bluuebunny/crossref_metadata_2025 huggingface dataset:
Orthogonally spin‐adapted coupled‐cluster equations involving singly and doubly excited clusters. Comparison of different procedures for spin‐adaptation
Authors: Piecuch Piotr , Paldus Josef
Published: January 2005

A particularly compact form of the orthogonally spin‐adapted coupled‐cluster equations involving all singly and doubly excited clusters is derived for the general case of a non‐Hartree–Fock closed‐shell reference determinant. The diagrammatic approach based on the graphical methods of spin algebras is applied. The relationship of different diagrammatic procedures for spin‐adaptation, employing both bare and spin‐adapted two‐electron interaction vertices, is discussed. A comparison with the results obtained with algebraic spin‐adaption approaches is also given.
Fractal displays of genomic DNA. I. Eco RI fractal lattice of buffalo rat
Authors: Finkel David L.
Published: January 2005

A complex multidimensional DNA structure is obtained when treating the elements of the Eco RI restriction set of buffalo rat as a cascade of self‐similar units. Fifteen of the 16 (about 94%) Eco RI repeat families, ranging in length from 2300 to 83 nucleotides and in frequency from 500 to > 106 units, are sorted into a three‐tiered well‐ordered fractal/Fibonacci lattice. On the first level, 11 fractal dimensions are defined as unique collinear collections of repeat DNA families in a “graph‐invariant” plot of log(fragment length):log(frequency). On the second level, the individual dimensional arrays are clustered as groups, each sharing, at last, one common vertex. On the third level, the entire display is scaled along a stepwise log to the base 6 expansion. This study represents the first successful attempt to unify an “entire” set of repeat DNAs into a single motif, in effect confirming the contention that select repeat DNAs provide a framework for the discrete packaging and unpackaging of genomic information.
Bonded tableau unitary group approach to the many‐electron correlation problem
Authors: Li Xiangzhu , Zhang Qianer
Published: January 2005

A spin‐free symmetry‐adapted valence bond (VB) state, named bonded tableau (BT), is deduced from the classical bonded function and labeled by an at most two‐column Weyl tableau. The complete set, which is composed of the BT basis or canonical bonded tableau (CBT), can be constructed from an overcomplete set of BT states. CI CBT and VB CBT are two kinds of complete sets that are constructed in this paper. They can be used, respectively, in the CI and VB theory. It is shown that there is a one‐to‐one correspondence between the labeling scheme for CI CBT and the Gelfand–Tsetlin (GT) basis. This relationship enables an efficient generation and compact representation of the BT basis if one desires to use the known global representation scheme for the GT basis. Effective algorithms for the matrix element evaluation of unitary group generators and products of generators between BT states are presented. In the formulation, the action of a generator on a BT state yields another BT state times a coefficient, so that the matrix elements of an arbitrary multiple product of generators are reduced to a calculation of the overlaps between BT states. The evaluation of the overlaps leads to a simple factorization into cycle contributions, whose values are given explicitly and only depend on the length parameters of the cycles. It is hoped that the presented formalism can facilitate the procedures for handling of the many‐electron correlation problem.
The multireference constant denominator perturbation theory for one‐particle systems and its application to the anharmonic oscillator
Authors: Lan Zhida , Cullen John M.
Published: January 2005

In this paper a multireference constant denominator perturbation theory (CDPT) is developed to reduce incomplete basis set errors arising when solving the Schrödinger equation with a finite basis set. The advantage of this method is that very few basis functions are needed, and all calculations if carried out to high enough order in the perturbation treatment effectively use a complete basis set. As a first step the theory has been restricted to one‐particle Hamiltonians and applied to the anharmonic oscillator to study the convergence properties. For perturbation calculations carried out to fifth order, results from Pade approximates show an improvement in accuracy of between one and three orders of magnitude.
Two‐center molecular repulsion integrals over slater functions
Authors: Maslen E. N. , Trefry M. G.
Published: January 2005

For the general two‐electron two‐center integral over Slater functions, use of the Neumann expansion for the electron‐electron interaction term yields the standard auxiliary functions. These are expanded and integrated explicitly by two independent methods. The resulting simple analytic formula for the total integral is completely general, requiring only the Slater function quantum numbers and exponents and the internuclear separation. Hence all two‐electron hydrid, coulomb, exchange, and one‐center integrals are considered. The efficiency of calculation of this expression is compared with those of other methods, indicating an order of magnitude improvement in speed over recursion for the exchange integral.
Scalar nonlinearities in physics and chemistry
Authors: Heimsoeth Bettina
Published: January 2005

Scalar nonlinearities are operators of the form N(u) = 〈Au, u〉Bu, where A, B are linear operators, and 〈·,·〉 is the inner product in a Hilbert space ℋ︁. This paper reviews applications of scalar nonlinearities. We show that operators of the form N(u) are found in equations that describe phenomena of classical mechanics, open systems of quantum mechanics, and chemical physics.
Comparability graphs and molecular properties III. C9 and C10 alkanes
Authors: Bonchev D. , Kamenska V. , Mekenyan O.
Published: January 2005

