bluuebunny/crossref_metadata_2025 huggingface dataset:
Crossed‐Molecular beam studies of the state‐to‐state reaction dynamics of charge transfer at low and intermediate energy
Authors: Futrell Jean H.
Published: January 2005

The microscopic, state‐to‐state reaction dynamics of charge transfer reactions are reviewed for the two system N(N2, N2)N and Ar+(N2, Ar)N. The crossed molecular beam method demonstrates that the symmetric, molecular case proceeds by two mechanisms at low collision energy (< 0.7 eV). One of these involves an orbiting complex in which the total available energy is redistributed, essentially statistically. The second mechanism is a direct mechanism which involves nearly rectilinear trajectories and corresponds to exactly resonant charge transfer at low energy (0.7 eV) and charge transfer accompanied by vibrational excitation at higher energy (> 10 eV). The orbiting mechanism is dominant at low energies and disappears entirely with increasing collision energy. The unsymmetric, nearly resonant charge transfer reaction is direct at all energies. At very low collision energy (0.6 eV) and moderate collision energy (> 1.5 eV) the dominant reaction channel is the generation of N2+(X2 g, v = 1) with little rotational excitation. At 0.6 eV substantial angular scattering (including back‐scattering) is observed; above 1.5 eV the trajectories are essentially rectilinear. At intermediate energies (0.8 < E < 1.3 eV) a very different mechanism is observed. All the energetically accessible vibrational states of N are formed, each with a preferred scattering angle at a given energy. Related experiments and theoretical models which rationalize some, but not all, of these results are described. The most intriguing puzzle is the “energy window” in which quantum‐state specific, angular‐specific reactive scattering is observed.
Cation binding effect on imidazole tautomerism
Authors: Basch H. , Krauss M. , Stevens W. J.
Published: January 2005

Binding of Zn2+ to imidazole (Im) and methyl imidazole (MeIm) is studied by ab initio methods as a model for the effect of cation binding on tautomeric energies. Gradient energy optimized conformations were obtained for all neutral and ionic structures, including the deprotonated molecules and the ylides. The N3H tautomer of MeIm is calculated to be more stable than N1H by about 1 kcal/mole. However, binding of a Zn2+ cation to the available nitrogen site is found to reverse the order of binding, leaving N1H more stable by 1 kcal/mole. Binding of Zn2+ produces a significant perturbation in the electronic structure, a smaller shift in the equilibrium conformation of the imidazole ring, and only a small absolute shift in the relative tautomer energies. Methyl substitution at C5 has a small effect on both conformation and energetics.

A high‐energy ylide tautomer is produced by moving the proton bound to C2 to the N1 atom. The binding of Zn2+ to the C2 site is substantially stronger than to the N1 site, yielding nearly isoenergetic ZnIm2+ conformations for binding to either N or C atoms. For the deprotonated salts the lowest energy conformation has the C2N3 bond bridged by Zn2+.
Stationary properties of δE/δR
Authors: Epstein Saul T.
Published: January 2005

It is made clear that two different statements in the literature concerning energy derivatives are completely compatible by deriving them as two different interpretations of the same equation. Some other aspects of these results are also discussed.
Configuration selection and extrapolation in multireference configuration interaction calculations: The (H2)2 van der waals complex as a benchmark example
Authors: Shavitt Isaiah , Brown Franklin B. , Burton Peter G.
Published: January 2005

Multireference configuration interaction (MR‐CI) calculations using energy‐selected and complete‐active‐space reference configurations are compared for a large‐basis‐set treatment of the (H2)2 van der Waals complex in a T geometry. Procedures for estimating truncation errors arising from energy threshold selection of configuration state functions and for extrapolating to the full CI limit are tested by comparisons with untruncated calculations and with the full CI energy. The results of these tests show that great care is required in the use of the correction and extrapolation formulas. Optimization of the orbitals of the reference wave function (compared to the use of SCF orbitals) is found to produce order‐of‐magnitude improvement in the results.
A computational study of some isomerization equilibria and their possible relation to vinyl chloride carcinogenicity
Authors: Murray Jane S. , Politzer Peter
Published: January 2005

