bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparative analysis of the electrostatic potentials of some structural analogues of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin and of related aromatic systems
Authors: Murray Jane S. , Evans Patricia , Politzer Peter
Published: January 2005

We have carried out an ab initio STO‐5G computational analysis of the electrostatic potentials of four structural analogues of the highly toxic 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) and four related aromatic systems benzo[a]pyrene, benz[a]anthracene, and two isomeric benzoflavones. These systems, to varying degrees, induce aryl hydrocarbon hydroxylase activity and are believed to interact with the same cytosolic receptor in initiating their biochemical responses. Our present results for the TCDD analogues support and have allowed us to further qualify our early observations regarding factors which are linked to high biological activities in the dibenzo‐p‐dioxins and structurally similar systems. We find that a high degree of activity appears to require nonoverlapping negative potentials above all or most of the lateral regions, with an observed optimum range of magnitudes. In systems with central oxygens, it is required that the negative oxygen potentials be small and weak; however, oxygen negative regions in the molecule are not necessary for high activity. The observed differences between the potential patterns of the four aromatic systems and those of TCDD and its active analogues may reflect an inherent dissimilarity in the nature of their interactions with the cytosolic receptor. That the carbonyl oxygen negative potential of 7,8‐benzoflavone is significantly more negative than its isomer's supports earlier speculation concerning the former's more effective inhibiting effects on metabolic oxidation.
The origin of PPP theory
Authors: Pople John A.
Published: January 2005

An historical account is given of the part played by the author in the development of PPP theory in the early 1950s.
On the connection between semiempirical molecular orbital theories and effective hamiltonians: A constrained variational point of view
Authors: Khan P. , Das K. K. , Bhattacharyya S. P.
Published: January 2005

The idea of interpreting the semiempirical MO theories from a constrained variational point of view is explored. A density‐based approach is developed for calibrating the parameters of useful semiempirical Hamiltonians of molecular electronic structure theory. Results of some numerical experiments with model Hamiltonians of constrained variational origin are reported.
Two colorful applications of the PPP method
Authors: Balasubramanian K.
Published: January 2005

Two different color‐related applications of the Pariser‐Parr‐Pople method are described. The first one deals with an explanation of the UV‐visible absorption spectrum of the curcumin pigment in turmeric and the color change of curcumin upon addition of OH−. The other application deals with blue and red shifts in poly‐diacetylene to polybutatriene transformation. The applications of the FEMO and PPP methods to curcumin facilitate assignment of the two observed bands of curcumin (UV and visible) to the π → π* (UV) and n → π* (visible) transitions, respectively. A graph‐theoretical formulation of the PPP method is also considered.
Studies on large electronic systems with PPP: Testimony of PPP's activity
Authors: I'Haya Y. J.
Published: January 2005

It is nearly 40 years since the PPP method was proposed, and we demonstrate here that the method is still active in studying the electronic structures of very large conjugated systems and/or periodic systems. The kink (solition) problems in polyacetylene are solved with the UHF‐PPP Hamiltonian, demonstrating that there appear several new features which have not been obtained without considering the interelectronic interactions of the PPP type. Next, the electron correlation effect in polyacetylene is discussed through the crystal orbital‐PPP CI calculations. Finally, three‐dimensional crystal orbital‐CNDO calculations on graphite and black phosphorus are reported.
Peel, a modified PPP method, applied on the spectra of some nucleic acid bases
Authors: Fischer‐Hjalmars Inga , Henriksson‐Enflo Anita
Published: January 2005

A comparison between some different modifications of PPP methods is made. It is noticed that each modification may have its advantages but that some may be more suitable for interpretation of electronic spectra than others. It is also pointed out that PPP methods may allow more accurate descriptions of π properties than all‐valence methods, such as CNDO/S.

The Peel method, a PPP method especially adapted to reproduce the lowest π → π* transitions of a few reference molecules, has been applied to studies of the pyrimidine bases cytosine, uracil, and thymine. The obtained frequencies and oscillator strengths compare favorably with experimental evidence. The calculated polarization directions are in general agrreement with measurements. In case of cytosine a reinterpretation of measurements is suggested.
Nonlinear phase excitations in the PPP model of polyacetylene
Authors: Sasai Masaki
Published: January 2005

The Pariser‐Parr‐Pople (PPP) method has revealed rich effects of the π electron Coulomb interaction in (CH)x. We discuss results obtained by the PPP model with the unrestricted Hartree‐Fock (UHF) approximation and we show that concepts introduced by the PPP‐UHF model give a new insight on Franck‐Condon‐type electronically excited states in (CH)x. In the first part of this paper, a brief review of the PPP‐UHF model is given. We introduce phase and amplitude variables and show that solitons in the PPP‐UHF model are phase excitations. In the second part of this paper, the effective Hamiltonian describing phase excitations is derived. We show that the phase Hamiltonian derived by the time‐dependent Hartree‐Fock approximation is the classical limit of the Hamiltonian derived by the bosonization method of Tomonaga, Luther, and others. The phase Hamiltonian has a family of new soliton solutions. These solitons are pure electronic excitations and correspond to the exciton and the magnon in (CH)x.
A computational analysis of the electrostatic potentials and relative bond strengths of hydrazine and some of its 1,1‐dimethyl derivatives
Authors: Murray Jane S. , Sukumar Nagamani , Ranganathan Shoba , Politzer Peter
Published: January 2005