The graph‐theoretical approach recently developed for comparing and ordering isomeric chemical compounds is extended and applied to the calculation of various molecular properties of C9 and C10 alkanes (critical densities, volumes, and pressures; heats and entropies of vaporization; standard entropies in liquid and gas state). The key to the approach is in specifying subsets of comparable isomers which are located in the same paths in the so‐called comparability graphs. Optimized samples for structure‐property correlations are thus obtained.
Ab initio studies of arginine and deaminoarginine
Authors: Sapse Anne‐Marie , Strauss Rosalind
Published: January 2005

L‐Arginine and Deaminoarginine were studied via quantum chemical ab initio calculations using the STO‐3G basis set for arginine and the STO‐3G and 3–21G basis sets for deaminoarginine. It was found that the most stable conformations are the ones featuring the carboxyl group slightly twisted versus the rest of the molecule, which adopts an extended conformation. It was also found that the cyclic zwitterion is less stable than the neutral cyclic conformation and that there is a barrier to the proton transfer from the carboxyl's oxygen to one of guanidine moiety's nitrogens.
Inclusion of relativistic effects into ZDO methods. IV. Relativistic CNDO/1
Authors: Boča Roman
Published: January 2005

A relativistic, four‐component version of the CNDO (complete neglect of differential overlap) method is introduced. This method spans the class of zero‐differential‐overlap approximation and it utilizes a nonempirical parametrization based on results of atomic Dirac‐Fock calculations. The spin‐orbit splitting is included implicitly using the relativistic basis set which distinguishes Slater‐type functions of the lower and higher component spinors. The method is applicable to closed‐shell as well as to open‐shell systems. The actual version used is the R‐CNDO/1χ with a variable scaling approach. Applications to molecular geometries, ionization potentials, and energies of spin‐orbit splitting are demonstrated for AH, AH2, AH3, MH, InX, and HgI2 molecules at the self‐consistent‐field level as well as in the second order of the many‐body perturbation theory.
Differentiability of degenerate electronic wave functions with respect to parametric variables
Authors: Yin Li , Goscinski Osvaldo
Published: January 2005

The present paper is aimed at differentiability of electronic wave functions, with respect to parametric variables, in the presence of electronic degeneracy. An analysis is made of a wave function, constructed so that it has the largest domain of parameter space in which it is differentiable, with the help of Berry's formula for the geometric phase. In particular, the electronic wave functions, in presence of a double degeneracy in two‐ and three‐dimensional parameter spaces, are studied in detail. It was found that the three‐parameter‐dependent wave function is differentiable everywhere except along an axis starting from the degenerate point where it is discontinuous. The two‐parameter‐dependent wave function is differentiable everywhere except at the degenerate point where it is disocontinuous. These singularities are expected to have consequences on wave functions having parametric variables as arguments.
The atomic three‐body problem. An accurate lower bond calculation using wave functions with logarithmic terms
Authors: Kleindienst H. , Emrich R.
Published: January 2005

Accurate lower and upper bounds for the nonrelativistic ground state energies E0 of the real systems 4He, H−, D−, and T− were calculated by the method of variance minimization using wave functions which include logarithmic terms. In addition, an analogous treatment with an infinite mass approximation for H−, He, and the isoelectronic series up to Z = 10 was carried out. Especially for H− and He the results (a.u.) are given by

These values have an absolute error smaller than 1.28 · 10−4 cm−1 for H−1and 3.10 · 10−9 cm−1 for He. Moreover it is shown that a variation of nuclear mass mn for the H− species does not produce a second discrete eigenvalue.
Partitioning of the Hamiltonian operator in the lie algebra formalism. Applications to the generalized brillouin theorem and the open‐shell Hartree‐Fock approximation
Authors: Koutecký Jaroslav , Scheuch Carsten
Published: January 2005

The Born‐Oppenheimer spin‐conserving Hamiltonian operator expressed in terms of the Lie algebra generators is partitioned according to the number of connected lines in the corresponding graphical representation. A simple derivation of some basic relations with the generalized Brillouin theorem, HF approach, and random phase approximation is presented.
Theoretical basis and design of the PPP model Hamiltonian
Authors: Jug Karl
Published: January 2005

The LCAO method is used for a discussion of the π electron approximation. The Hückel method is presented and rephrased on the basis of Löwdin orthogonalized orbitals. The background of the Pariser‐Parr‐Pople model Hamiltonian is illuminated and the method is derived from first principles. The parametrization is discussed and the scope reviewed.
Comparison of GVB and AMO approaches to the study of the PPP model of butadiene
Authors: Pauncz Ruben , Číž;ek JiřÍ
Published: January 2005

The purpose of this article is the comparison of generalized valence bond (GVB) and the alternant molecular orbital (AMO) techniques both in the localized and delocalized form. The model we have chosen is the PPP model of cis‐butadiene which is relatively simple and transparent. For equal bond lengths the AMO using delocalized orbitals is better than the GVB. For physical distances of the atoms these methods are of comparable accuracy. With the extension of the length of the central bond the GVB becomes better and is exact in the limit of two ethylenes. The AMO method using localized orbitals always gives the worst result of the three. In the limit of two ethylenes AMO in the localized and delocalized forms give the same error.
Application of the PPP method to the calculation of ionization potentials and electron affinities of conjugated organic molecules
Authors: Selsby R. G. , Pennance Philip , Barnhard K. I.
Published: January 2005