There is considerable evidence indicating that the primary in vivo DNA alkylation product of vinyl chloride, believed to be responsible for its carcinogenicity, is the 7‐N‐(2‐oxoethyl) derivative of guanine. Using a representative keto‐enol system, we have studied computationally two possible equilibria involving this adduct. The first is between the 2‐oxoethyl derivatives of the keto and enol forms of the representative system, whereas the second is between the 2‐oxoethyl derivative of the keto form and the corresponding hemiacetal. In the case of the analogous guanine derivatives, such equilibria could lead to disruption of the hydrogen bonding between guanine and cytosine in DNA and could cause miscoding and replicational and transcriptional errors. An ab initio self‐consistent field molecular orbital study has been carried out using the GAUSSIAN 82 system of programs. Optimized structures and energies have been calculated at the 2‐21G level for the representative keto‐enol system and its 2‐oxoethyl and hemiacetal forms. The formation of the 2‐oxoethyl adduct was found to have no significant effect upon this keto‐enol equilibrium, which strongly favors the keto form. Our calculations further show hemiacetal formation to be an unlikely possibility for this system. On the basis of these results and recent data on guanine tautomers, we speculate about the analogous equilibria involving the guanine derivatives.
Selection rules in alternant systems
Authors: Živković Tomislav P.
Published: January 2005

The CI space Xn generated by n electrons moving over 2n spin orbitals is considered. It is shown that the transitions between different eigenstates ψi ϵ Xn of alternant and weakly alternant Hamiltonians are governed by some special selection rules. These selection rules are characteristic to alternant systems, and they do not apply to nonalternant systems. The set of all such selection rules can be easily derived from the splitting theorem. In particular, the selection rules associated with spin independent alternant systems are considered. As an example, the PPP Hamiltonian Ĥp describing netural alternant hydrocarbons is treated. In the case of electron dipole transitions between eigenstates ψi ϵ Xn of the Hamiltonian Ĥp, the selection rules obtained are in agreement with the selection rules derived previously by Pariser and McLachlan.
Generalized Hellmann–Feynman theorem in the Xα method
Authors: Gáspár R. , Nagy Á
Published: January 2005

In the Xα method a generalized Hellmann–Feynman theorem has been derived, and its most important applications are presented. A formula for electronegativity using the definition of electronegativity given by Iczkowski and Margrave and the magnetic nuclear shielding factor and its relation to the electronegativity have been obtained. The virial theorem of the Xα method has also been derived with the aid of the Hellmann–Feynman theorem.
Intermolecular interactions: Reliability of results obtained for small hydrogen bonded dimers by Claverie's method
Authors: Czermiński Ryszard
Published: January 2005

Energies and geometries for a number of small hydrogen bonded dimers have been calculated by semiempirical method based on a perturbation approach. Results have been compared with experimental and the best theoretical data. A quite good description of equilibrium configurations has been obtained in every case when local multipoles from sufficiently accurate wave functions have been used. Hydrogen bond lengths have not been predicted with sufficient precision. Results indicate, however, that it should be possible to achieve improvement in the framework of the applied calculation scheme by modification of the parameter values.
Some iterative techniques for large CI matrices. II. Singular systems of linear equations
Authors: Bendazzoli G. L. , Evangelisti S. , Palmieri P.
Published: January 2005

An interative technique to solve nearly singular systems of linear equations of quantum chemical interest is described and tested. The solution is split into regular and singular parts, the last being the resonating eigenvector.
Spectroscopic order out of spectroscopic chaos
Authors: Taylor Howard S.
Published: January 2005