We have carried out a computational study of hydrazine and five of its 1,1‐dimethyl derivatives, focusing on their electrostatic potentials and relative bond strengths. Our approach has involved the calculation of ab initio self‐consistent‐field molecular orbital wave functions and molecular properties using the GAUSSIAN 82 system of programs. The electrostatic potentials of the hydrazines possess negative regions of varying sizes and strengths associated with the nitrogens of the α‐diamino linkages. Through an analysis of the positions of the most negative potentials of these regions, we have obtained directly the dihedral angles between the nitrogen lone pairs in these systems. Our use of the electrostatic potential to obtain these angles is a direct and general approach, in contrast to indirect procedures used in the past. We find this dihedral angle to be close to 90° in hydrazine, with variations in the substituted hydrazines that depend on the nature of the substituents. A highly polar structure is found for 1‐chloromethyl‐1‐methylhydrazine, which involves a delocalization of electronic charge from the substituted nitrogen towards the CH2Cl group. We find that substituents able to withdraw significant amounts of electronic density from the central nitrogen lone pair regions, either through resonance or by induction, have a slight bond strengthening effect on the central N‐N bond. This is attributed to a decrease in the repulsion between the weakened nitrogen lone pair regions. The difficulties encountered in seeking the controlled oxidation of hydrazine to nitro derivatives may be due, in part, to the fact that two factors which would favor this, highly negative nitrogen potentials and strong N‐N bonds, are opposing in nature.
Potential surfaces and nonadiabatic couplings in ionic systems: A study of the (O2H)+ interactions
Authors: Gianturco F. A. , Palma A. , Schneider F.
Published: January 2005

The adiabatic potential energy surfaces (PES) which are most likely to be involved in the elementary mechanism presiding over charge‐exchange and direct inelastic collisions between O2 molecules and collimated beams of protons are discussed. The general behavior of Diatomics‐in‐molecule (DIM) model interactions is analyzed in great detail as a function of the molecular vibrational coordinate and of the other internal nuclear coordinates. The general features of the lower two PES are discussed, and the corresponding nonadiabatic coupling terms between these surfaces are also computed and analyzed. These model results turn out to provide very useful indications on the specific dynamical features that are to be considered responsible for the inelastic, vibronic transitions observed in the target molecule during collisional experiments.
Roothaan's open shell theory from the viewpoint of an orthogonal group
Authors: Klimko G. T. , Mestechkin M. M.
Published: January 2005

Explicit equations for the constantsaandbof the energy as the average expectation value for the minimal domain of states giving the Roothaan energy functional are derived in terms of the total spin value and the “seniority” quantum number. The state with off‐diagonal long‐range order can appear in the framework of the Roothaan scheme for the quasidegenerate system as an alternative to the Hund rule. Admissible many‐electron states are established for all configurations of the icosahedral symmetry group systems, and correspondingaandbcoefficients are calculated.
The symmetrized basis function method for SU(2) and its application to the total spin states of many‐electron systems
Authors: Ding Peizhu , Wang Yi , Mu Yingkui
Published: January 2005

In this paper, the symmetrized basis function method is extended and used for the classification of the total spin states of many‐electron systems. The reduction matrix, which is expressed as a series of products and direct products of matrices is derived. It is very advantageous that the method can be completed by computer. As an example, the reduction matrix for a 5‐electron system calculated by IBM PC computer and the classification of the total spin states is presented.
Theoretical investigation of the hydration properties for the trans and gauche rotamers of succinonitrile
Authors: Walker G. A. , Bhatia S. C. , Hall J. H.
Published: January 2005

Ab initio SCF were performed on the trans and gauche rotamers of succinonitrile (SN) and on the monohydrated rotamers to determine the most probable hydration site(s) of water with SN and the relative strengths of these interactions and to examine the effect of one water molecule on the relative stability of the rotamers. MOPAC was used to determine the location of water on the rotamers of SN; the geometries of the hydrated rotamers were then optimized at the HF/4–31G level followed by single‐point calculations at the HF/6–31G and HF/6–31G* levels. The results show that trans SN is more stable than is gauche SN by 1.2 kcal/mol (at the HF/4–31G level with zero‐point energy corrections) and 0.8 kcal/mol (at the HF/6–31G* level, excluding zero‐point energy corrections). The hydrogen bond formed between the methylene hydrogen of trans SN and the oxygen of water is more stable than is the hydrogen bond formed between the nitrogen of SNt and the hydrogen of water by 0.63 kcal/mol. The most stable hydrogen‐bonding interaction for the gauche rotamer occurs for the methylene—OH2 hydrogen‐bonding interaction, which also has the largest hydration energy (ΔE = −4.39 kcal/mol), but it is less stable than is the most stable trans hydrogen‐bonding interaction (SNt—OH2) by 0.63 kcal/mol. Our results suggest that the trans rotamer of SN should associate with more water molecules than should the gauche. The relative rotamer stability between the trans and gauche rotamers of SN decreases by 0.2 kcal/mol (at the HF/6–31G* level) when water is hydrogen bonded to the methylene group of SN. A mechanism is also proposed to explain the phase‐separation behavior of this system.
Applications of the MBPT in the localized representation
Authors: Kapuy Ede , Bartha Ferenc , Bogár Ferenc , Csépes Zoltán , Kozmutza Cornelia
Published: January 2005

