bluuebunny/crossref_metadata_2025 huggingface dataset:
Configuration interaction calculations of the valence and the inner valence levels of the CS molecule
Authors: Müller Jiri
Published: December 2004

CI calculations were performed on the energies and relative intensities of the outer and inner valence levels of the CS molecule. Breakdown of the one‐particle model is discussed in terms of CI, and the results are compared with the analogous investigations using the Green's function technique.
Ab initio calculations on sulfur‐containing compounds. I. Uniform quality basis sets for sulfur: Total energies and geometries of H2S
Authors: Poirier R. A. , Daudel R. , Mezey P. G. , Csizmadia I. G.
Published: December 2004

Uniform quality basis sets (UQ‐NG; N=3, 4, 5), with s = p and s ≠ p, and a 6‐31 G* basis set have been optimized for the sulfur atom. These uniform quality basis sets in their uncontracted and contracted forms were used, together with other basis sets reported in the literature (a total of 40 basis sets), to study their accuracy in predicting the bond length and bond angle of H2S.
Cluster CPA calculations for infinite and half–infinite chains and applications to coupled polymer chains
Authors: Day R. S. , Ladik J.
Published: December 2004

It is shown how the cluster CPA can be used to obtain the local density of states p(E) in a half‐infinite or infinite random polymer chain. Calculations were performed for p(E) in a system composed of two coupled random chains with nearest neighbors interaction.
Interpretation of the antitumour activity of some diacridines
Authors: Abdul‐Ahad P. G. , Webb G. A.
Published: December 2004

Relationships are formulated in an attempt to explain the variation in the antitumour activity and toxicity of a series of diacridines. Three types of conformation are considered, these being relevant to the three types of double intercalation of DNA. The CNDO/2 method, with the incorporation of symmetry adapted orbitals, is used to obtain electron reactivity indices. Significant correlations are presented between the biological activities and the electronic and steric indices employed.
Molecular orbital study on the active center of lactate dehydrogenase
Authors: Krechl Jiří , Kuthan Josef
Published: December 2004

In the active site of lactate dehydrogenase important roles are given to aminoacids His195 and Arg171. The coenzyme nicotinamide adenine dinucleotide (NAD) is required in oxidized form (NAD+) for the enzymatic oxidation of the substrate L‐lactate. Molecular orbital methods CNDO/2, INDO, and EHT were applied to the study of the model of active center of lactate dehydrogenase. An energetically preferred arrangement of the models of His195, Arg171, NAD+, and lactate was found. Possible biochemical aspects of these arrangements are discussed.
Theoretical study of the interaction of nonactin with Na+, K+, and NH
Authors: Gresh Nohad , Pullman Alberte
Published: December 2004

In an attempt to account for the preferential binding to nonactin of K+ relative to Na+, theoretical computations are performed using the intermolecular interaction energies of the ionophore with the two cations. Both K+ and Na+ liganding conformations are considered, and an evaluation is made of the intramolecular energy expenditure caused by the reduction of the size of the cavity. The energy balance for the complexation of the two cations computed by taking into account the cation–ionophore interactions, the interactions between the liganding groups, as well as the desolvation enthalpies of the cations in methanol, favors K+ over Na+ by 4 to 5 kcal/mol, in fair agreement with the difference in the measured enthalpies of binding. The binding of NH to nonactin is also investigated.
Angular integrations in m‐dimensional spaces and hyperspherical harmonics
Authors: Avery John , Wen Zhen‐Yi
Published: December 2004

A general formula for angular integrations in many‐dimensional spaces (derived in a previous paper) is applied to several problems connected with solution of the Schrödinger equation for many‐particle systems. Matrix elements of the Hamiltonian are derived for cases where the potential can be expressed in terms of functions of the generalized radius multiplied by polynomials in the m coordinates. The theory of hyperspherical harmonics is reviewed, and a sum rule is derived relating the sum over all the harmonics belonging to a particular eigenvalue of angular momentum to the Gegenbauer polynomial corresponding to that eigenvalue. A formula is derived for projecting out the component of an arbitrary function corresponding to a particular eigenvalue of generalized angular momentum, and the formula is applied to polynomials in the m coordinates. An expansion is derived for expressing a many‐dimensional plane wave in terms of hyperspherical harmonics and functions which might be called “hyperspherical Bessel functions.” It is shown how this expansion may be used to calculate many‐dimensional Fourier transforms. A formula is derived expressing the effect of a group‐theoretical projection operator acting on a many‐dimensional plane wave. Finally, the techniques mentioned above are used to expand the Coulomb potential of a many‐particle system in terms of Gegenbauer polynomials.
Matrix element factorization in the unitary group approach for configuration interaction calculations
Authors: Payne Philip W.
Published: December 2004

