bluuebunny/crossref_metadata_2025_split huggingface dataset:
A spin‐free form of valence bond theory
Authors: McWeeny Roy 
Published: January 2005

AbstractClassical valence bond theory is recast in a spin‐free form which provides a practicable route to ab initio calculations of molecular electronic structure. The approach is simple and direct and requires only efficient algorithms for the generation and processing of permutations and the handling of Rumer diagrams: it makes modest demands on computing power and pilot calculations have indeed been performed entirely within the fast memory of a personal computer, which should be sufficient for dealing with systems possessing up to 10 electrons outside a closed shell. Simple applications confirm the conclusion of Cooper et al. [1] that, by using strongly overlapping orbitals, a small number of classical (nonpolar) structures can give results close to those obtained in a “full CI” calculation.
Theoretical studies on conformational preferences of model modified nucleic acid base N6‐(N‐alanylcarbonyl) adenine
Authors: Tewari Ravindra 
Published: January 2005

AbstractConformational preferences of model modified nucleic acid base N6‐(N‐alanylcarbonyl) adenine, ac6Ade, have been investigated using the quantum chemical PCILO (perturbative configuration interaction using localized orbitals) method. The multidimensional conformational space has been searched using selected grid points formed by combining the various torsion angles that take favored values derived from energy variation with respect to each torsion angle individually.The preferred molecular structure is stabilized by an intramolecular hydrogen bond from N(11)H of the amino acid to N(1) of the adenine. The observed crystal structure conformations for the naturally occurring, anticodon adjacent, threonyl analogs, tc6Ade, correspond to the predicted most stable conformation for the model modified base ac6Ade. Three stable, low energy conformations differing in the orientations of the carboxyl group and the amino acid side chain are predicted within 1 kcal/mol of the most stable structure. The possible bifurcated hydrogen bonding of N(11)H with N(1) and either of the carboxyl oxygens is of minor significance.The indicated orientational flexibility for the carboxyl group and the amino acid side chain may enable convenient probing of the molecular environment, in the vicinity of the anticodon in tRNA, by the amino acid substituent, with only modest changes in energy stabilization.
The role of hydrophobicity in the Ames test. The correlation of the mutagenicity of nitropolycyclic hydrocarbons with partition coefficients and molecular orbital indices
Authors: De Compadre Rosa L. Lopez , Shusterman Alan J. , Hansch Corwin 
Published: January 2005

AbstractA set of 20 nitropolycyclic aromatic hydrocarbons, whose mutagenicity has been determined in the Ames test, has been studied using octanol‐water partition coefficients (P) as a measure of relative hydrophobicity and molecular orbital energies to account for variation in their electronic characteristics. A good structure‐activity relationship was found using log P and ϵLUMO. The latter were taken from the results of ab initio calculations performed by Maynard, Pedersen, Posner, and McKinney [7] and were also calculated by the MNDO method. The dependence of mutagenicity on hydrophobicity was found to be similar to that observed for triazenes [2]. ϵLUMO values calculated by MNDO and STO‐3G were found to be strongly correlated, and the role of hydrophobicity in correlating mutagenicity was not significantly affected by the molecular orbital model employed.
Shape wave in density functional theory
Authors: Tachibana Akitomo 
Published: January 2005

AbstractThe shape wave u(r) is defined in such a way that the square of its absolute value gives the shape of the electron charge density for a nonrelativistic many‐electron system: |u(r)|2 = ρ(r)/∫ρ(r) dr, where ρ(r) is the total electron charge density. The u(r) is treated as the probability amplitude for the shape of the electron charge density in the real three‐dimensional space. The secular equation of the u(r) is given as L(r)u(r) = λu(r), where L(r) and λ are a real Hermitean operator and the eigenvalue, respectively. This equation has the form of the spinless one‐particle Schrödinger equation. The potential is local and is a functional of ρ(r), and so we need a self‐consistent field procedure to solve it. For equilibrium state shape wave u = ueq, which is temperature‐dependent, and for stationary state shape wave u = u0, respectively, Leq(r)ueq(r) = λequeq(r) and L0(r)u0(r) = λ0u0(r). For one‐electron systems, the latter equation reduces to the Schrödinger equation for a stationary state, with the usual eigenvalue λ0 = E0. In general, λ is identified with the Gibbs chemical potential μG. Boundary conditions such as the cusp condition at a nucleus and exponential decay at an infinitely large distance from the nucleus are easily implemented. In case L(r) allows complex eigenfunctions, u(r) = A(r)eiS(r), the hydrodynamical potential appears in the secular equation for the real amplitude A(r). The real phase S(r) satisfies the equation of continuity. By introducing «apparatus» operators [16], the excited state shape wave is also obtained. Comparisons are made with a recent discussion of Levy, Perdew, and Sahni [3].
Usage de l'algèbre de lie SU(n) dans l'etude des systèmes quantiques a n etats. VII: Sur le “problème inverse”
Authors: Tillieu Jacques , Van Groenendael Augustin 
Published: January 2005