Diagrammatic formulation of the MBPT is applied when the occupied and the virtual canonical orbitals are separately localized by unitary transformations. In this localized representation, due to the off‐diagonal Fock matrix elements, the perturbation operator contains extra terms generating the so‐called localization corrections. These corrections enter the perturbation energy in third and higher orders. Their magnitude depends on the type of localization, but they represent only a small fraction of the canonical corrections. The calculation of the localization corrections, however, does not need a significant amount of extra computer time. It is shown that by introducing an “order of neighborhood” local and nonlocal effects of the electron correlation can be separated and the contribution of the nonlocal effects can be neglected to a good approximation. Ab initio calculations have been carried out for the normal saturated hydrocarbons: C2n+1H4n+4 and for the all‐trans conjugated polyenes C2n+2H2n+4. As to the ratio of the local and nonlocal corrections, it is shown that there is only a quantitative difference for these two kinds of systems (strongly or weakly localizable). Neglecting nonlocal effects, considerable amount of computer time can be saved.
Spectroscopic studies on the protonation of Schiff bases
Authors: Bicca Alencastro R. De , Badilescu S. , Lussier L. S. , Sandorfy C. , Thanh H. Le , Vocelle D.
Published: January 2005

This short review concerns the protonation of Schiff bases by halogenic or carboxylic acids with a bias toward the chromophore of visual and bacterial rhodopsins. It is pointed out that the weak acids available in these pigments could not protonate the retinyl Schiff base 100% and that a supporting mechanism is needed to achieve full protonation. Our Fourier transform infrared studies both at room and low temperatures relating to this problem are summarized, and the propable role of water is emphasized.
Intermolecular interactions with the direct reaction field method
Authors: Van Duijnen P. Th. , Rullmann J. A. C.
Published: January 2005

The direct reaction field (DRF) method, developed to incorporate the effects of a (large) semiclassical environment into the Hamiltonian of a quantum mechanical system, is briefly reviewed. It is shown that the DRF method behaves—at least—like a supermolecule SCF calculation. With the water dimer as an example, the similarity with the SCF procedure is demonstrated, and an application to the interaction between the active site of papain and the remaining 3000 or so atoms of this protein shows the inadequacy of dielectric constant models and the necessity of including atomic polarizabilities in model force fields.
Nonlinear coupling in the quantum statistical theory of proton transfer dynamics
Authors: Baňacký P. , Zajac A.
Published: January 2005

The possible role of the nonlinear coupling on the character of the dynamics of particle transfer process is investigated. The analysis and solutions of the kinetic equation indicate that nonlinear coupling causes symmetry breaking of particle transfer potential and determines possible equilibrium structure of the system. Dissipative coupling characterizes the rate of the system to reach thermodynamic equilibrium and along with nonlinear coupling and parameters of the system determines in a unique way the resulting equilibrium structure of the system.
MNDO calculation of reaction heats determininig the mechanism of antioxidant action of the phenols in polyolefins
Authors: Bolotin A. , Pipiraité P.
Published: January 2005

The quantum chemical substantiation of antioxidant action of the substituted phenols in polyolefins is presented. The electronic structure of the phenols, phenoxy radicals, and model products of polyolefin destruction was calculated by the MNDO method. On the basis of the presumed chemical reactions, the electronic structure criteria determining the antioxidant activity are selected. These are the O—H bond dissociation energy in phenols and the charge on the atoms of π‐system in phenoxy radicals. The competition between these reactions takes place, and the calculation of the reaction heats shows that the reactions with the formation of the substituted quinones are in preference to the reactions in which the peroxycyclo‐hexadienones are formed.
Optimization of molecular electronic structure calculations. 1. Local symmetry and local symmetricized orbitals
Authors: Kovrikov A. B.
Published: January 2005

A formalism is suggested of the so‐called local symmetricized orbitals to be used for the construction of a symmetricized basis in molecular electronic structure calculations. The local symmetricized orbitals are defined as additive contributions to the symmetry orbitals of a molecule that arise from symmetry operations of a corresponding atom. The local symmetricized orbitals are transformed according to the irreducible representations of the molecular symmetry group. This approach appears to be most suitable for the optimization of quantum mechanical calculations accounting for the spatial symmetry of compounds under consideration. This fact is due to the formalism of the local symmetricized orbitals that explicitly accounts for the local symmetry of basis function centers, which is essential for such optimization.
Sur une tentative d'unification des théaories quantiques de la cancèrisation par les polyacènes: I. Thèorie des règions M, L, et B
Authors: Gayoso Jose , Kimri Said
Published: January 2005