By making the atomic effective charge self‐consistent with the molecular charge density, it is shown that the PPP method can accurately predict the ionization potential and electron affinity computed as an energy difference of separately minimized ground and ionized states. The calculation is applied to a large variety of conjugated organic molecules, including heterocyclic systems and π‐electron‐contributing substituent groups.
A tensor representation of the real spherical functions for the continuous group O in cartesian coordinates: Example of eulerian rotations
Authors: Boudeville Yves , Rousseau‐Violet Jeanne
Published: January 2005

The aim of this article is to give a practical way for the use of real spherical functions in another frame than the frame in which they have been defined. For instance, we calculate physical properties from a local frame and use them in the general frame, deduced one from the other by Eulerian rotations on the coordinates. The power of the method is in the use of cartesian coordinates and in the definition of a scalar product between these functions to set up the complete vectorial space generated by these representations of the group O.
Dependence of valency on geometry for configuration interaction wave functions
Authors: Siddarth Prabha , Gopinathan M. S.
Published: January 2005

Earlier definitions of valencies of atoms, molecules, and molecular orbitals are extended to configuration interaction (CI) wave functions. Using these definitions, valencies both at equilibrium and nonequilibrium geometries of molecules are calculated at the CI level and compared with non‐CI results. CI valency correlation diagrams are obtained. Valency variation with bond length using correlated wave functions is found to behave properly unlike in the case of SCF wave functions.
Upper and lower bounds to the Schrödinger equation eigenvalues
Authors: Fernández Francisco M.
Published: January 2005

A method is presented for obtaining rapidly convergent upper and lower bounds to the eigenvalues of the Schrödinger equation for one‐dimensional and central‐field models. The logarithmic derivative of the wave function is written as a Padé approximant and the bounds are obtained by simply counting the real zeroes of the denominator.
A CI pseudopotential‐based description of the low‐lying states of Ag O2
Authors: Bravo G. , Flores‐Riveros A. , Novaro O.
Published: January 2005

Extensive SCF‐LCAO‐MO variational and perturbative configuration interaction (CI) calculations framed within an effective core potential approximation have been performed to determine the two experimentally observed geometrical isomers of Ag O2 and the interconversion route between them. These structural forms, associated to the ground‐state local minima, yield virtually the same energy, and their spontaneous interconversion is strongly indicated, which agrees fairly well with the experimental measurements. The reaction Ag + O2 → Ag O2 was theoretically analyzed along a CI fully optimized energy pathway for the ground and various excited states, within C2v and Cs symmetry. Although a tight‐ion pair (A O) character is predicted for the ground state at the equilibrium geometries, its dissociation leads to neutral rather than to ionic fragments. The study of the reaction path within Cs symmetry shows an avoided crossing between the ground state and another 2A″ potential curve where the former correlates adiabatically with the reactants Ag(2S) + O2(1Δg). This indicates that the formation of the complex proceeds via a reactive state of molecular oxygen. The higher 2A″ electronic curves correlate with the metal 2P excited state, and the oxygen binding is found to be less favorable. The present results are shown to have an important bearing on the experimentally known catalytic properties of oxygen adsorbed on silver surfaces.
Non‐radiative deactivation of molecules. II. Theoretical study of the internal conversion rates in azulene
Authors: Gustav Klaus , Storch Michael
Published: January 2005

Based on completely optimized molecular geometries of the photophysically relevant electronic states, the corresponding vibrational energies and wave functions and the theoretical rate constants for the internal conversion processes of azulene within the Sp, term system are presented. The influence of the promoting and accepting modes on the radiationless deactivation rates is discussed. An analysis of the active nonradiative modes is given. In context with the respective theoretical radiative rates, the efficiency of the different azulene fluorescences is interpreted.
Electrostatic potential mapping of polycyclic aromatic hydrocarbon diol epoxides using a dipole
Authors: Kumar Anil , Mishra P. C.
Published: January 2005

Electrostatic potential maps of benz(a)anthracene diol epoxide, benzphenanthrene diol epoxide, chrysene diol epoxide, and tryphenylene diol epoxide, which are ultimate carcinogens derived from the corresponding polycyclic aromatic hydrocarbons, have been studied using a dipole of length 1 Å and strength 1 Debye. The net charge distributions in the molecules were obtained using the MNDO molecular orbital method. The maps were drawn for two closest distances of approach between the charged ends of the dipole and atoms of the molecules. Using the electric field directions and values of electrostatic potentials, reactive sites and relative reactivities of these molecules have been examined. Existence of a bay region appears to be important for the carcinogenic activity of a diol epoxide in this class of carcinogens.
Solvent effects on mechanism of tetramethylene polymerization
Authors: Jug Karl
Published: January 2005

