bluuebunny/crossref_metadata_2025_split huggingface dataset:
Application of AGP wave functions to the ground states of Li2 and CH+
Authors: Elander Nils , Sangfelt Erland , Kurtz Henry , Goscinski Osvaldo 
Published: December 2004

AbstractThe AGP wave function is used to describe the ground state potential energy curves of Li2 and CH+. Moderate size basis sets are used. The computational aspects of the AGP optimization are discussed and the nature of the AGP wave function versus internuclear separation is examined.
Treatment of aperiodicity and correlation in polymers
Authors: Ladik János J. 
Published: December 2004

AbstractThe negative factor counting method (in its simple and matrix block form) for the determination of the density of states of disordered polymer chains and its applications to different aperiodic organic polymers are reviewed. The problems of the calculation of the correlation energy in large systems like polymers will be discussed. Different ways for the partitioning of an energy band into regions make it possible to perform Møller–Plesset perturbation theoretical calculations on polymers. Applications to hydrogen chains are presented. Finally, possible applications to disordered polymers are also discussed.
Studies in the configuration interaction method. II. Generating functions and recurrence relations for the number of many‐particle configurations
Authors: Katriel J. , Pauncz R. , Mulder J. J. C. 
Published: December 2004

AbstractGenerating functions are constructed and recurrence relations are derived for the number of configurations of any number of particles with an arbitrary elementary spin for both fermions and bosons. Similar results are obtained for the number of configurations specified by the total weight and by the degree of excitation.
Nonadiabatic singer polymal wave functions for three‐particle systems
Authors: Poshusta R. D. 
Published: December 2004

AbstractA method for nonadiabatic many‐particle quantum‐mechanical calculations is described and illustrated for the special case of three particles. The method uses a basis of explicitly correlated Singer‐type exponential quadratic function (polymals) of the internal degrees of freedom. Rigorous symmetry states are projected from the basis: linear momentum of the center of mass, total angular momentum, and permutational symmetry under interchange of indistinguishable particles. The nonadiabatic wave functions are interpreted via purely quantum‐mechanical criteria of interparticle correlation as measured by average values of powers of interparticle distances and angles. The illustrations are made on H2+ which is easily treated in the Born–Oppenheimer and adiabatic approximations, on helium, muonic helium, and on (e+, e−, e+) which are poorly described in adiabatic methods. The ground and lowest bound excited states of these systems are studied with up to 256 tempered Singer polymals for which we find energies too high by 0.0011 a.u. in H2+, 0.0017 a.u. in muonic helium, 0.0009 a.u. in 4He, and 0.0002 a.u. in (e+, e−, e+); the corresponding relative errors are 1800, 4, 300, and 200 ppm, respectively.
Perturbation theory for open‐shell systems: Simulation of the UHF states by means of the perturbed RHF wave functions
Authors: Hubač Ivan , Čársky Petr 
Published: December 2004

AbstractSecond‐, third‐, and selected fourth‐order contributions to the correlation energy were calculated for a series of simple open‐shell systems by means of the previously developed double‐perturbation theory in the restricted MO formalism. It was found possible to assign some of the diagrams to self‐consistency effects and to approximate in this way the E – E energy difference. A comparison is made with a more rigorous approach, in which the UHF ground‐state wave function is expressed as a first‐order perturbation expansion based on the RHF reference wave function. Distinguishing between “self‐consistency” and “correlation” diagrams for open‐shell systems in the RHF formulation represents a special case of a more general problem met in any double‐perturbation treatment, such as, e.g., treatments of systems in the external field or perturbation expansions with noncanonical orbitals.
The electronic structure and properties of nonmetallic rare earth monochalcogenides
Authors: Anisimov F. , Dagys R. , Sargautis A. 
Published: December 2004