This study is a unified approach to quantum theories of polyacen carcinogenesis. Part I discusses the M, L, and B regions theory. Although the M region of polyacens is much less reactive than is the K region, the chemical reactivity of these two regions parallels each other. Consequently, the K‐L Pullman's theory may be reformulated by replacing the K region by the M region. The M‐L regions theory improves all the quantum structure‐carcinogenic activity relationships obtained previously in the K‐L framework. Moreover, it would appear that in these correlations, the most important role is played by nonconcerted electrophilic reactions. We suggest that the M‐L modified Pullman's theory and the Bay (B) region theory, proposed by Jerina et al., are related to different steps of the metabolic activation mechanism leading to ultimate carcinogen: the first step for the Pullman's theory and the carbocation step for the theory of the Bay region. We have termed the “M, L and B regions theory” as this unified approach to the problem of polyacens carcinogenesis.
A study of hydration effects on the conformational aspects of gaba mediators
Authors: Majumdar D. , Guha Sephali
Published: January 2005

The conformation of numerous chemical compounds is strongly influenced by solvents. Knowledge of their structure in solution is necessary, especially for a discussion of the biological and pharmacological activity of the molecules. The neurotransmitters and their agonists and antagonists are known to be flexible molecules that interact with quite distinct receptors. The conformational properties of several GABA (γ‐aminobutyric acid) mediators have been studied by the MNDO technique. The optimized geometries of the molecules have been used to study the solvent effects on their conformational properties considering the supermolecule approach for their first hydration shell. A conjectural pharmacophoric pattern for several GABA inhibitors has already been suggested from the molecular electrostatic potentials (MEP) of several molecules using a localized bond orbital technique. In the present work, MEP calculations have been carried out considering a solvent effect on the MNDO optimized geometries to investigate any deviation from the earlier results.
The modeling of nucleophilic and electrophilic additions and substitutions using extended Hückel‐based reaction potentials
Authors: Daul Claude , Goursot Annick , Morgantini Pierre‐Yves , Weber Jacques
Published: January 2005

A new formalism has been developed in order to evaluate intermolecular interaction energies for organometallic complexes including electrostatic, polarization, and orbital contributions based on extended Hückel molecular orbital (EHMO) theory. The electrostatic interaction is evaluated using (i) a multipolar expansion of EHMO charge density, or (ii) by calculating directly the electrostatic integrals in the basis of atomic orbitals. The polarization effects are evaluated by introducing a perturbation into the Hamiltonian. The orbital interaction is calculated by considering a supermolecule made of the organometallic substrate and a model electrophile or nucleophile. To provide the shortest possible response time on an interactive computer graphics facility, this model should require the minimum amount of computer time, which explains why approximate procedures are used to evaluate the dominant contributions to the interaction energies. Preliminary results show that these interaction energies lead to reaction potentials in good agreement with experiment for a broad series of nucleophilic and electrophilic addition or substitution reactions involving organometallic complexes. In addition, it is shown that the method can easily be extended for the calculation of solvent effects. To this end, developments considering the supermolecule surrounded by a polarizable continuum are in progress.
Excited and ionized states of RuO4 and OsO4 studied by SAC and SAC‐CI theories
Authors: Nakatsuji H. , Saito S.
Published: January 2005

The excitation and ionization spectra of RuO4 and OsO4 are studied theoretically by the symmetry‐adapted cluster (SAC) and SAC‐CI theories. This is the attempt to assign whole of the spectra by ab initio calculations including electron correlations. In the ground state, electron correlations work to reduce the polarity of the M–O bond overestimated in the Hartree–Fock calculation. The Os–O bond is stronger than is the Ru–O bond, which is reflected in the differences of the excitation and ionization spectra of RuO4 and OsO4. The excitation energies of the experimental spectra are well reproduced by the SAC‐CI theory, though the calculated intensities of some peaks are very small in comparison with the experiments. The outer‐valence ionization spectra calculated by the SAC‐CI theory agree well with the experimental photoelectron spectra. Some shake‐up peaks that are accompanied with an electron‐transfers from oxygen to metal are also calculated.
The dynamics of equally spaced multilevel model systems
Authors: Amirav A. , Even U. , Jortner Joshua
Published: January 2005

Coherent dynamics of multilevel systems is of interest for the elucidation of energy disposal and multiphoton chemistry of polyatomic molecules. We compare the analytical solutions for coherent multiphoton excitation of two sparse multilevel model systems: (1) The spin‐J system consisting of N = (2J + 1) equally spaced levels with the radiative coupling being determined by the xth component of the angular momentum. (2) The equally coupled N level system. The transition amplitudes for system (1) are always periodic, which is in contrast to the nonperiodic behavior exhibited by system (2) for N > 3.
Van der waals modulated spectral intensities: Application to infrared absorption
Authors: Linder Bruno
Published: January 2005

A formulation is presented of the effects of van der Waals interaction on infrared intensities. The theory takes account of induction and dispersion interactions (including charge interpenetration) and is applicable to fluids of arbitrary density. The transition moments are developed from generalized van der Waals‐induced moments, and expressions are obtained for the adsorption coefficient in terms of generalized charge densities and charge‐density (hyper)susceptibilities. Explicit expressions are given also of the integrated absorption coefficient in terms of multipoles, and (hyper)polarizabilities and their derivates, and the usefulness of this formulationin interpreting experimental data is illustrated by several examples. The relevance of the induction and dispersion interactions to allowed and to forbidden transitions made allowed is discussed.
Dirac‐Fock self‐consistent field method for closed‐shell molecules with kinetic balance and finite nuclear size
Authors: Mohanty A. , Clementi E.
Published: January 2005