AbstractThrough a formalism developed in formr papers, some problems, set by the determination, from experimental data of the evolution equation of an n‐states quantum system are considered.
Application of the Mermin entropy principle to the “apparatus” density functional theory
Authors: Tachibana Akitomo 
Published: January 2005

AbstractAn “apparatus” operator approach is presented for the extension of the density functional theory of Hohenberg, Kohn, and Mermin. Using the Mermin entropy principle, a one‐to‐one correspondence is established between the density matrix for the system and the electron charge density for a finite‐temperature system in the presence of an apparatus. In the zero‐temperature limit in the absence of an apparatus, the Hohenberg–Kohn theory is recovered. The central aspect of this new density functional theory is that the principle of maximum entropy is applied to the system plus its surroundings under the additional constraint that the electron charge density is given. The system is treated as a subsystem of a composite system and is not necessarily in the equilibrium state as in the Mermin theory. As an example, it is shown how, in principle, excited states are encompassed by the theory.
Exact ground state for a Herndon–Simpson model via resonance–theoretic cluster expansion
Authors: Klein D. J. , Schmalz T. G. 
Published: January 2005

AbstractThe Herndon–Simpson model for a particular catacondensed polyphene chain is considered as a nontrivial many‐body Hamiltonian, defined on a space with a basis of orthonormal Kekulé structures. An Explicitly correlated cluster expanded resonance–theoretic wave function is described for this model, and its quality is judged by calculation of the standard deviation for the energy expectation. The quality is found to be high. Indeed, for a particular parameter ratio within the range of experimental interest, the wave function ansatz is found to be exact. This very accurate solution is then used to gauge the quality of the common ansatz with equally weighted Kekulé structures, and it is found to be reasonably good.
Conformational studies on L‐serine phosphate and hydrated L‐serine phosphate using ab initio SCF calculations
Authors: Lunell S. , Nakamura Y. 
Published: January 2005

AbstractAb initio MO‐LCAO‐SCF calculations using an STO‐3G basis set were performed to find the most stable conformations of L‐serine phosphate and hydrated L‐serine phosphate. The most favorable conformation of L‐serine phosphate is found to be one where the bond sequence OCCC is trans and POCC gauche, and a very short hydrogen bond is formed between an oxygen atom of the phosphate group and a hydrogen atom of the ammonium group.For hydrated L‐serine phosphate, a bridge‐type hydration in which a water molecule links a phosphate oxygen and an ammonium hydrogen displays particularly low energy. In the four‐hydrated L‐serine phosphate anion, the most favorable conformation is such a bridged one having a rather extended configuration with regard to the bond sequences OCCC and POCC.
Time‐dependence of wave functions and the calculation of transition intensities
Authors: Sadlej Andrzej J. 
Published: January 2005

AbstractThe standard formalism of the perturbation theory for time‐dependent perturbations follows from the assumption that the exact solution of the unperturbed time‐independent problem is known. The consequences of this assumption are analyzed.Particular attention is given to the methods used for the evaluation of transition intensities. It is shown that the traditional intensity formula in most cases violates the assumptions underlying its derivation. It is shown that for the given ansatz concerning the form of the approximate wave function, the transition intensities should be determined from the variation treatment of the corresponding time‐dependent problem. The wave functions of both states involved in the transition should be given the same amount of variation flexibility. This condition is satisfied if the transition intensity expression follows from the complete timedependent perturbation treatment of the perturbed system.
Density matrix approximation as a probe to the interaction range analysis of electronic structure
Authors: Nevalainen Vesa , Pakkanen Tapani A. , Lindblad Marina 
Published: January 2005