AbstractThe electronic structure and properties of rare earth monochalcogenides LnX (X = S, Se, and Te) have been investigated. To explain the peculiarities of long‐wave absorption spectra of nonmetallic LnX, the spectra of 4ƒ‐5d excitations were calculated in two extreme representations. It was shown that the structure of the spectra, the positions of the bands, the almost constant red‐shift of LnX spectra as compared to one of corresponding Ln2+ impurities, may be rather well explained in the framework of the localized electron model. The problems concerning the pecularities of the electrical properties of LnX are discussed.
Semiempirical MO study of pyruvate substrate behavior in the active center of lactate dehydrogenase
Authors: Krechl Jiří , Kuthan Josef 
Published: December 2004

AbstractIn a previous paper [J. Krechl and J. Kuthan, Int. J. Quantum Chem. 21, 1029 (1982)] the energetically preferred configuration of the supermolecular His 195, Arg 171, NAD+, and lactate system was approximatively calculated. In this paper the EHT, CNDO/2 and INDO methods were used for an analogous reacting complex of lactate dehydrogenase involving His 195, Arg 171, NADH, and pyruvate models. Calculated quantum chemical data for the supermolecule I—II—III—IV are discussed in relation to a possible course of the enzymatic reaction.
Theoretical study of the interaction of tetramethylammonium with double‐stranded oligonucleotides
Authors: Gresh Nohad , Pullman Bernard 
Published: December 2004

AbstractComputations are performed on the interaction specificities of tetramethylammonium (TMA) for double‐stranded oligonucleotides held in the B conformation. The effects of base sequence and chain length are investigated. In the short oligomers (helices formed from dinucleoside monophosphates and trinucleoside diphosphates), the interaction energies of TMA are larger in the major groove of (dG)n · (dC)n than in the minor groove of either (dA)n · (dT)n or (dA—dT)n. Upon lengthening the oligomers, and owing to the gradual shaping of the grooves of the helix and cumulative effect of the phosphates, TMA is shown to increasingly favor the minor groove of (dA)n · (dT)n with respect to the major groove of (dG)n · (dC)n, with a sizeable energy difference computed at the pentanucleoside hexaphosphate level. The binding of TMA in the minor groove of (dA)n · (dT)n involves stabilizing contacts with several sites, on the bases and on the deoxyriboses. Configurations locating the cation closer to the thymine strand are slightly preferred over configurations locating it closer to the adenine strand.
Quantum‐mechanical and statistical mechanical studies of the torsional barrier of H2O2 in aqueous solution
Authors: Jönsson B. , Romano S. , Karlström G. 
Published: December 2004

AbstractMonte Carlo simulations were carried out on clusters consisting of one hydrogen peroxide molecule and n water molecules, n being 100 and 250. Quantum‐chemical potentials were used for the interactions between the species. The torsional angle was allowed to change in the simulation and its equilibrium value was found to change from ∼115° in the dilute vapor phase to ∼80° in the aqueous surrounding. At the same time the cis barrier became smaller than the trans barrier, as opposed to in the isolated molecule. A continuum model using the reaction field technique was investigated parallel with the simulations; with a reasonable size of the spherical cavity, containing the H2O2 molecule, it was found possible to qualitatively reproduce the Monte Carlo results.
A general formulation of the “quantum chemical le Chatelier principle”
Authors: Mezey Paul G. 
Published: December 2004

AbstractRegularities observed in the variations of calculated energy components, for example, those of electronic and nuclear repulsion energies in the analysis of conformational changes and in studies of the propagation of basis set errors in ab initio calculations, are found to be related to the variational principle and to the boundedness of energy expectation value functionals. These relations are analogous to the le Chatelier principle of equilibrium thermodynamics, and may be formulated as a general “compensation principle” for two sets of general parameters of the molecular total energy functional.
A unitary group formulation of many‐body theory: The spin‐shift formalism
Authors: Pickup B. T. , Mukhopadhyay A. 
Published: December 2004