We present a general method of implementing the kinetic balance condition within the Dirac‐Fock (DF) self‐consistent field (SCF) formalism for closed‐shell molecular structure. We review the steps leading to the derivation of DF SCF equations for closed‐shell molecules, particularly as formulated by Matsuoka et al. In the present approach, the large component of the molecular spinors are expanded in terms of atomic basis spinors of spherical‐type Gaussian functions, with the small component related to the large component by the kinetic balance condition. It is shown that imposing the kinetic balance condition on geometric Gaussian‐type basis functions allows us to obtain the Fock matrix elements, involving both the large and the small components, form the standard nonrelativistic Cartesian‐type matrix elements. By using properties of orthogonal polynomials, the solid spherical harmonics are expressed in Cartesian form, thus providing a general basis for transformation of one‐ and two‐electron‐matrix elements, obtained from a Cartesian Gaussian‐type basis, to a spherical Gaussian‐type basis. The advantages of using kinetically balanced geometric Gaussian‐type basis functions in molecular DF calculations including finite‐size nucleus effects are emphasized. For the sake of completeness, we have added in an appendix corrections to the nuclear attraction matrix elements for the finite‐size nucleus already derived by Matsuoka.
Non‐local relation between kinetic and exchange energy densities in Hartree–Fock theory
Authors: March N. H. , Santamaria R.
Published: January 2005

A non‐local generalization K(r, r') of the kinetic energy t(r) such that t(r) = ∫K(r, r') dr' is defined using the idempotency property of the Hartree–Fock first‐order density matrix. This is, in turn, related by means of an explicit differential equation to the non‐local exchange energy density X(r, r'). The relationship is illustrated for a couple of examples: with the Fermi‐hole in a uniform electron gas, of importance in the local density version of density functional theory, and with inhomogeneous electron systems.
Bonding in heteronuclear transition‐metal diatomics: NbIr
Authors: Castro Miguel , Keller Jaime , Mareca Pilar
Published: January 2005

Bonding in transition‐metal molecules presents novel features: (i) s electron bonding is overcome by multiple d electron bonding, (ii) intraatomic exchange favoring atomic magnetization competes with bonding that tends to pair the electrons, and (iii) in the heteronuclear dimers, the ionic terms may be important due to strong charge‐transfer effects. The NbIr heteronuclear diatomic molecule shows all these features clearly. The cellular multiple scattering‐xαβ calculation presented in this paper shows the ground state to correspond to antiferromagnetic coupling between the highly magnetic Nb atom and the Ir atom. A one‐electron charge transfer from Nb to Ir was found; the result is an ionic structure, Nb+Ir−, for the dimer. The computed equilibrium distance, 4.100 a.u., corresponds to a region where d bonding strongly overcomes the s bonding, which alone would have stabilized the molecule at 5.950 a.u. At intermediate interatomic separations, 5.35 a.u., the NbIr system has a state in which all molecular orbitals are bonding with a high hybridization between the ns and (n − 1)d electrons of each atom, resulting in a (almost entirely) covalent high multiple‐bond formation for this meta‐stable state of the dimer.
Electronic aspects of LADH catalytic mechanism
Authors: Tapia O. , Cardenas R. , Anders J. , Krechl J. , Campillo M. , Colonna‐Cesari F.
Published: January 2005

Electronic aspects of the catalytic mechanism of liver alcohol dehydrogenase (LADH) are studied with the help of ab initio analytical gradient SCF MO methods. Three points are considered: (i) role of the catalytic zinc; (ii) geometry and electronic structure of the transition state for the hydride transfer reactions; and (iii) factors affecting the energy gap for the hydride transfer step, namely, substrate binding to zinc, reaction field, and serine 48 effects on the potential energy profile. The coordination sphere of the catalytic zinc has been modeled with an ammonia molecule and two SH− groups; complexes with CH3O−, CH3OH, and CH2O have been studied; a (6, 2, 2, 2, 1/6, 2, 1/3, 2) basis set has been used for Zn++; a (5, 2, 1, 1/3, 2) was used for oxygen, carbon, and sulfur; and a (3, 1) was used for hydrogen atoms. The hydride step was studied with two model systems: pyridinium cation/1,4‐dihydropyridine coupled to the CH3O−/CH2O reaction, and cyclopropenyl cation/cyclopropene coupled to the CH3O−/CH2O system. For the latter, the role of Ser48 has been studied at the supermolecule level. The calculation on the hydride transfer step has been done at a 4–31G basis set level. The results obtained shed new light on the sources of catalytic activity of liver alcohol dehydrogenases.
Hydrogenlike atoms in uniformly charged sphere model of atomic nucleus. II. Application of basis‐set expansion method
Authors: Matsuoka O. , Fujimura N. , Hasegawa T.
Published: January 2005

The basis‐set expansion method has been applied to the nonrelativistic and relativistic ground states of one‐electron atoms H, Ar17+, Xe53+, Hg79+, and U91+ in the uniformly charged sphere (UCS) and the point‐change (PC) models of atomic nucleus, using the Gaussian‐type basis functions. The energies and the radial expectation values, especially 1/r2, converges faster in the UCS model than in the PC model. In the PC model, larger values of the exponent parameters of the basis functions are required both in the nonrelativistic and the relativistic calculations. Even in the UCS model, the larger values of the exponent parameters are needed in the relativistic calculations.
On a canonical functions approach to the elastic scattering phase‐shift problem
Authors: Kobeissi Hafez , Fakhreddine Khaled , Kobeissi Majida
Published: January 2005

