is calculated for a random distribution of donor‐pairs of hydrogenlike impurities in three‐ and two‐dimensional systems. Recent investigations of the hydrogen molecule in the alternant–molecular–orbital approximation are here extended. We found that the lowest excited state 1Σu (i.e., H+H−), which is optically connected to the ground state, plays a relevant role in the absorption spectra of semiconductor systems.
Broken symmetry in the electronic structure of the ferrocene molecule
Authors: Cook David B. 
Published: January 2005

AbstractAttempts to calculate the metal–ring distance in the ferrocene molecule using a well‐tried minimal basis reveal serious deficiencies in the minimal‐basis SCF model of the electronic structure of organometallics. It is found that the lowest state of the molecule in this approximation is a triplet and that there is a whole manifold of “states” in the ground‐state region that have broken spatial symmetry and high‐spin multiplicity.
Variation–iteration method for one‐dimensional two‐electron systems
Authors: Katyurin Svyatoslav V. , Glinkin Oleg B. 
Published: January 2005

AbstractThe variation–iteration method of Svartholm has been applied to the momentum‐space Schrödinger equation for one‐dimensional two‐electron systems. The first and second iteration momentum‐space wave functions have been evaluated in analytical forms. The momentum representation of the exact Hartree–Fock ground‐state wave function is chosen as the initial function. The influence of electron correlation on the distribution of momentum‐space probability density has been studied. It is shown for the one‐dimensional McWeeny–Coulson problem that the numerical value of the ground‐state energy of the one‐dimensional two‐electron atom is between the minimum energy values ε1/2 and ε1.
Green's function calculations using non‐Hartree–Fock orbitals
Authors: Holleboom L. J. , Snijders J. G. 
Published: January 2005

AbstractThe single‐particle Green's function is used to generate a new zero‐order Hamiltonian. The idea to generate a new zero order from the previous zero order by incorporating perturbative corrections up to certain order is attractive since it allows an iterative procedure to repeatedly improve the results by decreasing the perturbation. In particular, in those cases where the Hartree–Fock Hamiltonian is not a good approximation to the full Hamiltonian and where perturbation theory usually does not produce sufficiently accurate results, one might hope that such a repetitive procedure ultimately yields an improved zero order and accurate perturbative corrections from this newly generated zero order. Two such approaches are investigated: first, one in which the ω‐independent part of the self‐energy is fully incorporated in the zero order and, second, one in which the correlation energy is incorporated in a one‐electron potential in an average way. Numerical calculations are reported.
Algebraic formulae of matrix elements between valence‐bond determinants for periodic systems: The one‐band linear case
Authors: Fortunelli Alessandro 
Published: January 2005

AbstractA simple valence‐bond approach is considered for treating extended systems of equivalent atoms. In this approach, a limited nonorthogonal basis set of atomic orbitals is utilized and only nearest‐neighbor interactions are supposed. By exploiting then the translational invariance of the system, expressions for the matrix elements between antisymmetrized products of such nonorthogonal orbitals are derived in an algebraic form suitable for the numerical implementation on a computer. The one‐band linear case is explicitly considered. Some possible applications of the proposed approach and its connections with model Hamiltonians such as the Hubbard and the Heisemberg ones are also discussed.
Applications of n‐electron wave functions built up with two kinds of geminals
Authors: Liu Chengbu , Deng Conghao , Jin Beiyan , Hu Haiquan 
Published: January 2005

AbstractThe ground states of atoms and molecules Li−, Be, LiH, LiH, and Li2 have been calculated using the n‐electron wave functions built up with two kinds of geminals. As a comparison, the above systems have been calculated with the Hartree–Fock self‐consistent field and the multi‐configuration self‐consistent field method as well. The results show that the wave functions in this work are capable of describing the electron correlations, and both kinds of geminals can be taken as a starting point in building up n‐electron ground states.
Different forms of perturbation theory for the calculation of the correlation energy
Authors: Murray Christopher , Davidson Ernest R. 
Published: January 2005

