bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nonbonded interactions in membrane active cyclic biopolymers. II. Ion binding by valinomycin
Authors: Sundaram K. , Tyagi R. S. 
Published: December 2004

AbstractA possible mode for the capture of a cation by the carrier molecule valinomycin has been simulated. The intramolecular nonbonded energy and the binding energy for a centrally located potassium ion at different stages in the capture process have been evaluated using a method proposed earlier. Contrary to intuitive expectations the open conformation of valinomycin is also found to have a strong affinity for K+. There appears to be a rather high potential barrier between the open and closed symmetric conformations.
Singer polymals. I. Evaluation of matrix elements
Authors: Poshusta R. D. 
Published: December 2004

AbstractHamiltonian matrix elements between pairs of explicitly correlated Singer polymals are derived. A new method for 1/r12 matrix elements is found which avoids the 6 by 6 matrix inversion required at each quadrature point in the old method. The new formula requires about half the computer time of the old one. The electron repulsion and nuclear attraction matrix elements are shown to be related to incomplete elliptic integrals. Simplified formulas are derived for the special cases of cylindrical symmetry, spherical symmetry, and single‐center expansions. An upper bound approximation formula for the purpose of neglecting matrix elements is derived. These bounds may be useful for single‐center expansions but are found to be too high for the general case.
The role of model systems in the few‐body reduction of the N‐fermion problem
Authors: Percus J. K. 
Published: December 2004

AbstractThe problem of upper and lower ground state energy bounds for many‐fermion systems is considered from the viewpoint of reduced density matrices. Model density matrices are used for upper bounds to, first uncoupled, then coupled fermions. Model Hamiltonians are developed for lower bounds in corresponding fashion. Both mathematical and physical models are constructed for setting up universally valid inequalities on density matrices. These are joined by both inequalities and equalities in which the explicit form of the system at hand is used. A few illustrative examples are presented.
Inequalities for fermion density matrices
Authors: Garrod Claude , Hannon J. Michael 
Published: December 2004

AbstractA partial trace over the occupation numbers of all but k states in the density matrix of an ensemble with an arbitrary number of single‐particle states is defined as the (reduced) k‐state density matrix. This matrix is used to obtain a complete, practical solution to the problem of determining the representability of the diagonal elements of the one‐ and two‐particle (reduced) density matrices. This solution is expressed as a series of linear inequalities involving the density‐matrix elements; the inequalities are identical with those derived previously by Davidson and McCrae by a different method. In addition, our method is used to obtain nonlinear, matrix inequalities on the off‐diagonal elements of the density matrices.
Highly correlated particle densities and idempotent one‐densities
Authors: Henderson George A. 
Published: December 2004

AbstractThe problem of determining idempotent one‐densities which integrate to the exact or to a highly correlated particle density is considered. A method for obtaining the minimum energy idempotent one‐density integrating to a given correlated particle density within a finite basis is described. The implications of this are twofold. First, Hartree–Fock accuracy can be exceeded in describing the electron density with an idempotent one‐density; this is particularly relevant to the problem of constructing orbitals from experimental x‐ray scattering data. Second, electron densities from analytic CI or MCSCF wave functions can be made available in a form as compact as the Hartree–Fock density by reporting the orbitals which define the correlated density via an idempotent one‐density. A numerical example of the new method is given in which an accurate correlated density for He is “fitted” by an idempotent one‐density represented in a finite (near Hartree–Fock) basis. Considering the deficiencies of the basis for this purpose, a technique is suggested for constructing basis sets optimized for prediction of one‐electron properties rather than for energy.
Vers une nouvelle interprétation de la liaison chimique?
Authors: Julg Andre , Julg Pierre 
Published: December 2004