The determination of the phase‐shift δρ(E) (related to a central potential V(r), a total energy E, and an angular momentum ρ) is considered. The “canonical functions” approach already used for the eigenvalue problem is adapted to that of δ. The conventional approach computes the radial wave function yρ(E;r) starting at rs ∼ 0 (with convenient initial values) and stepping on toward a large value r = R ∼ ∞, where yρ is matched to its asymptotic value yρ(R) ∼ a sin(kR – ρ π/2 + δρ) and δ is deduced. The present approach starts at any “origin” r0, replaces the use of the wave function y by that of the “canonical functions” α and β (well defined for given V, E, and ρ) and defines two functions q(r) and Q(r) in terms of α and β. When r → O, q(r) approaches a constant limit giving Q(r0), and thus the starting problem is avoided. Using this value Q(r0), the function Q(r) is generated for r > r0. The function Q(r) reaches a constant limit when r → ∞; this limit is precisely tan δ; thus, the “final” matching problem is avoided. The present method is applied to the Lennard–Jones potential function for low and high E and for low and high ρ. The comparison of the results of the present method with those of confirmed numerical methods show that the present method is competitive.
The rigorous (nonvariational) solution of the Schrödinger equation for a molecular potential of arbitrary shape. I. General formulation
Authors: Gegusin I. I.
Published: January 2005

An approach allowing us to solve exactly a Schrödinger eigenvalue problem for a molecular potential of arbitrary shape is developed. The method is based on an integral equation. The conditions, under which the original eigenvalue problem may be transformed into an algebraic one, are discussed. Owing to a special partitioning of space into cells, only some absolutely and uniformly convergent Green's function expansions are involved. As a consequence, all the algebraic manipulations are rigorously justified, in constrast to some direct generalizations of multiple scattering technique. Nevertheless, the method offered results in much the same secular equation, as one previously obtained [R. G. Brown, J. Phys. B 21, L309 (1988)]. The relationship between the present method and other approaches is discussed. The matching conditions between any two local representations of a true wave function, which are believed to be some extra and independent ones [E. Badralexe and A.J. Freeman, Phys. Rev. B 37, 10469 (1988)], are shown to result from a corresponding set of algebraic equations. Some computational aspects are also discussed. A preliminary result of the H2+ energy spectrum calculation is reported.
The rigorous (nonvariational) solution of the Schrödinger equation for a molecular potential of arbitrary shape. II. The basis functions, discrete spectrum, and the special case of MT potential
Authors: Gegusin I. I.
Published: January 2005

This paper continues Part I of this paper, referred to in the following as I. The construction of a basis set is explicitly presented. The special case of MT potential is considered, and the corresponding secular equation is shown to match exactly the one resulting from the well‐known scattered wave method. Finally, the bound states of a molecule are obtained as the solutions of a real‐valued matrix eigenvalue problem.
Higher analytic derivatives. I. A new implementation for the third derivative of the SCF energy
Authors: Colwell Susan M. , Jayatilaka Dylan , Maslen Paul E. , Amos Roger D. , Handy Nicholas C.
Published: January 2005

This is the first of a series of papers on the ab initio calculation of the second, third, and fourth derivatives of the energy with respect to nuclear coordinates. The knowledge of these derivatives yields anharmonic spectroscopic constants. Here, we present efficient formulae for the analytic evaluation of these derivatives for closed‐shell SCF wave functions. We discuss our implementation of the third derivative formula, in particular the integral and vectorization procedures. Applications are reported for H2S, CHOF, and HCCF.
Quasi‐particle equation from the configuration‐interaction (CI) wave‐function method
Authors: Casida Mark E. , Chong Delano P.
Published: January 2005

The Green‐function method is a well‐known way to reduce the quantum mechanical problem of n electrons moving in the field of clamped nuclei to the problem of solving a one‐electron Schrödinger equation (the quasi‐particle equation) involving a pseudopotential (the self‐energy). This method is widely used in solid‐state, low‐energy electron‐molecule scattering, ionization, and electron attachment theory, and much work has focused on finding accurate self‐energy approximations. Unfortunately, the operator nature of the fundamental quantity (Green function) in the usual quasi‐particle equation formalism significantly complicates the derivation of self‐energy approximations, in turn significantly complicating applications to inelastic scattering and multiconfigurational bound‐state problems. For these problems or wherever the operator approach becomes inconvenient, we propose an alternative quasi‐particle equation derived wholely within a configuration interaction wave‐function formalism and intended to describe the same phenomenology as does the Green function quasi‐particle equation. Our derivation refers specifically to electron removal but is readily generalized to electron attachment and scattering. Although the Green function and wave‐function quasi‐particle equations are different, we emphasize the parallels by rederiving both equations within the equations‐of‐motion formalism and then producing a wave‐function analog of the Green function two‐particle‐hole Tamm–Dancoff approximation.
Symmetry adapted formulation of the generalized Brillouin theorem
Authors: Godefroid M. , Lievin J. , Metz J.‐Y.
Published: January 2005