AbstractAn interaction range analysis of electronic structures has been developed with density matrix approximation techniques. A successive error analysis of electronic interaction of differing ranges reveals clear pattrns. Aromatic hydrocarbons and heterocycles are used as examples. The developed technique is proposed to be used as an illustrative tool for the description of delocalization and as a guide for studies where delocalized systems are modeled by small subsystems.
A theoretical study of the diffuseness of the V(1B1u) state of planar ethylene
Authors: Lindh Roland , Roos Björn O. 
Published: January 2005

AbstractThe ground (N) state and the 1B1u(V) excited state of planar ethylene have been studied at the CPF and MR‐SDCI levels of theory, using an extended CGTO basis set of the ANO type. The investigation especially addresses the near‐degeneracy problem in the ground state and the coupling between the diffuse character of the π* orbital and the amount of correlation included in the wave function of the V state. The MR‐SDCI results yield a vertical excitation energy in the range 7.8–8.0 eV, whereas the CPF result is 7.9 eV. The best MR‐SDCI result for 〈1π*‖z2‖1π*〉 is 7.8, whereas CPF calculations based on MR‐SDCI INOS give the value 6.7. It is clear from the results that these numbers have not converged and that more extended calculations than was possible in the present work would yield an even more compact wave function.
Reaction ergodography for the additions of HLi and its dimer to acetylene
Authors: Zhao Cheng‐Da , Chen Bin , Huang Jing‐An 
Published: January 2005

AbstractThe ab initio calculation has been performed with the addition pathways of HLi and its dimer to acetylene at the RHF/3‐21G basis set. It shows that the reaction mechanisms of these two reactions are rather similar. In either of two reaction pathways, there is a meta‐stable molecular complex near the isolated reactant state. This kind of addition can be treated approximately as the unimolecular reaction in which the molecular complex rearranges into the product. We have estimated the activation entropies and the statistical A factors of these two reactions by the use of RRKM theory. Frontier molecular orbital analysis of these two transition states reveals their HOMOS to be formed from both HOMO‐LUMO and HOMO‐HOMO interactions.
Some properties of the bivariational Hartree–Fock scheme for complex symmetric many‐particle operators
Authors: Löwdin Per‐Olov , Froelich Piotr , Mishra Manoj 
Published: January 2005

AbstractThe bivariational Hartree–Fock scheme for a general many‐body operator T is discussed with particular reference to the complex symmetric case: T† = T*. It shown that, even in the case when the complex symmetric operator T is real and hence also self‐adjoint, the complex symmetric Hartree–Fock scheme does not reduce to the conventional real form, unless one introduces the constraint that the N‐dimensional space spanned by the Hartree–Fock functions ϕ should be stable under complex conjugation, so that ϕ* = ϕα. If one omits this constraint, one gets a complex symmetric formulation of the Hartree–Fock scheme for a real N‐electron Hamiltonian having the properties H = H* = H†, in which the effective Hamiltonian Heff (1) may have complex eigenvalues εk. By using the method of complex scaling, it is indicated that these complex eigenvalues—at least for certain systems—may be related to the existence of so‐called physical resonance states, and a simple example is given. Full details will be given elsewhere.
Pseudopotential investigation of some alkali metal molecules
Authors: Tamássy‐Lentei I. , Derecskei‐Kovács Á. 
Published: January 2005

AbstractThe effect of electron correlation on the results of pseudopotential calculations was examined using a simple analytical semiempirical pseudopotential and a correlated floating‐type one‐center wave function. Investigations were performed for the XH alkali metal hydride molecules (X  Na, K, Rb, Cs). The inclusion of the electron correlation in the ground state proved important for the calculation of the dissociation and ionization energies, but it is less significant for the determination of the equilibrium nuclear distances. The ground state potential energy curves are also determined.
Computational chemical studies of chiral stationary‐phase models: The nature of the Pi interaction in complexes of methyl N‐(2‐naphthyl) alaninate with N‐(3,5‐dinitrobenzoyl)leucine n‐propylamide
Authors: Sabio Michael , Topiol Sid 
Published: January 2005