Techniques of diagrammatic spin algebra are employed to derive segment factorization formulas for spin‐adapted matrix elements of one‐ and two‐electron excitation operators. The spin‐adapted basis is formed by the Yamanouchi–;Kotani geneological coupling method, and therefore constitutes an irreducible basis of the unitary group U(N), as prescribed by Gel'fand and Tsetlin. Several features distinguish this paper from similar work that has recently been published. First, intermediate steps in the derivation of each segment factor are fully documented. Comprehensive tables list the spin diagrams and phases that contribute to the possible segment factors. Second, a special effort has been made to distinguish between those parts of a segment factor that can be ascribed to a spin diagram and those parts which arise from the orbitals. The results of this paper should thus be useful for those who wish to extend diagrammatic spin algebra to evaluation of matrix elements for states built from nonorthogonal orbitals. Third, a novel graphical method has been introduced to keep track of phase changes that are induced by line up permutations of creation and annihilation operators. This technique may be useful for extension of our analysis to higher excitations. The necessary concepts of second quantization and diagrammatic spin algebra are developed in situ, so the present derivation should be accessible to those who have little prior knowledge of such methods.
Classical limit of quantum mechanics in the large
Authors: Thomas L. H.
Published: December 2004

Hilbert space may be regarded as a convenient standard approximation by interpolation and extrapolation to a unitary space, in general decomposable into a sum of semisimple spaces. In the limit we expect a particle theory expanded in powers of h according to the number of interacting particles. In the classical limit h tending to zero, phase space, with equations of motion reducible to Hamiltonian form, replaces Hilbert space. The modification of states by observing them is taken care of by considering probability distributions of petty ensembles. If the equations for any single observer can be made autonomous by replacing empirical time by universal time with an arrow we have a causal system. We then obtain the relation between probability and negentropy required for the second law of thermodynamics. An approximate Newtonian theory provides proximate particles with internal and external variables and admits the Poincaré group. For the internal variables we have approximately the Breit interaction. For the external variables we have equivalence for observers of the homogeneous Lorentz group of relativity. We introduce grand ensembles of ultimate particles, and nebulae as proximate particles in the large. We assume the Einstein principle of equivalence for the ten parameter set of observers suggested by relativity and suppose the second law of thermodynamics holds for each observer. The Einstein law of gravitation follows in classical theory to the order of the reciprocal of the large constant, in general with positive natural curvature as well as that corresponding to mass. Replacing particle interactions by fields we include them in classical theory.
Stability of chemical species
Authors: Leroy G.
Published: December 2004

In the framework of our theoretical approach of the structure and reactivity of chemical intermediates we have been led to reexamine the concept of stability which is widely used by the experimentalists often without specifying its true meaning. In this work we propose a more general definition of the concept of stabilization energy, namely,

where ΔHa is the heat of atomization of the species under consideration and the EAB's are standard bond energy terms derived from the heats of atomization of reference compounds. Using experimental heats of formation or semiempirical ones deduced from theoretical heats of reaction of appropriate isodesmic processes, we have calculated the stabilization energies of various types of chemical species: saturated, unsaturated and conjugated molecules, free radicals, carbocations, and carbanions. The results obtained can be rationalized in terms of steric hindrance, angular strain, polar interactions, electron delocalization, and substituent effects. Moreover, we have shown that heats of hydrogenation and bond dissociation energies do not provide accurate information on the thermodynamic stabilization of unsaturated compounds and free radicals, respectively. Among other applications the concept of stabilization energy allowed us to propose a detailed classification of free radicals and to rationalize their reactivity. Considering the particular case of radical recombination reactions we have been able to deduce interesting equations showing the relations between the concept of stabilization energy and other quantities commonly used in chemical physics, namely, the bond cleavage enthalpy [BDE(CC) if one considers alkane thermolysis], the thermodynamic stability measured by the free enthalpy change of a given reaction, and the kinetic stabilization related to the activation energy of a chosen chemical process