The singly excited functions satisfying Brillouin theorem are expressed as linear combinations of configuration‐state functions for any spin and spatial symmetries (atomic or molecular) and for any reference wave function. The generality of the formulation is ensured by the use of the irreducible tensor method that can be adapted to any symmetry point group of interest. The expansion coefficients are simply written as products of fractional parentage coefficients, spin‐ and orbit‐recoupling coefficients, and phase factors. The formalism is illustrated for some atomic (Kh) and molecular (C∞v, C3v, and Td) configurations. Group theoretical techniques are also used to correlate the Brillouin conditions within a chain of groups.
Midgap levels of photoexcited conductive polymers. I. A simple description of the midgap levels based on molecular orbital interaction
Authors: Tanaka Kazuyoshi , Yoshizawa Kazunari , Yamabe Tokio
Published: January 2005

We studied the midgap levels appearing in the photoexcited conductive polymers such as trans‐ and cis‐polyacetylenes, poly(p‐phenylene), polypyrrole, and polyacene based on the molecular orbital analysis. The midgap levels are constructed from the transformation of the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the ground state. As the result of the localization of the wave functions associated with these midgap levels, large polarization is induced between adjacent carbon atoms. Based on the examination of the energy gap between the two midgap levels, the polymers with a nondegenerate ground state such as cis‐polyacetylene, poly(p‐phenylene), and polypyrrole would show no sizable photoconductivities.
Approximate noninteracting kinetic energy functionals from a nonuniform scaling requirement
Authors: Ou‐Yang Hui , Levy Mel
Published: January 2005

We have recently proven an inequality for the exact noninteracting kinetic energy density functional \documentclass{article}\pagestyle{empty}\begin{document}$ T_s [n]:\mathop {\lim }\limits_{x \to \infty } T_s [n_\lambda ^x] \le T_s [n_s^y [n] + T_S^2 [n] < \infty,where{\rm }n_\lambda ^x (x,{\rm }y,{\rm }z) = \lambda n(\lambda x,{\rm }\lambda {\rm y, }\lambda z) $\end{document}. It is known that the gradient expansion through fourth order, T[n], violates this inequality. Toward improving TsGE[n], we have constructed two new functionals, Ts1[n] and Ts2[n], by keeping the zeroth and second orders in TsGE[n] and replacing the fourth order with two simple terms, respectively, so that these new functionals satisfy the inequality. Numerical tests are presented for Ts1[n], Ts2[n], and TsGE[n] and for the gradient expansion through second order. Hartree–Fock and hydrogenic atomic densities are employed.
Phase associated with the single‐particle density of many‐electron systems
Authors: Sukumar N. , Deb B. M.
Published: January 2005

In excited states of atoms and molecules, as well as in time‐dependent situations, the one‐electron density no longer suffices to completely characterize the electronic state; in addition, one now requires information about the electronic phase or the current density. We show that, for a stationary electronic state, the continuity equation of quantum fluid dynamics represents a differential equation for the electronic phase, which must be solved subject to certain periodicity conditions. These periodicity conditions arise from the nodal topology of the wave function and give rise to quantized vortices of current. The consequences of writing an electronic “wave function” for a many‐electron system directly in terms of the single‐particle density and phase have been investigated. We have shown that such a procedure leads to the appearance of an “internal magnetic vector potential.” We also establish the connection between the electronic phase and the geometrical (“Berry”) phase accompanying the adiabatic transport of a quantal system around a closed loop in parameter space. This leads to a generalization of the current density concept and allows us to discuss the geometrical phase in terms of the circulation of this current in parameter space.
Multidimensional wave‐packet analysis: Splitting method for time‐resolved property determination
Authors: Pernot P. , Lester W. A.
Published: January 2005

The splitting method is an approach for preventing unphysical behavior of a wave‐packet at the edges of its coordinate space representation grid. We examine here the possibilities and limits of using this method to perform time‐resolved analysis of the products of molecular processes.
Harmonic oscillator tensors in contact transformation theory
Authors: Palting Pancracio , Villa Maria
Published: January 2005

The application of contact transformation theory to the perturbed harmonic oscillator is reexamined in the light of the harmonic oscillator tensors previously presented. It is found that the recasting of the formalism of this problem in terms of harmonic oscillator tensors results in great simplifications, most of which stem from the introduction of the additional algebraic quantum numbers (l, m). The order of magnitude of each fragment of the Hamiltonian is easily recognizable, and the diagonal and nondiagonal parts contained therein are readily identifiable. The determination of the contact transformation operator is reduced to a simple formula. First, an analysis is made for a single mode of vibration, and it is subsequently extended to a multimode case. The perturbed diatomic vibrator is presented as an example.
A quantum‐chemical study of the LiH + LiH⋅− ⇄ LiH2− + Li⋅ reaction
Authors: Hess B. Andes , Zahradník R.
Published: January 2005