AbstractThis paper lays the algebraic foundation of a unitary group approach to many‐body theory. We define a set of second quantized spin‐shift operators which are used to construct spin‐adapted many‐electron configuration functions. We investigate the particle‐hole transformation, normal ordering, and contraction of spin shifts. This gives an orbital Wick's theorem reflecting the permutational structure of the states spanning the irreducible representations of the spatial unitary group. We study the spinless Hamiltonian, and the relationship between spin shifts and unitary group generators.
A classification of polyenes into 4L + 2‐ and 4L‐classes on the basis of Coulson's bond orders and information theory and its application to the interpretation of electrocyclic reactions
Authors: Fratev F. , Enchev V. , Karadakov P. , Castan̄o O. 
Published: December 2004

AbstractIt is shown that in the framework of the π‐electron approximation even polyenes can be unambiguously divided into 4L‐ and 4L +2‐ classes. The classification scheme is based upon the sign alternation of the bond‐order between the first and the last atoms and upon the different information content of the bond‐order distribution for 4L‐ and 4L +2‐ polyenes, respectively. The classification has been shown to hold for the four lowest‐lying electronic states: the ground state 1A, the spectroscopically permitted excited state 1B, the spectroscopically forbidden state 1A, and the lowest triplet state 3B at all levels of sophistication in the π‐electron approximation, ranging from the simple Hückel Hamiltonian to the PPP‐multi‐CI approach. It is shown that the proposed classification is valid also for heteropolyenes and polyenelike/heteropolyenelike fragments from complex organic molecules. The classification has been shown to be useful for the theoretical interpretation of thermo‐ and photoelectrocyclic reactions. The sign of the bond order between the first and the last atoms (or the average information content) determines unambiguously the topology of the forming cyclic transition state and, thence, the course and the mechanism of these reactions. The results obtained indirectly support the hypothesis that the lowest excited states in butadiene and hexatriene are of the type 1B.
Anomeric effect in phosphate moieties of carbohydrates: An Ab initio study
Authors: Kaliannan P. , Vishveshwara S. , Rao V. S. R. 
Published: December 2004

AbstractAb initio SCF MO calculations at the STO‐3G level have been carried out on CH2OHOPO3−H and CH2OHOPO32−, which have been considered as model systems for the Cl‐phosphate moiety of sugars. The results predict higher anomeric energy for the phosphate moiety at Cl atom of pyranosides. Also a trans arrangement of the exocyclic O—P bond is preferred rather than a gauche arrangement, thus exhibiting a reverse exo‐anomeric effect. A complete potential energy map has been constructed for CH3OPO3−H, a model system for C6‐phosphate moiety. It is seen that the bond angle optimization brings down the relative energies of various conformations. The effect of the phosphate group on the preferred conformation of phosphate containing polysaccharides is also discussed.
Calculation of internal rotation spectra of three molecules with increasing asymmetry factors
Authors: Fernández M. , Góamez P. , Serna P. 
Published: December 2004

AbstractInternal rotation energy levels for phenol, fluoracetaldehyde, and difluoracetaldehyde are estimated with two methods: first, by means of the reduced inertia moment formula introduced by Pitzer; second, with use of an internal axis system and by taking into account the influence of the overall rotation through the inertia factor. Two methods have been used to estimate potential barriers (CNDO/2 and PCILO). Results are related to the asymmetry of the tops. Theoretical far IR spectra for the three molecules are calculated. In the case of phenol, good agreement is found with experimental data.
The valence bond description of conjugated molecules. II. A very simple method to approximate the structural weights of a fully correlated valence bond wavefunction
Authors: Hiberty P. C. , Ohanessian G. 
Published: December 2004

AbstractA very simple method to get the same structural weights as those provided by a fully correlated valence bond function is proposed. It applies to the ground states of delocalized systems, and only requires the knowledge of the occupied molecular orbitals. The SCF wavefunction is projected on a valence bond basis, and the SCF weights thus obtained are multiplied by some sets of multipliers which are transferable from one molecule to another. Some numerical examples are displayed, showing a very satisfying agreement between the structural weights thus estimated and the ones resulting from a rigorous calculation.
On the SCF theory of continuum solvent effects representation: Introduction of local dielectric effects
Authors: Contreras Renato , Aizman Arie 
Published: December 2004