The polymerization mechanism of tetramethylenes was reinvestigated under inclusion of solvent effects. The approach of a methanol molecule to a borderline diradical, a typical diradical, and a typical zwitterion was studied by a valence, charge, and dipole moment analysis of SINDO1 calculations. Whereas the solvent molecule has no effect on the character of the zwitterion, the borderline diradical was found to switch to a zwitterion at the approach of the methanol molecule if the distance between the donor carbon and the methanol oxygen is below 2 Å. A similar switch of character was observed for the typical diradical at CO distances below 1.5 Å. From energy considerations it is concluded that borderline diradicals can follow a zwitterionic polymerization mechanism in polar solvents, whereas typical diradicals are much less likely to do so.
Theoretical approach to reactions of polyatomic molecules
Authors: Zülicke L. , Merkel A.
Published: January 2005

A scheme for systematic reduction of the theoretical treatment of elementary reactions involving polyatomic molecules is described; it consists of (1) limitation to the energetically relevant regions of the nuclear configuration space (the reaction path and its near environs) and (2) restriction to the dynamically relevant subspace of the nuclear configuration space (the active modes). Starting from a generalized reaction path Hamiltonian of Nauts and Chapuisat allowing for the use of arbitrary curvilinear coordinates and several large‐amplitude modes, the realization of the above‐sketched scheme is discussed. A compilation of recent work along these lines, mostly based on the simplified Miller‐Handy‐Adams reaction path Hamiltonian, is given with particular emphasis on applications of a statistical adiabatic model.
MNDO CI study of the photoisomerization about polar double bonds
Authors: Ertl Peter
Published: January 2005

Cis‐trans photoisomerization of ethylene, some of its methyl and fluoro derivatives, methaniminium, and aminoborane was studied and twisted biradical geometries of these molecules in So, S1, and S2 states were optimized using the half‐electron version of the MNDO method with configuration interaction. The results are quite satisfactory, in good agreement with available ab initio data.
Quantum chemical calculations of silicon clusters in silicon dioxide and point defects in β‐cristobalite
Authors: Gadiyak G. V. , Korolenko I. V. , Morokov Yu. N.
Published: January 2005

A local electronic‐state density in β‐cristobalite in the empirical tight‐binding approximation has been calculated by the recursion method. In particular, silicon clusters have been considered in β‐cristobalite containing up to three coordination spheres of silicon atoms.
Graph theory of synthons
Authors: Kvasnička V. , Pospíchal J.
Published: January 2005

A graph‐theory model of synthons is suggested. A synthon is a special kind of the molecular graph in which some vertices are distinguished from other ones, and they are called the virtual vertices. The most important property of the synthons is that the constraint of strict stoichiometry is removed and the virtual vertices formally correspond to functional groups that are not closely specified.
Ground‐state multiplicity by spin‐extended Hartree–Fock method
Authors: Mestechkin M. M. , Whyman G. E.
Published: January 2005

New possibilities of the spin‐extended Hartree–Fock method in determining the sequence of energy levels are analyzed and demonstrated by the example of some oxygen‐contained compounds.
QCT study of isotopic effects in H + HBr abstraction and exchange reactions
Authors: Tiňo Jozef , Urban Ján , Klimo Viliam
Published: January 2005

The method of quasi‐classical trajectories on an LEPS hypersurface was used for studying the influence of the exchange of one or both of the hydrogen atoms for deuterium in the reaction H1 + H2Br. As expected, the reaction cross sections of the exchange and abstraction reactions were found to increase if H1 was replaced by D and decrease if H2 was replaced by deuterium. A similar change in the reaction cross sections have also been observed for vibrationally excited reactants. The distribution of vibrational (rotational) energy is related to the ωe (Be) values of the respective reactants and products.
Theoretical study of the reaction Ne + H+2 → NEH+ + H in the 2A′ ground state
Authors: Urban J. , Jaquet R. , Staemmler V.
Published: January 2005

A three‐dimensional potential energy surface for the 2A′ ground state of the system (NeH2)+ (2Σ+ in collinear geometry) has been calculated at SCF and CEPA levels. This surface describes the abstraction reaction

which is endoergic by 0.57 eV (ΔH00) and has been studied recently by different experimental groups at low collision energies. Our CEPA calculations yield an endoergicity of 0.55 eV (ΔH00). The 2A′ surface has a minimum at collinear geometry with RNe—H = 2.29 a0 and RHH = 2.08 a0 and a well depth of 0.49 eV relative to Ne + H+2. The effects of electron correlation on the shape of the surface and on the well depth are discussed.

An analytic fit of the collinear part of the surface has been constructed based on Simon's proposal of using polynomials in the coordinates (RRe)/R instead of (RRe). The fitted potential is used for quantum mechanical scattering calculations with the finite element method (FEM). Preliminary results for reaction probabilities for H+2 in different vibrationally excited states are given and compared to the experimental results.
Density matrix of crystalline systems. I. Long‐range behavior and related computational problems
Authors: Pisani C. , Aprà E. , Causà M.
Published: January 2005

The one‐electron density matrix (DM) of crystalline systems is discussed, especially concerning its longrange behavior; reference is made throughout to systems treated at a Hartree–Fock–LCAO–SCF level of approximation. The analysis is performed on the assumption of generally smooth behavior of eigenvalues and eigenvectors in k (reciprocal) space, so that they can be expressed by means of a truncated Fourier expansion. This assumption allows us to obtain analytic approximations for the DM, on the basis of the information collected at a few, suitably selected sampling k points. It is therefore possible at the same time to discuss the influence of structural parameters (dimensionality of the system, existence and shape of the Fermi surface, structure of the chemical bonds) and to set up a computational scheme that is general and simple enough.
Physical quantification of the biological effectiveness of ionizing radiations
Authors: Watt D. E. , Kadiri L. A.
Published: January 2005

The role, and limitations, of the fundamental physical quantities used in our current system of dosimetry for the protection of individuals against the hazardous effects of ionizing radiation is discussed briefly. A major limitation is the inability to correlate biological data, in a unified way, as a function of linear energy transfer with the consequent necessity for quality factors.