AbstractDifferent forms of perturbation theory for the calculation of correlation energy in both closed‐and open‐shell systems are discussed. For closed‐shell systems, Epstein–Nesbet perturbation theory is compared with Møller–Plesset (MP) perturbation theory based on canonical Hartree–Fock orbitals and with MP theory based on internally consistent SCF orbitals. The traditional MP theory gives superior results despite its use of an inferior zeroth‐order Hamiltonian. This behavior is rationalized in terms of the larger denominators present in the traditional MP theory. These conclusions are used to support the restricted open‐shell perturbation methods proposed recently by Murray and Davidson, and these new methods are compared with spin‐restricted Epstein–Nesbet theory and the unrestricted MP (UMP) approach. © 1992 John Wiley & Sons, Inc.
Formulation of N‐ and ν‐representable density functional theory. V. Exchange‐only self‐consistent field
Authors: Kryachko Eugene S. , Ludeña Eduardo V. 
Published: January 2005

AbstractThe concept of a self‐consistent field is developed within the version of density functional theory based on local‐scaling transformations. It is shown that in this context there arise two types of consistency: one relating to the charge‐consistency within an orbit and another to “orbit jumping.” The latter is analyzed in terms of one‐particle equations. The connection with other methods is discussed. © 1992 John Wiley & Sons, Inc.
Proton affinities of molecules containing nitrogen and oxygen: comparing ab initio and semiempirical results to experiments
Authors: Ozment Judy L. , Schmiedekamp Ann M. 
Published: January 2005

AbstractWe report a comparison of theoretical and experimental proton affinities at nitrogen and oxygen sites within a series of small molecules. The calculated proton affinities are determined using the semiempirical methods AM1, MNDO, and PM3; the ab initio Hartree–Fock method at the following basis levels: 3‐21G//3‐21G, 3‐21+G//3‐21G, 6‐31G*//6‐31G*, and 6‐31+G(d, p)//6‐31G*; and Møller–Plesset perturbation calculations: MP2/6‐31G*//6‐31G*, MP3/6‐31G*//6‐31G*, MP2/6‐31G+(d, p)//6‐31G*, MP3/6‐31G+(d, p)//6‐31G*, and MP4(SDTQ)/6‐31G+G(d, p)//6‐31G*. The semiempirical methods have more nonsystematic scatter from the experimental values, compared to even the minimal 3‐21G level ab initio calculations. The thermodynamically corrected 6‐31G*//6‐31G* proton affinities provide acceptable results compared to experiment, and we see no significant improvement over 6‐31G*//6‐31G* in the proton affinities with any of the higher‐level calculations. © 1992 John Wiley & Sons, Inc.
A BSSE‐free SCF algorithm for intermolecular interactions. II. Sample calculations on hydrogen‐bonded complexes
Authors: Vibók Á. , Mayer I. 
Published: January 2005

AbstractThe usefulness and reliability of the recent BSSE‐free SCF algorithm based on the “chemical Hamiltonian approach” (CHA/F) is demonstrated by calculating potential curves for several hydrogen‐bonded complexes with 4‐31G, 6‐31G, and 6‐31G** basis sets. It is concluded that the CHA/F scheme gives results that are numerically close to those of the Boys–Bernardi a posteriori correction scheme but are free from the “overcompensation” characteristic of the latter at smaller distances and given basis sets. © 1992 John Wiley & Sons, Inc.
Comparative semiempirical calculations of tetrazole derivatives
Authors: Ivashkevich O. A. , Gaponik P. N. , Koren A. O. , Bubel O. N. , Fronchek E. V. 
Published: January 2005

AbstractHeats of formation, protonation energies, charge distributions, dipole moments, and geometrical structures of thirty‐two 1‐, 2‐, 1,5‐, and 2,5‐substituted tetrazoles have been investigated by different semiempirical SCF MO calculations. MNDO and, in some cases, AM1 methods are the most acceptable for tetrazole derivatives. A number of linear correlations between available experimental data and the calculated energies and charge characteristics have been established. © 1992 John Wiley & Sons, Inc.
CASSCF optimization problem for a group of excited states
Authors: Khait Yu. G. , Panin A. I. 
Published: January 2005