AbstractThe introduction of local dielectric effects within the SCF theory of continuum solvent effects representation is examined at a semiempirical level. The formalism is developed in the frame of the reaction field theory within the effective charge approximation. The solvation free energies of Li+, Na+, F−, and Cl− ions in water were calculated in order to illustrate the reliability of the proposed model. The extension to molecules and molecular ions was performed including a desolvation corrective term related to the specific neighborhood of each atomic center. The results show a qualitative agreement with experimental data. A comment on the solvatonlike models for incorporating the solvent effect into the Hamiltonian is also given.
New concepts in bonded functions theory. I. alternative way of matrix elements evaluation
Authors: Golȩbiewski A. , Broclawik E. 
Published: December 2004

AbstractA new method of evaluation of energy matrix elements has been derived for bonded functions of Boys and a spinless Hamiltonian. In configuration‐driven CI algorithm the new approach is two to four times faster than the original Reeves' one, the relative speed depending on the case. In integral‐driven CI the new approach is about 10 to 12 times faster than the equivalent approach based on the Reeves' scheme.
Statistical representation of atomic systems structure. II. classification of the representation; entropy dependence of the total energy for isoelectronic series
Authors: Fraczak Marek 
Published: December 2004

AbstractThe energies of the ground states of the mononuclear atomic systems, until now determined merely by approximate methods, turn out to exhibit some almost exact interdependencies. A simple statistical functional of the electronic structure (the “γ representation”) turns out to be decisive for the system energy. In this paper that interdependence is further traced for the N‐electron systems in isoelectronic series (with constant N and varying Z). The resulting “combinatorial formula” reproduces the experimental data with the errors at least ten times smaller than those of the conventional Hartree–Fock approximation. The reason why there is such an exact formula for the ground‐state energy remains to be clarified. The limiting behavior of our energy formula for large Z exhibits consistency with the Thomas–Fermi and the Z−1perturbation expansion models.
Chemical ordering of molecules: A graph theoretical approach to structure‐property studies
Authors: Grossman Stuart C. 
Published: December 2004

AbstractThe problem of ordering of chemical structures and subsequent comparison of their relative properties is initiated here by an approach which uses chemical points of view as well as graph theoretical considerations. Briefly, a collection of structurally related compounds (such as isomers, but even more broadly, compounds of similar chemical architecture even if involving different atoms) is examined pairwise and the degree of structural similarity is deduced from a comparison matching of their skeletons. Depending on the nature of the similarity found, some compounds qualify as related and are depicted as vertices of a graph with the corresponding connection. The derived “comparison graph” represents the framework for subsequent comparison of selected molecular properties. “Comparison graph” can also be viewed as depicting a hierarchy of dominance for the compounds and can be drawn with arcs (directed edges), or alternatively as a poset (partially ordered set). Usefulness of such viewing on a collection of structures is illustrated on hexane and heptane isomers and their properties. The results are discussed, particularly with respect to alternative ordering schemes. It was found that the present approach complements and does not duplicate other results, thus offering some novel structural insights.
Maximization of atomic information‐entropy sum in configuration and momentum spaces
Authors: Gadre Shridhar R. , Bendale Rajeev D. 
Published: December 2004

AbstractThe maximum entropy procedure (MEP) of Jaynes has been extended to the case involving constraints in complementary spaces. It has been rigorously shown that the sum of information entropies in position and momentum spaces is invariant to uniform scaling of the electron coordinates. A new MEP procedure requires that this sum of entropies must be maximized subject to the known constraints in both spaces. A specific application of this maximization procedure for synthesizing atomic‐electron densities in coordinate and momentum spaces has been outlined.
Gaussian vs. Slater representations of d orbitals: An information theoretic appraisal based on both position and momentum space properties
Authors: Regier Philip E. , Fisher Jacob , Sharma B. S. , Thakkar Ajit J. 
Published: December 2004