AbstractThe pi interaction in complexes of (S)‐methyl N‐(2‐naphthyl)alaninate with (S)‐ and (R)‐N‐(3,5‐dinitrobenzoyl)leucine n‐propylamide was investigated with Hartree–Fock and second‐order Møller–Plesset perturbation theory calculations using the STO‐3G basis set. For each complex, the geometry of the model of the pi interaction (i.e., naphthalene and 1,3‐dinitrobenzene) was derived directly from the complex geometry which was relaxed by the semiempirical quantum‐mechanical AM1 method. At the level of treatment used herein, our results are in agreement with our earlier AM1 interaction energy calculations in which the pi interaction, one of the three primary interactions proposed in models of the aforementioned complexes, is attractive and of equal strength (ca. 0.8 kcal/mol) in both complexes.
The many‐body expansion of the potential energy function for elemental clusters
Authors: Murrell J. N. 
Published: January 2005

AbstractThe many‐body expansion of the potential energy function of elemental clusters is examined in general terms in regard to its convergence for microclusters and the bulk phase. The systems Ben and Lin are examined in detail. For Li it is clear that the many‐body expansion has no low‐order convergence, but it is shown that a potential of the form

gives good binding energies for Lin (3<n⩽9) and also a good value for the heat of atomization of the bcc crystal.
Pairing theorem of graph eigenvalues: Its new proof and a generalization
Authors: Shen Mingzuo 
Published: January 2005

AbstractEach undirected graph has its own adjacency matrix, which is real and symmetric. The negative of the adjacency matrix, also real and symmetric, is a well‐defined mathematically elementary concept. By this negative adjacency matrix, the negative of a graph can be defined. Then an orthogonal transformation can be readily found that transforms a negative of an alternant graph to that alternant graph: (−G) → G. Since the procedure does not involve the edge weights, the pairing theorem holds true for all edge‐weighted alternant graphs, including the usual “standard” graphs.
N‐band Hubbard models for copper oxides and isoelectronic systems. New models for organic and organometallic magnetic conductors and superconductors
Authors: Yamaguchi Kizashi 
Published: January 2005

AbstractThe electronic structures of undoped and doped copper oxides and other related oxides are investigated on the basis of the N‐band Hubbard models. The Hubbard Hamiltonians for clusters of transition metal oxides are exactly diagonalized by the full valence‐bond (VB) configuration interaction (CI) method in order to elucidate populations of doped holes, electronic excitation energies, etc. Possible mechanisms of the high‐Tc superconductivity for oxide superconductors are discussed on the basis of the calculated results, together with available experiments. The analysis of correlation and spin correlation effects on doped copper oxides indicates theoretical possibilities of new models for organic and organometallic magnetic conductors and superconductors. Organic and organometallic analogs to copper oxides are therefore proposed on the basis of these results.
Partitioning of electronic properties in two‐center, one‐electron coulombic systems
Authors: Cioslowski J. 
Published: January 2005

AbstractSeparability of the Hamiltonian describing a two‐center, one‐electron Coulombic system in the prolate spherical coordinates allows for a detailed comparison between the partitioning of electronic properties based on Bader's quantum theory of atoms in molecules and the newly developed GAPT partitioning. The one‐electron systems can serve as the simplest models of the covalent and ionic bonds. The GAPT partitioning correctly describes the effects associated with the infinite range of Coulombic interactions.
The Hubbard connection
Authors: Matsen F. A. 
Published: January 2005

AbstractHubbard theory is an approximation to the Pariser‐Parr‐Pople theory which presents many of its important features in a relatively simple form. Our Hubbard Hamiltonian employs a single electron‐correlation parameter: 0 ⩽ x ≡ U/(t + U) ⩽ 1, where t is the one‐electron hopping integral (the negative of the Hueckel β), and U is the Hubbard one‐center, two‐electron repulsion parameter. This parameter provides the Hubbard connection between the following:

Molecular orbital and valence bond theory
The exchange approximation and full CI
Conductors and insulators
Fermi‐Dirac and Boltzmann statistics
Normal and superconductors
We present Hubbard theory in the freeon (spin‐free) formulation where each N‐electron state is pictorially represented by a Gel'fand diagram which specifies both the orbital configuration and the multiplicity. The simplicity of the freeon formulation makes the Hubbard connection quite transparent.
Calculation of band structure of doped polyacetylene
Authors: Yang Cao , You‐Liang Wang , Bo Chen 
Published: January 2005