From consideration of the various interaction processes undergone by charged particles in the equilibrium slowing down spectra generated by the incident radiation field, it is shown that good correlation of biological effects, for all radiation types, can be achieved in terms of linear primary ionization. The implication is that delta‐ray effects play at most a very minor role, at moderate fluences, and that, consequently, the absorbed dose is an unsuitable parameter for describing radiation effects. Structure in the radiosensitive targets is observed to have a critical dimension of about 2 nm. It occurs only when double‐stranded DNA is present and the magnitude of the inactivation probability is consistent with double‐stranded breaks being the significant lesion. Calculation is made of the yield of lesions as a function of the mean free path for primary ionization.

Differential spectra of radiation quality are obtained for monoenergetic electrons (0.2 keV to 30 MeV); for characteristic X‐rays from carbon, aluminium, neon, potassium, copper, silver, and tungsten; for 50 kV X‐rays, 250 kV X‐rays, 241Am, 137Cs, and 60Co gamma rays; and for neutrons with energies between 0.1 and 50 MeV. Damage by electrons is shown to be predominant at the ends of their tracks between 50 and 200 eV, whereas this is not so for the proton recoil spectrum generated by neutrons above 1 MeV.

Proposals are made for a unified system of dosimetry that is independent of radiation type and that obviates the need for quality factors. The method is thought to be of general applicability to irradiations by nuclides incorporated into mammalian cells, by ingested emitters, and by external sources.
Tunnel effect in proton transfer
Authors: Esquivel J. L. , Mata‐Segreda J. F.
Published: January 2005

Semiempirical computations were carried out to determine the tunneling rates in the case of coupled motion of two protons along the reaction coordinate. The following molecular systems were studied for medium intermolecular distances (A—B = 2.72 or 2.75 Å); +AH—BH—A, where A was NH3 or H2O and BH was HF or H2O. In the cases where the bridge was HF, solvation was modeled with just one water molecule attached to each side of the perpendicular axis through HF at 2.75 Å. Coupled motion of three protons was also included in the case of H3O—H2O—H2O—H2O.
Application of molecular orbital graph theory to vibrational problems of finite chain systems
Authors: Lü Tianxiong , Tachibana Akitomo , Yamabe Tokio
Published: January 2005

A generating function approach based on molecular orbital graph theory is presented that provides a straightforward way of obtaining the secular polynomials and energy bands for repeated unit systems from polynomial recurrence expressions. The possibility of obtaining the analytical energy‐level spectrum of the system can also be predicted. These results are then used to discuss the vibrational problems of finite chain systems with single‐ and double‐component lattices. It seems to be the first report describing the vibrational states of an (AB)N chain.
Modeling of real 20‐component protein chains: Determination of the electronic density of states of aperiodic seven‐component polypeptide chains containing strongly different amino acid residues
Authors: Bakhshi A. K. , Otto P. , Liegener C.‐M. , Rehm E. , Ladik J.
Published: January 2005

To model a real 20‐component protein chain, the results of the electronic density of states (DOS) of seven‐component periodic and aperiodic polypeptide chains calculated on the basis of ab initio matrix block negative factor counting method are presented. The seven amino acid residues taken into account are rather different: glycine, serine, cysteine, asparagine, histidine, aspartic acid, and tryptophane. In the case of aperiodic chains, the energy states are found to be distributed over a much wider range of energy than in the corresponding periodic chains. In the intrinsic state, both periodic and aperiodic polypeptide chains are predicted to be insulators. The effect of the block copolymerization on the DOS distribution and the band gap values of both periodic and aperiodic polypeptide chains has been also investigated. Finally, the methods to calculate from the DOS curves hopping conductivities and the possibilities to compare them with experiment are outlined.
Theoretical study of single‐electron capture in the N5+ + He and O6+ + He collisions by means of ab initio methods
Authors: Bacchus‐Montabonel M. C. , Amezian K.
Published: January 2005

Partial cross sections of single‐electron capture on the n = 3 levels have been determined theoretically for the N5+ + He and O6+ + He collisions by means of a semiclassical method using ab initio potential energy curves and radial and rotational coupling matrix elements. The different behavior of these two isoelectronic systems is fairly well reproduced by our calculations.
Density functional calculation of transition metal cluster energy surfaces
Authors: Beznosjuk S. A. , Dajanov R. D. , Kuldjanov A. T.
Published: January 2005