AbstractLe découpage de la densité électronique totale dans une molécule, en contributions provenant des diverses orbitales, permet de définir des régions orbitalaires de caractère conventionnel dont la charge totale est égale à la somme des populations de Mulliken des orbitales constituant les diverses régions. La fluctuation relative de la charge de ces régions s'exprime simplement en fonction des populations de Mulliken et des indices de liaisons. Cette analyse de la densité dans des molécules variées (saturées ou conjuguées) permet de retrouver les liaisons chimiques de la graphie classique comme correspondant à des régions orbitalaires de charge voisine de 2 et présentant une fluctuation relative fiable (⩽0, 10). Compte tenu du caractère dynamique que contient la notion de liaison, les résultats obtenus trouveraient une interprétation simple dans l'espace des phases. L'impuissance de la Mécanique quantique de donner une description dans cet espace expliquerait l'impossibilité de déduire de calculs quantiques la notion měme de liaison chimique.
Thermodynamics of the DNA helix‐coil transition
Authors: Porta Jordi 
Published: December 2004

AbstractThe thermodynamics of the DNA helix‐coil transition is studied, starting from the thermodynamical potential difference between the states helix and coil; this potential difference is understood as the difference in free energies. With only three parameters obtained from experimental data different quantities of the T2 phage DNA molecule are calculated. It is observed that the phase transition is of second order.
Relativistic symmetry orbitals for the double groups C2v, C∞v, D∞h, and Oh
Authors: Rosén Arne 
Published: December 2004

AbstractRelativistic symmetry orbitals are given for molecular LCAO calculations for the double groups: C2v, C∞v, D∞h, and Oh (6‐fold) coordination. The atomic orbitals used in the LCAO are of the four component form. A discussion of the comparison between nonre ativistic and relativistic molecular eigenvalues is presented.
Floating spherical gaussian orbital model study of some organometallic systems. I. LiCH3, HBeCH3, and CH3BeCH3
Authors: Ray N. K. , Mehandru S. P. , Bhargava Shobha 
Published: December 2004

AbstractThe floating spherical Gaussian orbital method has been used to study the equilibrium geometries and electronic structures of some organometallic systems like LiCH3, HBeCH3, and CH3BeCH3. The predicted geometries are in good agreement with other theoretical estimates and available experimental results. Electron transfer values from carbon to metal atom and bond energies of LiC and BeC bonds are estimated and discussed. The wave functions obtained here have also been used to predict the electron momentum distributions and Compton profiles for LiCH3, HBeCH3, and CH3BeCH3.
Dependence of the electrostatic molecular potential upon the basis set and the method of calculation of the wave function. Case of the ground 3A1(π → π*) and 1A1(π → π*) states of formaldehyde
Authors: Daudel R. , Le Rouzo H. , Cimiraglia R. , Tomasi J. 
Published: December 2004

AbstractThe dependence of the molecular electrostatic potential V on the method of calculating the wave function and on the basis set has been examined for three electronic states of H2CO. The calculations analyzed here refer to two different basis sets: a minimal set and a split‐valence shell set, both supplemented with diffuse p orbitals. The methods of calculation include the SCF procedure (for the ground state), CI procedures of increasing complexity, which practically reach their asymptotic limits, and, for the excited states, simpler methods (rigid orbital excitation, complete CI of single excitations, electron‐hole potential methods). It appears that in the ground state the two bases give equivalent descriptions of V when the CI process reaches the asymptotic limit and that the SCF descriptions approximate fairly well the more accurate ones. For the 3A1(π → π*) state the conclusions on the two basis sets are similar and, in addition, it is shown that a simple method (EHP) gives an approximation of V that reproduces the essential features of the complete CI calculations. The inadequacy of both basis sets for the representation of the 1A1(π → π*) state turns out to be evident. A discussion is presented concerning the possibility of adopting the calculation of V as an auxiliary tool to compare the accuracy of different descriptions of the same electronic state.
Quasidegenerate coupled‐cluster approach with hermitian model hamiltonian
Authors: Kvasnička Vladimír 
Published: December 2004

AbstractThe quasidegenerate coupled‐cluster approach with a hermitian model Hamiltonian is constructed. This theory provides an efficient method for simultaneous construction and evaluation of the hermitian diagrammatic model Hamiltonian up to an arbitrary high order of perturbation theory and prescribed form of topology.
On the electrostatic properties of papain in relation to its enzymatic activity
Authors: Lavery R. , Pullman A. , Wen Y. K. 
Published: December 2004