This analysis allowed us to give a new interpretation of the adjectives transient, persistent, and stable introduced by Griller and Ingold and to show that the persistence of a radical may be due to other factors than steric ones. In conclusion, the concept of stabilization energy appears to be a good tool for rationalizing the static and dynamic properties of chemical species.
Exponential factor optimization in characteristic function method for electrically charged particles
Authors: Demiralp Metin
Published: December 2004

In this work, the simplest basis function on hyperspherical coordinates with an optimization parameter is used to construct the characteristic function for a system of electrically charged particles. The first three coefficients of the serial expansion of the characteristic function in powers of eigenvalue parameter are evaluated in terms of well‐known functions. The construction of a Padé table for this expansion with the aid of these coefficients, a theoretical discussion, the optimization of the exponential parameter, evaluation of ground state energies for certain systems, and finally comparison of accuracy increments due to optimization for several systems complete the work.
Improved SCF interaction energy decomposition scheme corrected for basis set superposition effect
Authors: Sokalski W. A. , Roszak S. , Hariharan P. C. , Kaufman Joyce J.
Published: December 2004

A modified scheme for SCF interaction energy decomposition has been proposed where the nonphysical basis set superposition error (BSSE) has been corrected by means of the counterpoise method. A new procedure to separate the exchange and induction energy terms free of nonphysical BSSE has been tested in the case of the H2O dimer. The first order BSSE appears to be non‐negligible for strong hydrogen bonded complexes. In addition the scheme allows separation of the long‐controversial charge‐transfer contribution within the induction term, which has been considerably overestimated in previous studies.
Electronic structure and luminescent properties of Cu+ and Ag+ impurity centers in NaCl
Authors: Pedrini C. , Chermette H.
Published: December 2004

Fluorescence studies of Cu+ and Ag+ impurity centers embedded in NaCl are reported. The multiple scattering Xα method is used to describe the electronic structure of these ions in NaCl matrix. It is shown that taking into account the external lattice potential is necessary to obtain good results which can be compared to experiment. This has been done through the computation of an embedded NaCl cluster potential. Moreover, in NaCl:Ag+, it is also necessary to include relativistic corrections in order to obtain a reliable energy diagram. This is easily done by use of the Wood Hamiltonian, which allows the self‐consistent inclusion of Darwin and mass–velocity terms in MS Xα codes. A good quantitative agreement with experiment is finally obtained for the optical excitations and the emission mechanism of the luminescent centers. The metal–chlorine distance is predicted slightly larger than the NaCl bulk value and the harmonic force constant of the a1g vibrational breathing deformation of the cluster is calculated and found in good agreement within experiment. For the first time the Stoke shift of such systems is evaluated.
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

The 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

The 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

Generating 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

A 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

Second‐, 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

The 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

In 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

Computations 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.
On construction of diatomic potential energy functions
Authors: Kryachko Eugene S.
Published: December 2004

The Clinton function αn(R) corresponding to the model potential energy function Un(R), is proposed as a test of applicability of Un(R). Such an analysis is given for some model potential energy functions.
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

Monte 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

Regularities 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

This 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

It 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

Ab 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

Internal 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

A 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

The 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

A 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

The 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

The 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

The 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

A 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

Closed 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

General 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

The 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

The 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.
Heavy fermions and heavy atoms
Authors: Smith J. L. , Michael Boring A. , Weinberger P.
Published: December 2004

The heavy fermion state is a recently discovered ground state for the electrons in some cerium and light actinide compounds at low temperatures. Their properties are briefly reviewed and the calculational problems they present are discussed.
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

The 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.
Acetaldehyde photochemistry: Testing the additivity assumption and polarized basis set effects
Authors: Yadav J. S. , Goddard J. D.
Published: December 2004