AbstractIn the present paper, the band structures of valence‐band and conduction‐band for polyacetylene and Lidoped polyacetylene were calculated by LCAO‐SCF ab initio crystal orbital (CO) method. The influences of doping on the geometric structures, the band structures, and the changes of atomic charges were analyzed and discussed to explain the insulator‐conductor transition and electric conducting mechanism of doped polyacetylene.
Investigation of the influence of the substitutes on the intensity of the absorption band of some polymethine dyes
Authors: Balevičius L.‐M. , Bolotin A. , Shatkovskaya D. 
Published: January 2005

AbstractThe dependence of the long‐wave absorption‐band intensity on the structural peculiarities of a number of polymethine dyes has been investigated using the PPP method. It has been shown that the analyzed dependence is caused by the electronic properties of the substitutes.
The method of moments and chemical bonding: Interaction of two adatoms on simple cubic and B.C.C. transition metal (001) surfaces
Authors: Pick Štěpán , Tomášek Mojmír , Luchini Marco U. 
Published: January 2005

AbstractThe generalization of the well‐known Hückel rule of quantum chemistry is combined with the crossing theorem of the method of moments to interpret the calculated properties of two adatoms on the (001) surfaces of the simple cubic metal and the B.C.C. transition metal. The relation between the fourth‐order energy moment and the repulsive interaction between adatoms is emphasized. Some conclusions are drawn on the mechanism of the W (001) surface reconstruction.
Density matrix of crystalline systems. II. The influence of structural and computational parameters
Authors: Pisani C. , Aprà E. , Causà M. , Orlando R. 
Published: January 2005

AbstractThe one‐electron density matrix (DM) of a number of crystalline systems—lithium, graphite, boron nitride, silicon, and beryllium—are considered here, as resulting from Hartree–Fock–SCF‐LCAO calculations. The influence of structural and computational parameters is discussed. It is shown in particular why the structure of chemical bonds in semiconductors leads to an oscillating long‐range behavior of the DM, similar to that observed in metals, where these oscillations are related to the very existence of a Fermi surface. Concerning computational parameters, the influence of the density of sampling k points and of basis set on the calculated DM is considered; it is shown that the choice of the basis set is not a very critical one as far as the DM range is concerned. Some critical aspects of the interrelation between DM range and exchange part of the Fock Hamiltonian are analyzed.
Analysis of molecular shape changes along reaction paths
Authors: Arteca Gustavo A. , Mezey Paul G. 
Published: January 2005

AbstractIn this work, a detailed discussion and illustrative example are given for a new conceptual approach to the notion of similarity between nuclear configurations. We analyze the changes in molecular shape, as described by a topological characterization of molecular envelope surfaces, along reaction paths. This approach provides an independent definition of formal species along the path, defined in terms of molecular shape, rather than based on the curvature properties of the potential energy function. The two approaches are compared in this work, using the [1,3] hydrogen‐shift in formic acid as an illustrative example. The reaction is studied at SCF and MP2 ab initio levels. Although the actual reaction paths are different, the molecular shape changes encountered along them are similar, indicating that the essential shape changes provide chemical information on a more fundamental level than do detailed reaction paths.
Exploring the potential energy hypersurface of histamine monocation: Tautomerism in gas phase
Authors: Tapia O. , Cárdenas R. , Smeyers Y. G. , Hernández‐Laguna A. , Rández J. J. , Rández F. J. 
Published: January 2005

AbstractThe potential energy hypersurface of the histamine monocation is determined by ab initio methods at the STO‐4G level using analytical gradient techniques. Three transition states and two minima have been found for the NτH tautomer. One of the transition states connects the trans conformational region with a minimum gauche structure, where the proton of the ammonium group is approximately halfway between the Nπ of the imidazole group and the N of the ammonium group, but nearer to the Nπ. This minimum connects the potential energy surface of the NτH tautomer with the imidazolium one. In the latter region, three transition states and two minima have been found. Critical points are discussed in relation with experimental data and histamine H2 receptor models.