AbstractThe electrostatic potentials and fields created by the thiol protease, papain, are computed on the surface envelope of the protein and in the region of its active site using a technique based on accurate multipole expansions of the electron density of appropriately chosen subunits. The effect of binding counterions to the protein is considered specifically. The possible role of the “electrostatic environment” in the functioning of the enzyme is discussed and it is found that this environment can favor the proton transfer between Cys 25 and His 159 that is thought to initiate its mode of action, but that the reasons for this appear more complicated than earlier models would suggest.
Bond length alternation in cyclic polyenes. II. Unrestricted hartree–fock method
Authors: Paldus J. , Chin E. , Grey M. G. 
Published: December 2004

AbstractThe problem of bond length alternation in cyclic polyene models as described by the Pariser–Parr–Pople π‐electron Hamiltonian, together with an empirical quasi harmonic σ‐core potential is investigated using the unrestricted Hartree–Fock wave function employing different spatial orbitals for different spins. It is shown that in contrast to the restricted Hartree–Fock method, which favors bond alternation in large cyclic polyenes, the unrestricted Hartree–Fock method stabilizes the symmetric structures with equidistant internuclear separation. An assessment of the amount of correlation error recovered by the unrestricted Hartree–Fock procedure is examined and the qualitatively different behavior of the cyclic polyene models when described by restricted and unrestricted Hartree–Fock wave functions is discussed from this viewpoint.
Bond length alternation in cyclic polyenes. III. Alternant molecular orbital method
Authors: Pauncz R. , Paldus J. 
Published: December 2004

AbstractThe bond length alternation problem in cyclic polyene models as described by the Pariser–Parr–Pople Hamiltonian and an empirical quasiharmonic π‐core potential is investigated using the one‐parameter alternant molecular orbital (AMO) method. It is shown that in contrast to the unrestricted Hartree–Fock (UHF) results, which lead to symmetric equidistant structures, the one‐parameter AMO results yield bond length alternating structures similar to those obtained with the restricted HF approach. The correlation energy recovered by the AMO method is examined for the symmetric polyenes in the whole range of coupling constants for both the Pariser–Parr–Pople and Hubbard Hamiltonians and compared with exact full configuration interaction (FCI) results. For the first member of the cyclic polyene series we also compared the FCI and AMO correlation energies for different nuclear framework distortions. This comparison indicates that in contrast to the UHF results the fraction of the correlation energy recovered by the AMO approach is very uniform over the range of nuclear distortions considered. The AMO results thus strongly indicate the dimerization in the polyenic chains.
Protonation of aminopyrone: A four centers problem, CNDO/2 versus MNDO study
Authors: Meghezzi H. , Boucekkine A. , Kolli B. , Hamdi M. 
Published: December 2004

AbstractThe mechanism of protonation of aminopyrones and the nature of the protonated forms are of some interest because of the biological importance of these compounds. The use of CNDO/2 molecular electrostatic potentials contour maps shows that the most reactive centre is the extracyclic oxygen. MNDO protonation energies confirm the protonation of this atom and elucidate the basicity of the nitrogen atom. The effect of a N substitution on the basicity of the N13 atom has been studied. The calculations are in agreement with preliminary experimental results.
Basis set quality. II. Information theoretic appraisal of various s‐ orbitals
Authors: Simas Alfredo M. , Thakkar Ajit J. , Smith Vedene H. 
Published: December 2004

AbstractThe expectation values 〈rk〉 (−2 ⩽ k ⩽ 4, k = 10), values of the charge density ρ(r) at selected points, and coefficients in the MacLaurin expansion of ρ(r) are used to test the quality of 71 orbital basis sets used for the atomic helium Hartree–Fock problem. These tests in position space are complementary to the momentum space tests previously carried out [Int. J. Quantum Chem. 21, 419 (1982)]. Information theoretic measures with respect to either or both position and momentum space properties are subsequently defined and the orbitals are ranked accordingly. These measures indicate that, for a given orbital, momentum space properties are more poorly predicted than position space ones. Moreover an improvement in the virial ratio does not necessarily lead to a more balanced orbital with respect to position and momentum space properties. Basis sets containing Slater‐type orbitals are again found to be rather accurate. The exponentially damped rational function is confirmed to be the outstanding two‐parameter unconventional orbital.
Ordering of Kekulé structures using nonadjacent numbers
Authors: El‐Basil Sherif , Osman A. N. 
Published: December 2004