Selected portions of the S0 and T1 potential energy surfaces of acetaldehyde surveyed in our earlier studies have been reexamined. The assumption of the additivity of basis‐set polarization and of electron correlation effects used extensively in our earlier work on acetaldehyde has been tested through explicit polarized basis‐set electron‐correlation calculations. The “additivity assumption” introduces average absolute errors in energy differences of only 1.9 (MP3) to 3.4 (MP2) kcal mol−1 in seven comparisons. The effects of using 6‒31G** SCF optimized geometries as opposed to single‐point calculations on 3‒21G SCF structures (6–31G**//3–21G) as in our previous papers were examined. In six comparisons, the average absolute error in relative SCF energies introduced by the use of the 3–21G geometries rather than the fully consistent 6–31G– ones was only 0.3 kcal mole−1. After a uniform scaling procedure, comparisons of the 6–31G** and 3–21G calculated vibrational frequencies with experiment for CH3CHO (S0), CH4, and CO (20 comparisons) yielded absolute differences of 41 cm−1 (6–31G**) and 57 cm−1 (3–21G). All these more elaborate calculations support for the specific case of acetaldehyde and various minima and transition states of relevance to its photochemistry, the commonly used and practically important approximations (e.g., additivity) made in our earlier studies.
Ab initio calculations of phenylene ring motions in polyphenylene oxide
Authors: Laskowski Bernard C. , Jaffe Richard L. , Komornicki Andrew
Published: December 2004

An important class of polymers is comprised, in part, of 1,4‐disubstituted phenylene groups. It is widely believed that large amplitude phenylene torsional motions play a critical role in determining such physical properties as the toughness and degree of crystallinity of these polymers. We have studied what is perhaps the simplest polymer in this class, polyphenylene oxide (PPO), using ab initio quantum chemistry methods to determine the conformational properties, torsional potential energy functions, and vibrational frequencies. From our calculations on dimer (diphenyl ether) and trimer (para‐diphenoxy benzene) fragments emerges a qualitative description of the mechanism of phenylene rotation in the polymeric material.
Analysis of nonadditivity effects and estimation of many‐body effects in linear water chains
Authors: Fülscher M. P. , Mehler E. L.
Published: December 2004

Calculations have been carried out on small linear water chains in order to investigate nonadditivity and cooperativity effects and to study the rate of convergence of the many‐body interaction potential. Ab initio wave functions were computed for the pentamer and all related subgroups by using the self‐consistent, nonorthogonal group function approximation. The interaction energy and its electrostatic, exchange, polarization, and charge‐transfer components were decomposed into a sum of n‐body terms. Simple formulas were developed for estimating the interaction energy of higher‐order complexes, which also allow an extrapolation to be made to infinite chain length. Comparison of the extrapolated interaction energies, with and without including the nonadditivity effects, showed that the latter increases the H‐bond's stability by more than 25%. The results further suggest that the interaction potential can be truncated at either the three‐ or four‐body level, depending on the accuracy required, but that, at best, only rough qualitative results can be expected when pair additivity is assumed. Finally, the dipole moment appears to be sufficiently converged with the inclusion of trimeric terms.
Ab initio calculations on the quenching of excited lithium atom by molecular hydrogen. I
Authors: Matsumoto Shiro , Mizutani Kimiko , Sekiguchi Atsushi , Yano Takakazu , Toyama Masayuki
Published: December 2004

MCSCF calculations were performed on the two lowest states of the LiH2 system, using a basis set of double‐zeta‐plus‐polarization quality. The characters of the wave functions in the region of the conical intersection of the potential energy surfaces were investigated in some detail. The depth of the minimum on the Ã2B2 surface was estimated to be 0.46 eV. The frequency of the a1 vibration at the center of the conical intersection was 3.76 X 1013 sec−1. Under the symmetry of the nuclear geometry lower than C2v in the immediate neighborhood of the center of the intersection, the upper state was characterized by a 3a′ natural orbital by which the more distant H is bonded to Li, while in the lower state the nearer H is bonded to Li. Thus, the process of the nonadiabatic transition caused by a nuclear motion of b2 symmetry is visualized as a smooth bond‐preserving process, and is expected to occur with high probability. For the correct description of the states in this region, MCSCF functions comprising limited CSF's were inadequate, and an additional process of nonorthogonal CI was required.
A theoretical examination of substituent effects on the detoxification reaction between glutathione and halogenated methanes
Authors: Luke Brian T. , Loew Gilda H. , McLean A. D.
Published: December 2004