Parts of the potential energy surface of the title process and related processes have been investigated at the SCF/6‐31G**, SCF/6‐31++G**, and MP2/6‐31++G** levels. The investigated reaction is exothermic (−6.23 kcal/mol, MP4/6‐31++G**//MP2/6–31++G** level, ZPE included): A linear intermediate radical anion, LiHLiH⋅−, is significantly stabilized with respect to LiH + LiH⋅− (−38.74 kcal/mol, the same level as above). The BSSE at MP2/6–31++G**//MP2/6–31++G** amounts to 1.8 kcal/mol. The title process seems to be suitable for experimental study in molecular beams.
A graphical approach to configuration interaction studies in molecules using determinants of nonorthogonal orbitals
Authors: Rettrup Sten , Thorsteinsson Thorstein , Sarma C. R.
Published: January 2005

A direct graphical approach is presented for the evaluation of matrix elements of a spin‐free Hamiltonian between Slater determinants of nonorthogonal orbitals. The matrix element contributions are obtained by repeated use of N – 2 electron minors of the overlap matrix that are generated by independent looping of reduced N – 2 electron graphs for the α‐ and β‐strings of the determinants. Because of the successive generation of the matrix contributions, the method is well suited for direct diagonalization schemes.
Two‐electron repulsion integrals over Gaussian s functions
Authors: Gill Peter M. W. , Johnson Benny G. , Pople John A.
Published: January 2005

We present an efficient scheme to evaluate the [0](m) integrals that arise in many ab initio quantum chemical two‐electron integral algorithms. The total number of floating‐point operations (FLOPS) required by the scheme has been carefully minimized, both for cases where multipole expansions of the integrals are admissable and for cases where this is not so. The algorithm is based on the use of a modified Chebyshev interpolation formula to compute the function exp(−T) and the integral Fm(T) = ∫01u2mexp(−Tu2) du very cheaply.
Self‐Consistent‐Field potential‐energy surfaces for hydrogen atom pairs within small palladium clusters
Authors: Anchell James , Gutowski Maciej , Nichols Jeff , Simons Jack
Published: January 2005

An ab initio electronic structure study is presented of hydrogen–hydrogen interactions in an electronic environment perturbed by the presence of palladium atom clusters. In particular, we investigated changes in the interatomic potential when the atomic centers are trapped inside an fcc palladium octahedral hole and when they are separated from each other by a (111) plane of palladium atoms. The (111) plane was modeled with a cluster of three palladium atoms. Self‐consistent field (SCF) level calculations were performed, and palladium atom pseudopotentials were employed to make the systems studied computationally tractable. For pairs of atoms placed within the octahedral hole, various lines of approach were considered [along the (100), (110), and (111) directions]. Lattice deformations and electronic excitations were examined for their effect on the interatomic potential.
A Laplace transform solution of Schrödinger's equation using symbolic algebra
Authors: Clarkson Michael E. , Pritchard Huw O.
Published: January 2005

A general solution to the one‐dimensional time‐independent Schrödinger equation is derived using the properties of the Laplace transform. The derivation assumes that the potential function is real and that it can be expressed as a Fourier series with a finite number of terms, which includes, but is not limited to, equations of the type of Hill's or Mathieu's equations. In the case where the Laplace transform of the potential function contains only simple first‐order poles, then a complete closed‐form solution is derived. In other cases, an approximate solution is derived and the symbolic algebra program MACSYMA is used to carry out the inverse Laplace transform. The MACSYMA implementation of this general solution is discussed and is applied to a sample problem, namely, the solution of eigenvalues of Mathieu's equation. Comparison is made between the eigenvalues obtained by this method and those obtained by another analytical solution of this equation, by Hill's method.
Application of an optimization technique to the discretized version of the Griffin–Hill–Wheeler–Hartree–Fock equations
Authors: Custodio R. , Giordan M. , Morgon N. H. , Goddard J. D.
Published: January 2005

Application of the SIMPLEX optimization method to define the mesh of the discretized version of the Griffin–Hill–Wheeler–Hartree–Fock (GHWHF) equations was studied. Improved discretization parameters with respect to the original method were obtained for atomic systems with two or four electrons and for the H2 molecule. For the atomic systems, the following correlations between the discretization parameters and the total energy were found: N = a · In(ΔE) + b; Ω0 = a′ · In(ΔΩ) + a″; and In(ΔΩ) = b′ · ln(N) + b″. These equations provide a systematic procedure to reach a desired degree of accuracy in the energy for the atomic systems studied as well as to fix the basis set to be employed. These equations are similar to those found earlier for eventempered basis sets and permit the establishment of a relationship between the two methods. The even‐tempered method is also an approximate solution of the GHWHF equations. The optimized integral discretized basis is more efficient in representing small basis sets for atoms and the basis for the hydrogen molecule in comparison to the even‐tempered one. The optimization procedure was successfully applied to generate the universal basis for the atomic systems studied.
Electron correlation in pericyclic reactivity: A similarity approach
Authors: Ponec Robert , Strnad Martin
Published: January 2005

The recently proposed topological approach to chemical reactivity in terms of the secondorder similarity index was applied to the detailed analysis of correlation effects in pericyclic reactivity. This analysis implies that (i) the electron correlation is generally more important in forbidden reactions than in the allowed ones; (ii) in contributing to the discrimination between the allowed and forbidden reactions, the Fermi correlation acts in parallel with the Coulomb one; and (iii) the so‐called multibond reactions are more sensitive to electron correlation than are the electrocyclic ones.