AbstractKekulé indices and conjugated circuits are computed for 36 Kekulé structures, together with two VB quantities associated with the corresponding factor graphs (previously called submolecules). These latter quantitites are nonadjacent numbers of Hosoya and the reciprocal of the connectivity indices of Randić. It was found that the index of Hosoya successfully orders a set of Kekulé structures belonging to the same hydrocarbon in a parallel order as their Kekulé indices and branching indices. This substantiates the relation between VB and MO theories. A code is derived by summing contributions of nonadjacent numbers in all Kekulé stuctures of a hydrocarbon. The order of the resulting codes is found to be identical to the order of the molecular properties (resonance energies, π‐energies, and eigenvalues) of the hydrocarbons.
Electron correlation effects on magnetic properties of BH
Authors: Iwai Masahiro , Saika A. 
Published: December 2004

AbstractThe nuclear magnetic shielding and magnetizability tensors for the BH molecule are calculated by the coupled‐Hartree–Fock method and many‐body perturbation theory. As in the case of H2, HF, and F2, the second‐order non‐CHF diagrams make an inappreciable contribution.
Calculation in k space of integrals arising in the theory of Van der Waals forces. II. Formulas for arbitrary atoms
Authors: Ishida Kazuhiro 
Published: December 2004

AbstractBasic integrals arising in the momentum space formulation of the van der Waals forces are derived for arbitrary two atoms. These integrals are essentially the matrix elements of the Fourier transformed Coulomb operator and the Fourier transformed square of the Coulomb operator between any two Slater orbitals. The derivation is completely analytic and the results are expressed as finite series expansions in terms of auxiliary integrals. Recursion relations among the auxiliary integrals are developed.
A possible interpretation of atomic valency as a spontaneously broken symmetry response
Authors: Prat R. F. 
Published: December 2004

AbstractThe concept of atomic valency is looked at as a specific response of an atomic system to an arbitrary environment. It is shown, on the basis of a mathematical result in group actions theory, how valency might be related both to previously found Hartree–Fock deformed states and to the isotropy groups (up to a conjugation) of the extrema of the electronic energy functional.
Perturbation theory and electron correlation in extended systems: Cyclic polyene model
Authors: Takahashi M. , Paldus J. , Čížek J. 
Published: December 2004

AbstractThe applicability of the finite‐order many‐body perturbation theory to the electron correlation problem in extended one‐dimensional systems is examined. The cyclic polyenes CNHN, N = 4ν + 2, ν = 1, 2, …, with the DNh geometry as described by both the Pariser–Parr–Pople and Hubbard Hamiltonians, are employed to model the metallic‐like one‐dimensional systems. The second‐order perturbation theory contributions to the correlation energy are obtained with three different partitionings of the Hamiltonian (Hückel, M⊘ller–Plesset, and Epstein–Nesbet). The third‐ and fourth‐order contributions are also calculated in special cases. A comparison with other methods is given wherever available. For the Hubbard Hamiltonian the asymptotic behavior of the perturbation theory expansion is examined numerically. It is shown that the finite‐order perturbation expansion can provide reliable results for the correlation energy of one‐dimensional systems even in the correlation region which corresponds to the spectroscopically determined physical value of the coupling constant.
Analysis of bonding properties in molecular ground and excited states by a Cohen‐type bond order
Authors: Dick Bernhard , Freund Hans‐Joachim 
Published: December 2004

AbstractWe propose a Cohen‐type bond order analysis in terms of orthogonalized atomic basis functions which can be used to analyze NDO wave functions of large organic and metal–organic molecules. It is shown that for small molecules the results gained with this method are in excellent agreement with the same analysis based on ab initio STO‐3G wavefunctions. For large planar aromatic systems these all‐valence electron bond orders are found to be a consistent generalization of the π‐bond order. A simple relation between these bond orders and the corresponding covalent bond energies is established. The method can be easily extended to study excited state multiconfiguration wave functions. We present calculations for C2H2, C2H4, C2H6, and Mn2(CO)10. The results indicate that the method can be used to discuss the photochemistry of organic and metal–organic compounds.
Density functional theory: Approximating quasiparticle density functional
Authors: Beznosjuk Serge A. , Kryachko Eugene S. 
Published: December 2004