A quantum field mechanics of an electron subsystem in 3D physical space as the topology of compact atomic clusters with spontaneously broken local canonical symmetry is used for investigation of different types of microdefects in the condensed state of transition metals. The theory is illustrated with results of calculation of small compact Fen clusters (n = 2, 6, 14).
Molecular point symmetry and the phase of the electronic wave function: Tools for the prediction of critical points of potential energy surfaces
Authors: Mezey Paul G.
Published: January 2005

By combining a well‐known theorem of Longuet‐Higgins on the phase changes of electronic wave functions along loops encircling a conical intersection of potential surfaces, and a recently proven “vertical symmetry theorem” on the relative distributions of point symmetry groups and potential surface critical points in configuration space, a new result is obtained, suitable for the prediction of the presence of a critical point within a domain of configurations.
On the long way from the Coulombic Hamiltonian to the intermolecular energy surfaces: Concluding remarks at the tromsø conference, June 20–22, 1989
Authors: Löwdin Per‐olov
Published: January 2005

After a brief review of the main results given at the conference, the general properties of the Coulombic Hamiltonian for a system of electrons moving in a framework of moving atomic nuclei—considered as point charges—are discussed. Since this Hamiltonian is invariant under translations and rotations, the total momentum and the total angular momentum are constants of motion, which means that it is possible to separate the motion of the center of mass and the rotation of the system as a whole. Even if these separations are simple in principle, they lead to a mixing of the electronic and nuclear coordinates that complicates the transformed Hamiltonian. The general features of this Hamiltonian are discussed both in pure quantum mechanics and general quantum theory dealing with wave functions Ψ respective density matrices ρ or system operators T. The principles of the latter are derived from five simple axioms, and it is shown that pure quantum mechanics is a special case of the general theory and that the analogy between these two approaches is essential for the “economy of thinking.” It is indicated that the general theory of the shape and topology of the energy surface 〈H〉 = TrHΓ and its critical points, as a function of the system operator Γ involving both electronic and nuclear coordinates, is a very difficult mathematical problem and that calculation of this surface even for simple molecular systems represents a formidable computational problem, which has to be solved in order to be able to understand the nature of chemical reactions from first principles.
The effect of interlayer interactions on chemisorption pattern stability: Hydrogen atoms on graphite
Authors: LaFemina John P.
Published: January 2005

Extended Hückel band calculations, frontier crystal orbital analysis, and degenerate‐level perturbation theory are used to evaluate the effect inclusion of bulk‐layer interactions has on the relative stabilities of ordered overlayers. Five patterns of hydrogen atoms chemisorbed on unreconstructed graphite (11% coverage) are examined for the Bernal (ABABAB…) and registered (AAA…) three‐dimensional stackings. The single‐layer two‐dimensional approximation for graphite is found not to be entirely satisfactory for the type of chemisorption pattern stability questions addressed in this paper.
Explicit algebraic form of coupled cluster equations for the PPP model of benzene with an approximate inclusion of triexcited clusters
Authors: Čížek J. , Paldus J. , Vinette F.
Published: January 2005

An important role of model Hamiltonians for quantum mechanical description of the molecular electronic structure is discussed, and the effectiveness of the Pariser–Parr–Pople (PPP) model in calculations of molecular properties, in building of other models, and in various theoretical exploits is emphasized. The striking simplicity and transparence, yet remarkable reality and credibility, of this model is illustrated by applying the basic coupled cluster approach (CCD), as well as its extension approximating triexcited clusters (CCSDT‐1), to the PPP model of benzene. The explicit final equations are obtained using the symbolic computation language MAPLE for both the orthogonal and nonorthogonal versions of the CC formalism. These equations are presented using the interface between MAPLE and the typesetting language TROFF. In this way, the explicit CC equations are obtained in terms of basic parameters of the model, starting from the CI description and showing their equivalence with those obtained via diagrammatic techniques. A new insight into the structure of CC equations at various levels of approximation is thus achieved and a useful model for electron correlation is provided.
Transferability of the electronic structure characteristics of saturated molecules
Authors: Gineitytė V. , Shatkovskaya D.
Published: January 2005

The common quantum mechanical problem for the total class of saturated hydrocarbons has been stated and solved within the framework of the effective Hamiltonian method. A quantum mechanical substantiation of the transferability of the electronic structure characteristics (bond dipole moments, bond energies, and bond force constants) as well as the investigation of the origin of transferability have been considered for the stated class of chemical compounds.
The fate of the dry electron—A preliminary investigation
Authors: Mann Ady , Green Alex
Published: January 2005

A set of electron impact cross sections for vibrational excitation of water in the gaseous phase are first presented. The associated loss function is used in conjunction with a rotational loss function to generate efficiencies for the conversion of low‐energy electron energy to rotational and vibrational excitation. An analogous calculation is carried out for intramolecular and intermolecular modes in the liquid phase leading to similar results. We also examine a simple model for the time dependence of the electron hydration process based on a square well with a time‐dependent radius and well depth. We can fit the time and wavelength dependence of the experimental absorption coefficients if we introduce another time‐dependent parameter that might be encompassed by a diffuse potential well model.
A non‐local pseudopotential in the FSGO model: Study of some organometallic systems
Authors: Kaur S. , Ray N. K.
Published: January 2005