AbstractA second‐order version of the CASSCF approach to the optimization problem for a single (ground or excited) state and a group of excited states (involving, if necessary, also the ground state) is proposed. In contrast to the already existing methods, in the frameworks of our approach, there arises no need in completing the set of states to be optimized to the full basis set of configuration function space. Generation of secondary orbitals in the course of orbital optimization is also not required. All necessary integral transformations are performed only with active orbitals. These certainly attractive features of our approach appear due to employing the Gauss parametrization of average electronic energy domain, which is nonstandard in quantum chemistry. © 1992 John Wiley & Sons, Inc.
A simple proof for the formula to get symmetrized powers of group representations
Authors: Planelles J. , Zicovich‐Wilson C. M. 
Published: January 2005

AbstractA general formula to decompose the p‐power of irreducible representations of an arbitrary space group into sum of sets of irreducible representations of such a group, having identical permutational symmetry, is presented. Its proof is based upon a straightforward application of the properties of the generalized projection (shift) operators. © 1993 John Wiley & Sons, Inc.
Relativistic theory for indirect nuclear spin–spin couplings within the polarization propagator approach
Authors: Aucar Gustavo A. , Oddershede Jens 
Published: January 2005

AbstractWe present a relativistic theory for the nuclear spin–spin coupling tensor within the polarization propagator approach using the particle‐hole Dirac–Coulomb–Breit Hamiltonian and the full four‐component wave function. We give explicit expressions for the coupling tensor in the random‐phase approximation, neglecting the Breit interaction. A purely relativistic perturbative electron–nuclear Hamiltonian is used and it is shown how the single relativistic contribution to the coupling tensor reduces to Ramsey's three second‐order terms (Fermi contact, spin–dipole, and paramagnetic spin–orbit) in the nonrelativistic limit. The principal propagator becomes complex and the leading property integrals mix atomic orbitals of different parity. The well‐known propagator expressions for the coupling tensor in the nonrelativistic limit is obtained neglecting terms of the order c−n (n ⩾ 1). © 1993 John Wiley & Sons, Inc.
Temperature dependence of hydrogen transfer reactions via tunneling at low temperatures
Authors: Chandra A. K. , Sreedhara Rao V. 
Published: January 2005

AbstractIn this paper, we present a symmetrical and adiabatic reaction path for hydrogen transfer from methane by methyl by an ab initio method using the 6‐31G** basis set with the MP2 corrections for UHF/6–31G** optimized geometries. The theoretical barrier is fitted with the symmetric Eckart function. We, then, take thermal averages of the rate constant (k) considering the discrete nature of the C–H oscillators and the continuous character of the various low‐frequency modes of the composite system. Finally, we present a linear relation between log k and temperature. It is shown that this relation gives a good description of the kinetic results of both thermal and photochemical H‐transfer reactions in the region of low temperatures. © 1993 John Wiley & Sons, Inc.
Electronic structures around the local defects in all‐trans‐polyacetylene: An analysis by the cluster‐series model
Authors: Maekawa Koji , Imamura Akira 
Published: January 2005

AbstractA new approach to analyze the electronic structure around the local defects in a polymer chain is developed based on the cluster‐series model. In this approach, by extending the cluster molecule to both sides of the defect, the size effect on electronic structure can be estimated systematically and efficiently. Moreover, through the extension process, the periodicity of electronic structure can be extracted in the form of the periodic pattern of stationary orbitals, which are molecular orbitals unaltered by the extension of the cluster. Such periodic orbitals characterize the bulk states of periodic polymer and the energy band structure can be reconstructed from them. Illustrative analyses of some kinds of local defects in all‐trans‐polyacetylene are presented at the ab initio STO‐3G level. The effects of local defects can be detected by the deviation from periodic bulk states. © 1993 John Wiley & Sons, Inc.
Nonlinear Roothaan's equations
Authors: Marañón Julio 
Published: January 2005

AbstractIn the regime of the molecular‐orbital theory, a new approach beyond of the Hartree–Fock approximations is proposed. By an appropriate introduction of the quantum fluctuation into the molecular electronic ground state, the Hartree–Fock equations plus Gaussian fluctuation are obtained. Basically, the procedure proposed consists of an adequate utilization of the Gaussian approximation of the Ising model on a lattice (K.G. Wilson et al., Phys. Rep. C 12, 76 (1974)). It leads to an effective nonlinear Hamiltonian for the molecule for studying nonlinear problems in the molecular electronic structure. © 1993 John Wiley & Sons, Inc.