A concept is advanced explaining how low resolution studies of chaotic spectral regions can result in assignable regular spectra. The concept draws on ideas from resonance scattering theory, and application is made to interpretation of vibrational spectra in the quasicontinuum.
Multiphoton ionization of many‐electron atoms
Authors: L'Huillier Anne , Jönsson Lars , Wendin Göran
Published: January 2005

We review the main experimental results obtained in the multielectron multiphoton ionization of rare gases. Multiple ionization is interpretated as a sequential process. We study the influence of many‐electron effects in multiphoton ionization within the framework of the random phase approximation. This is applied to a calculation of the two‐photon ionization cross section of xenon. Finally, the role of screening effects in the multiphoton stripping of heavy atoms (Xe) is estimated.
A mixed basis set of Slater and Gaussian functions. Integrals with modified Gaussian functions
Authors: Colle Renato , Fortunelli Alessandro , Salvetti Oriano
Published: January 2005

The analytic expressions of the integral prototypes involving both Slater and s‐type Gaussian functions, explicity derived in Ref. 1, are generalized to the case of higher order modified Gaussian functions [2].
The “measurement problem” in quantum chemistry and the interpretation of phenomenological damping terms in the method of classical trajectories for nonadiabatic molecular collisions
Authors: Dunne Lawrence J.
Published: January 2005

A “measurement” interpretation is given to the phenomenological damping terms invoked by Dunne, Stamper, and Murrell to resolve density matrices to pure states following time evolution in a nonadiabatic molecular collision.
Spin‐free quantum theory. XXV. The unitary‐group formulation of fermion many‐body theory
Authors: Matsen F. A.
Published: January 2005

The fermion unitary group formulation (UGF) of many‐body theory is based on the unitary group U(2n) where n is the number of freeon orbitals. This formulation, which conserves particle‐number but not spin, is isomorphic to the particle‐number‐conserving, second‐quantized formulation (SQF). In UGF we derive the familiar diagrammatic algorithm for matrix elements, M(Y) = (−1)H+L where H and L denote the numbers of hole lines and loops in the diagram D(Y) of M(Y). The unitary group derivation is considerably simpler than is the conventional, second‐quantized derivation that employs time‐dependence, Wick's theorem, normal‐order, and contractions. In neither fermion UGF nor SQF is spin conserved. We carry out in UGF the spin‐projection (symmetry adaptation to SU(2)) of the fermion vectors and obtain with a spin‐free Hamiltonian the same matrix elements as with the freeon UGF (part 24 of this series). The fermion unitary group formulation for a spin‐free Hamiltonian should be regarded as an alternate path to spin‐free quantum chemistry.
A revisit to molecular path codes: Ordering, comparability, and similarity of benzenoid, nonbenzenoid, and monoradical hydrocarbons
Authors: El‐Basil S. , El‐Enany M.
Published: January 2005

Molecular path codes are computed for 17 hydrocarbons, including benzenoid alternant, nonbenzenoid alternant, fully aromatic nonalternant, antiaromatic nonalternant, and monoradical hydrocarbons. Comparability conditions of Randić and Wilkins [9–18] based on the theorems of Muirhead and Karamata are imposed on the partial sums of the codes. The distance/similarity matrix is constructed using the individual paths as projections in Euclidean space. Both resonance energies and total pi energies correlate with partial ordering predicted from both comparability and similarity schemes. The normalized codes succeed in discriminating combinatorial properties of the various topologies studied such as conjugated circuits.
The decomposition of the SCF interaction energy in hydrogen bonded dimers corrected for basis set superposition errors: An examination of the basis set dependence
Authors: Alagona Giuliano , Ghio Caterina , Cammi Roberto , Tomasi Jacopo
Published: January 2005