A non‐local core pseudopotential has been used in the framework of floating spherical Gaussian orbital (FSGO) model to study the equilibrium geometries and valence electronic structures of some organolithium and organoberyllium systems. The calculated equilibrium geometries, heats of hydrogenation, average electric polarizabilities, and magnetic susceptibilities are in good agreement with the results of the all‐electron FSGO model calculations. Valence electron wave functions obtained here have been used to predict the valence electron Compton profiles (CP) and electron momentum distributions (EMD) of the systems studied. A good correlation has been shown among the peak height of the CP (J(0)), valence electron energy (Ev), and number of valence electrons (Nv).
Monohydration selectivity mechanism of alkali cations in the potassium channel of excitable biomembranes
Authors: Kariev A. M. , Ovchinnikov A. A.
Published: January 2005

Nonempirical quantum chemical method Hartree–Fock–Roothan LCAO SCF MO in a two‐exponent Dunning basis with the use of an extended set of Gaussian functions by Huzinaga–Dunning with consideration of electron correlation according to the Meller–Plesset theory of excitations of the second order was used to study monohydrates of Li+, Na+, K+, and HCOO− ions. The indicated basis was supplemented with polarization functions of d‐type on the O atom and of p‐type on the hydrogen atom as well as with diffusion functions of p‐type on the oxygen atom. It has been found that binding energies of the water molecule with Li+, Na+ appeared to be higher and with K+ lower than with HCOO− · H2O. Potential curve shapes of K+ + H2O and HCOO− + H2O reactions are shown to be similar. The molecular mechanism of K+ channel selectivity of an excitable membrane is explained on the basis of the obtained calculations.
A comment on classical rainbow angles for atom‐atom scattering
Authors: Murrell J. N. , Eriksson L. A. , Bosanac S. D.
Published: January 2005

The dependence of the rainbow angle on the depth of the potential well, the equilibrium distance, andthe force constant of the potential are discussed for several potential functions.
Exciton interactions in the spectrum of a dinaphthonorbornadiene
Authors: Strickler S. J. , Cormier Russell A. , Connolly John S.
Published: January 2005

The near‐ultraviolet spectrum of a dinaphthonorbornadiene is reported and is interpreted using simple exciton theory that treats this molecule as a dimer of naphthalene. It shows examples of weak, intermediate, and strong coupling for the interaction of the three observed bands of naphthalene.
Theory of ultrafast time‐resolved absorption spectroscopy
Authors: Yeh C. Y. , Chang W. L. , Ma H. , Lin S. H.
Published: January 2005

In this paper, the density matrix formalism has been applied to treat ultrafast time‐resolved absorption spectroscopy. We have shown that in the femto‐second (fs) pump‐probe experiments, the observed time‐resolved absorption spectra consist of the contributions from the population (i.e., incoherent contribution) and the coherence (i.e., the phase of the system). The adiabatic approximation has been used to derive the expressions for ultrafast time‐resolved spectra. We have also shown that the dynamics of the coherence will result in quantum beat. Numerical calculations have been performed to demonstrate the theoretical results.
Solvent effects in dense gases
Authors: McGlynn S. P. , Asaf U. , Kumar D. , Felps S. , Rupnik K. , Klasinc L.
Published: January 2005

The effects of perturber gases, partial pressures in the range 0–50 atm, on the absorption spectra of high‐n Rydbergs, n ≥ 10, in methyl iodide and benzene have been investigated. The perturbers were rare gases and H2. It has been shown that the spectroscopic energy shift Δ = Δ(ρ, n) is given by Δ = Δ°ρ, where ρ is the number density of the perturber, for n ≥ 10. The shift data can be interpreted using a Fermi model, and values of scattering lengths for perturbers can be extracted. These scattering lengths are internally consistent, independent of the absorber, and in excellent agreement with electron‐swarm results. Given that the Fermi model is nonspecific and microscopic, scattering shifts and lengths for mixtures of perturber gases can be predicted. These predictions, having been verified by experiment for He/Ar mixtures, provide a means of “tailoring” any required scattering length. Finally, it is shown empirically that the determinative scattering parameter is the polarizability of the perturber, and a universal relationship a = −0.26α + 1.18 between scattering length a, in Bohr radii, and polarizability α is found to exist.
Ab initio simulation of benzene Raman intensities
Authors: Ozkabak Ali G. , Thakur Surya N. , Goodman Lionel
Published: January 2005

C6H6 Raman scattering activities calculated from harmonic model ab initio Hartree–Fock 6–311 ++ G(d, p) polarizability derivatives (and harmonic force fields calculated at the same level) accurately simulate experiment (to within 1% for the a1g modes). Accurate predictions are also made for the e2g modes (to within 5% for ν7 and ν9, and more poorly for ν6 and ν8 [in Fermi resonance with ν6 + ν1]) and for the e1g out‐of‐plane mode, ν10. Only the ν6 in‐plane CCC bending mode scattering activity is found to be anomalous. Systematic variation of the basis set indicates that the benzene scattering activities and depolarization ratios are strongly dependent on inclusion of both carbon and hydrogen atom diffuse functions in the basis set. Predictions are also made for 12C6D6 and for unmeasured intensities in 13C6H6. Measurements of a1g mode scattering activities in the latter molecule are predicted to be useful in testing the harmonic Hartree–Fock Raman intensity model.
High‐resolution infrared analysis of the ν17 band of pyridine
Authors: Thiel Yvonne , Walters Valerie A. , Wiberg Kenneth B. , Colson Steven D.
Published: January 2005