An important metabolic pathway for halogenated methanes is the detoxification reaction with glutathione in the cell cytosol fraction. Experimental studies have shown that the rate of this SN2 displacement reaction is directly related to the leaving group ability of the dissociating halide; the rate increases in order of the ions F−, Cl−, Br−, and I− leaving. In this study, we examine the role of the other halomethane substituents on the rate of this reaction for compounds with a common leaving anion. To this end, reaction Cl− + CY3Cl → ClCY3 + Cl− (Y = H,F,Cl) is examined using ab initio methods. The barrier for this exchange process may arise from requiring an inversion of the CY3 group in the transition structure, the energy required to homolytically cleave the CCl bond, or from unfavorable steric and electronic interactions in the five‐coordinated transition structure. Of these three factors, only the third explains the calculated ordering of barrier heights. This suggests that successive halogenation of methane not only increases its activity as a substrate for anaerobic reduction, as was shown earlier, but also decreases its ability to take part in the detoxification reaction with glutathione.
Hydrogen chemisorption on, and diffusion through, palladium clusters
Authors: Baykara N. A. , Andzelm J. , Salahub D. R. , Baykara S. Z.
Published: December 2004

LCGTO‐VWN‐MP calculations have been performed for PdH, Pd3H, and for a series of Pd10H clusters representing hydrogen chemisorbed on a (111) surface and at various sites along a diffusion path through the cluster. The calculated chemisorption geometry (over a threefold site) has a PdH distance of 1.72 Å, slightly longer than that proposed by Eberhardt et al. (1.69 Å) on the basis of comparison of photoemission data with slab calculations. Agreement with known energy differences is very good: We find the chemisorption site 0.25 eV (exp. 0.26) more stable than the most stable (octahedral) bulk site. The top of the diffusion barrier, a triangular site, is calculated 0.34 eV (exp. 0.23) above the octahedral site. All the sites have a split‐off state of Pd d, H s character below the d bands, indicating that the ‘invisible’ state of H/Pd(111) recently (not) observed in UPS cannot straightforwardly be attributed to hydrogen incorporation at sites such as these.
On the role of symmetry in the ab initio hartree‐fock linear‐combination‐of‐atomic‐orbitals treatment of periodic systems
Authors: Dovesi Roberto
Published: December 2004

The symmetry properties of the mono‐ and bielectronic terms contributing to the Fock matrix in the ab initio Hartree–Fock treatment of periodic systems are discussed. A computational scheme which takes full advantage of the point symmetry is presented; in this respect, it represents a generalization of the scheme proposed in Int. J. Quantum Chem. 17, 501 (1980). Computational details and numerical examples are reported; it is shown that with respect to two of the bottlenecks of this kind of calculation, namely, computer time and backing storage required for the bielectronic integrals, it is possible to obtain saving factors as large as h and h2, respectively, where h is the order of the point group. Preliminary tests are reported which indicate that the study of relatively complicated systems, like quartz or corundum (9 and 10 atoms in the unit cell, respectively) at an ab initio Hartree–Fock level is now within reach.
On weighted löwdin orthogonalization
Authors: Bhatia Rajendra , Mukherjea Kalyan K.
Published: December 2004

Let f1, ⃛, fn be a basis of unit vectors and g1, ⃛, gn an orthonormal basis in a Hilbert space. We consider and solve the problem of finding an orthonormal basis e1, ⃛, en such that a weighted average of the distance of ej from fj and gj is minimized in the sense of least squares.
Structures and 1H‐chemical shifts of conjugation deficient hydrocarbons
Authors: Vogler H.
Published: December 2004

The geometrical and electronic structures and the 1H‐chemical shifts of some conjugation deficient hydrocarbons and their benzenoid isomers 1–3 which consist of two concentric perimeters have been studied by means of semiempirical procedures. Conclusions on the importance of macrocyclic annulenoid conjugation are drawn from the bond lengths pattern and the proton shifts in these compounds.
Electron correlation forms in the LiH molecule—fermi correlation
Authors: Feng Xuejun , Dahl Jens Peder
Published: December 2004

Correlation forms for electrons of the same spin small molecules are analyzed by taking the LiH molecule as an example. The analyses are carried out on the basis of the reduced density matrix theory and for both the ground state and the first excited state. It turns out that although there is a certain similarity, the correlated motions of parallel‐spin electrons in this four‐electron molecule are more complicated than those found for the two‐electron molecule. The correlation forms are not always constant. They vary essentially from one state to another, but are insensitive to fairly large changes of the internuclear distance for a given state.