AbstractA detailed appraisal of Gaussian‐type orbital (GTO) and Slater‐type orbital (STO) expansions of 3d orbitals is carried out for the 2S state of copper—a case that should be maximally unfavorable for STOs. The appraisal is based on a wide variety of both position and momentum space properties and utilizes an information theoretic quality assessment technique. It is found that GTO expansions are not as useful as STO expansions for the prediction of 〈p8〉, 〈p7〉, and 〈r−6〉 because these properties probe the functional deficiencies of GTOs at small r and large p. On the other hand, GTO expansions can predict accurate values of large r properties like 〈r8〉 despite the fact that their position space asymptotic decay is too fast. Unlike the case of s orbitals in helium, there does not seem to be any consistent ordering between accuracy in position space and accuracy in momentum space. The quality measures are found to be very useful for pinpointing the deficiencies of various expansions. This information enables us to construct easily a new GTO and a new STO expansion that are more accurate than any of the others in the literature. It is suggested that one STO is worth no more than two GTOs in the case of d orbitals.
Matrix elements between spin‐bonded functions in a hole‐particle formalism
Authors: Wormer Paul E. S. 
Published: December 2004

AbstractClosed formulas are given for Hamiltonian matrix elements between spin‐bonded functions in a holeparticle formalism. The derivation is based on Wick's theorem and the use of Jucys diagrams. The final formulas are only slightly more complicated than those for bonded functions in a particle formalism.
Theoretical approach to the evaluation of spin‐only magnetic form factors for many‐electron atomic systems
Authors: Glass R. 
Published: December 2004

AbstractGeneral expressions for evaluating spin‐only magnetic form factors for many‐electron atomic systems are derived using Racah algebra techniques. The formulas are derived in the |αLSMLMS〉 representation. The general formalism allows the evaluation of spin‐only magnetic form factors beyond the Hartree‐Fock approximation.
General features of geometries and potential constants of hydrogen bonded dimers
Authors: Millen D. J. 
Published: December 2004

AbstractThe interaction between theoretical and experimental investigations of hydrogen bonded dimers is examined. The influence on the interaction of the type of information available from current experimental techniqes is discussed, and new areas of profitable interaction are suggested. Dimer geometries, the directional character of hydrogen bonds, bond lengthening, and streching and bending force constants are considered in turn. Emphasis is placed on general features of hydrogen bonding rather than on properties of specific dimers.
The exact one‐electron model of molecular structure
Authors: Hunter Geoffrey 
Published: December 2004

AbstractThe Schrödinger equation satisfied by the electron density is derived without approximation from the theory of marginal and conditional amplitudes. The equation arises from a factorization of the total N‐electron wavefunction defined by the normalization appropriate to a conditional amplitude. This Schrödinger equation is an exact dynamical model for computing effective one‐electron potentials from known N‐electron wavefunctions. Results are presented for several small molecules. They display the shell structure of atoms, and the valence structure of both ionic and covalent molecules.
The minimal energy conformations of o‐benzosemiquinone anion radicals in the solvents containing the alkali metals and the alkali earth metals
Authors: Shinagawa Y. , Shinagawa Yasuko 
Published: December 2004

AbstractThe minimal energy conformations of o‐benzosemiquinone anion radicals in the solvents containing the alkali metals and the alkali earth metals were calculated by using the INDO method and the molecular geometry adjusting method. Our previous calculations of o‐benzosemiquinone anion radical in the lithium ion solution showed that the total energy of the minimal energy conformation in the case of the assignment |A3| > |A4| were lower by approximately 320 kcal/mol than that in the case of the assignment |A3| > |A4|. Therefore, the calculations of the minimal energy conformations of o‐benzosemiquinone anion radicals in the metal ion solutions were carried out in the cases of the assignment |A3| > |A4|. The conformations in both assignments A3 < A4 < 0 and (A3 < 0 < A4 and |A3| > |A4|) were calculated.Although the metal ions were not explicitly contained in the molecular geometries, the minimal energy conformations are considered to be close to the real minimal energy conformations of o‐benzosemiquinone anion radicals in the metal ion solutions. The CO bond lengths has a good positive rank correlation with the stabilities of o‐benzosemiquinone radicals in the divalent metal ion solutions and the bond angles ∠C3C4H4 had a good negative rank correlation with those stabilities.