AbstractA constructive approach for deriving the approximating quasiparticle energy density functional is proposed. As a matter of fact, the proposed approach is the direct development of the Kohn–Sham quasiparticle concept and the Levy–Valone approach. The approach presented takes into account a pseudopotential character of the exchange‐correlation part of the density functional and results in a system of functional equations to obtain ground‐state energies of many‐electron systems.
Fourier transforms of atomic orbitals. II. Convolution theorems
Authors: Niukkanen A. W. 
Published: December 2004

AbstractAnalytic expressions have been derived for tensor convolutions of basis functions of exponential class (the reduced Bessel functions, the Slater, and the hydrogen‐like functions). For reduced Bessel functions, in particular, the method suggested in the paper is a more clear and compact alternative to the laborious proof of the important Filter and Steinborn convolution theorem.
Diagrammatic valence‐bond theory for finite model Hamiltonians
Authors: Ramasesha S. , Soos Z. G. 
Published: December 2004

AbstractValence bond (VB) diagrams form a complete basis for model Hamiltonians that conserve total spin, S, and have one valence state, ϕp, per site. Hubbard and Pariser–Parr–Pople (PPP) models illustrate ionic problems, with zero, one, or two electrons in each ϕp, while isotropic Heisenberg models illustrate spin problems, with only purely covalent VB diagrams. The difficulty of nonorthogonal VB diagrams is by‐passed by exploiting the finite dimensionality of the complete basis and working with unsymmetric sparse matrices. We introduce efficient bit manipulations for generating, storing, and handling VB diagrams as integers and describe a new coordinate relaxation method for the ground and lowest excited states of unsymmetric sparse matrices. Antiferromagnetic spin‐½ Heisenberg rings and chains of N ⩽ 20 spins, or 2N spin functions, are solved in C2 symmetry as illustrative examples. The lowest S = 1 and 0 excitations are related to domain walls, or spin solitons, and studied for alternations corresponding to polyacetylene. VB diagrams with arbitrary S and nonneighbor interactions are constructed for both spin and ionic problems, thus extending diagrammatic VB theory to other topologies.
Calculation of periodic molecular systems with perturbed periodicity within the PCILO framework. II. Model systems
Authors: Lochmann R. , Weller Th. 
Published: December 2004

AbstractThe energetic behavior of one‐dimensional (HF)n and (H2O)n chains with added or inserted H2O or HF was studied by means of the PCILOPSI method presented in Part I. The results were compared with the ones by the original PCILO method. The stabilization energies are in good qualitative agreement; energy differences due to impurity effects even quantitatively agree. The results show that the PCILOPSI method is well suited for the study of perturbed periodic molecular systems.
A formulation of the quantum‐mechanical many‐body problem in terms of hyperspherical coordinates
Authors: Avery John , Zhen‐Yi Wen 
Published: December 2004

AbstractHyperspherical coordinates are used to construct a matrix representation of a general N‐particle Hamiltonian in the case where the interaction is electrostatic. The Yukawa interaction can be treated similarly, as is shown in an appendix. The basis functions used to construct the matrix representation of H are mononomials inthe 3N coordinates of the particles, multiplied by functions of the hyperradius. Methods for transforming from this representation to a symmetry‐adapted representation are also discussed.
Unitary group approach to the many‐electron problem. III. Matrix elements of spin‐dependent Hamiltonians
Authors: Gould M. D. , Chandler G. S. 
Published: December 2004

AbstractThis is the final paper in a series of three directed toward the evaluation of spin‐dependent Hamiltonians. In this paper we derive the reduced matrix elements of the U(2n) generators in a basis symmetry adapted to the subgroup U(n) × U(2) (i.e., spin‐orbit basis), for the representations appropriate to many‐electron systems. This enables a direct evaluation of the matrix elements of spin‐dependent Hamiltonians in the spin‐orbit basis. An alternative (indirect) method, which employs the use of U(2n) ↓ U(n) × U(2) subduction coefficients, is also discussed.