A method for analyzing asymmetric top rovibrational bands displaying both blended and resolved features is described. The two‐phase computational procedure uses a modified version of the asymmetric rotor‐band contour program FASTPLOT to generate a preliminary set of upperstate spectroscopic constants. The parameters are subsequently refined by employing the assigned line‐fitting formalism of the ASYROT program using both resolved and blended features. The technique is detailed in a comprehensive analysis of the ν17 band of pyridine. Inclusion of quartic centrifugal distortion constants was found to satisfactorily model a high‐resolution (0.004 cm−1) spectrum of this band, yielding a standard deviation of 0.00137 cm−1. The variation in the rotational parameters with vibrational quantum number is examined in terms of possible weak rovibrational perturbations to the ν17 state. An ab initio calculation of the ν17/ν27 Coriolis coupling constant indicates the observed results are consistent with the interaction of these two vibrational states.
Ab initio MRD‐CI calculations for breaking a chemical bond in a molecule in a crystal or other solid environment. III. Me2NNO2 decomposition of dimethylnitramine in a large crystalline environment
Authors: Chapman Douglas A. , Roszak Szczepan , Keegstra Phillip B. , Hariharan P. C. , Kaufman Joyce J. , Buenker Robert S.
Published: January 2005

Recently we extended our strategy for MRD‐CI (multireference double excitation‐configuration interaction) calculations, based on localized/local orbitals and an “effective” CI Hamiltonian, for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystalline or other solid environment.

Our technique begins with an explicit quantum chemical SCF calculation for a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized, and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD‐CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD‐CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general in that the space treated explicitly, as well as the surrounding space, may contain voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc.

Dimethylnitramine is the smallest prototype of the energetic R2N—NO2 nitramines, such as the 6‐member ring RDX or the 8‐member ring HMX. Decomposition of energetic compounds is initiated in the solid by a breaking of the target bond. Thus, it is crucial to know the difference in energy between breaking a bond in an isolated energetic molecule versus in the molecule in a solid. In the present study, we have carried out MRD‐CI calculations for the Me2N—NO2 dissociation of dimethylnitramine in a dimethylnitramine crystal. The cases we investigated were one dimethylnitramine molecule (surrounded by 53 and 685 neighboring dimethylnitramine molecules represented by multipoles), three dimethylnitramine molecules, and three dimethylnitramine molecules (surrounded by 683 neighbors). All multipoles were cumulative atomic multipoles up through quadrupoles. The MRD‐CI calculations on dimethylnitramine required large numbers of reference configurations from which were allowed all single and double excitations.
A partial recurrence relation for reduced class coefficients of the symmetric group
Authors: Katriel Jacob
Published: January 2005

An expression for the product of a single‐cycle class [(1)N ‐ P(p)]N and an arbitrary class [(1)l1(2)l …︁ (N)lN]N of the symmetric group has recently been conjectured. This expression involves a sum over a relatively small number of reduced class sums, depending on p indices. A further conjecture is formulated and demonstrated, according to which reduced class coefficients (RCCS) involving cycles whose length is expressed by means of a single index can be related to corresponding coefficients in the product of [(1)N ‐ P+1(p ‐ 1)]N with an arbitrary class sum. Consequently, the problem of evaluating the general class sum product reduces to that of obtaining a relatively small set of fundamental RCCS containing no single‐index cycles. The conjectures mentioned can be used to evaluate the product [(1)N ‐ p(p)]N · [(1)N ‐ q(q)]N in terms of fundamental RCCS that can all be obtained from the product [(r)]r · [(r)]r, where r = min(p, q). For the latter product, we use a result due to Boccara.
A counterexample and a modification to the adiabatic approximation theorem in quantum mechanics
Authors: Gingold H.
Published: January 2005

A counterexample to the adiabatic approximation theorem is given when degeneracies are present. A formulation of an alternative version is proposed. A complete asymptotic decomposition for n dimensional self‐adjoint Hamiltonian systems is restated and used.
SCF theory of molecular interactions
Authors: Grant A. J. , Pickup B. T.
Published: January 2005

Perturbation theory is used to analyze the interactions between two closed‐shell systems in the SCF approximation. The effects of orbital overlap are included. The dominant terms through second order are identified, including electrostatic, penetration, polarization, charge transfer, and SCF dispersion effects. The basis set limit is studied so that basis set superposition contributions may be identified. Calculations are presented for HeHe and Na+H2O.
The generalized Slater–Condon rules
Authors: Verbeek Jacob , Van Lenthe Joop H.
Published: January 2005

By relating the blocking structure of the relevant matrix of overlap‐integrals to its cofactors, the Slater–Condon rules for the evaluation of an element of a matrix representation of an electronic Hamiltonian in a Slater determinant basis are generalized to the case where not all orbitals are orthogonal. This yields a set of 33 rules, which allows for an efficient implementation of the valence bond theory.