A systematic examination of the components of the interaction energy, obtained with the Kitaura and Morokuma method, for nine H‐bonded dimers without and with counterpoise corrections (CP) is presented. The nine dimers Hn A …︁ HBHm correspond to all the possible combinations of HF, H2O, and NH3 as electron donors and electron acceptors. The interaction energy and the corresponding components have been computed over a sizable interval of intermolecular distances with five basis sets (STO‐3G, MINI‐1, 3‐21G, 4‐31G, 6‐31G**) selected among those most extensively used to study interactions in larger systems. The CP corrections to the ΔE components have been obtained with a method, implemented in our group, which permits assignment to the pertinent components of ΔE of a physically reasonable portion of the CP correction even though different CP corrections are adopted. We examine here three versions of the CP correction, namely, the full CP correction (i.e., the original version of Boys and Bernardi) and CP corrections limited to the virtual space of the partner or to the electron donor only. The resulting data are employed to assess the basis set dependence of several models of hydrogen bonding (the electrostatic model, the semiclassical model, etc.) both with and without CP corrections.
Symmetry orbitals for one‐electron band‐structure computations of one‐dimensional crystals
Authors: Koch Wolfhard , Seelig Friedrich Franz
Published: January 2005

The algorithm to derive symmetry orbitals for one‐electron band‐structure calculations of one‐dimensional crystals presented by Božović and Delhalle [1] has been applied to extend the explicit formulation of symmetry‐adapted linear combinations of Bloch sums to any line group isogonal to Cn or Cnv. We also point out how the transformation matrices that block‐diagonalize the one‐electron eigenvalue equation H(k)C(k) = S(k)C(k)E(k) can be determined. Finally, the extended‐Hückel block‐matrix dimensions of some typical quasi‐one‐dimensional transition metal compounds have been tabulated in order to give a measure for computational savings achievable by this technique.
Alternant systems and their properties
Authors: Živković T. P.
Published: January 2005

Neutral, ionic, and complete alternant systems are studied using the alternancy symmetry adapted (ASA) approach. This approach is based on an explicit construction of ASA operators that are up to the sign invariant with respect to the particle‐hole symmetry transformation. These operators serve as building blocks of alternant systems, and they determine their characteristic properties. All Hamiltonians describing neutral alternant systems are explicitly constructed. Up to some minor restrictions, all Hamiltonians describing ionic and complete alternant systems are also explicitly constructed. Inversely, given a Hamiltonian Ĥ in a second quantization notation, one can easily check whether or not this Hamiltonian describes a neutral (ionic, complete) alternant system. All linear properties characteristic to neutral (ionic, complete) alternant systems are obtained. In particular, all one‐ and two‐particle properties are derived in an explicit form. The properties obtained substantially generalize “classical” properties of alternant systems such as, in the case of neutral alternant systems, uniform charge density distribution, vanishing bond orders between atomic sites of the same parity, and alternancy selection rules for the electric dipole transitions.
The Coulomb Green's function in IR3 and its relation to the harmonic oscillator in IR4
Authors: Briels W. J.
Published: January 2005

A very simple derivation of the Coulomb Green's function in IR3 is presented, which is based on an application of the Kustaanheimo–Stiefel transformation directly to the equation defining the harmonic oscillator Green's function in IR4. The representation that we obtain makes it possible to Fourier transform the Green's function with respect to its space variables by means of a simple Gaussian integration. The method that we use can readily be applied to several other Hamiltonians.
Conformation dynamics of macromolecules
Authors: Luo Liaofu
Published: January 2005

The locality of conformational state with relation to the asymmetry in potential is proved. The adequacy of harmonic approximation of conformation wave function is discussed. Then, on the grounds of the rigorous definition of the conformation‐vibrational state, the role of vibration in the information transmission and cooperativity is discussed by the calculation of statistical mechanics. In the rest of this article, the conformational change as a quantum transition is investigated in detail. The viewpoint of conformational transition with respect to electronic motion is stressed. On the basis of proposed selection rules, the approach to the conformational change that consists if multiple steps is clarified.
Simple qualitative rules to study the surface electronic structure of some binary crystals
Authors: Tomášek Mojmír , Pick Štěpán
Published: January 2005

Based on the use of the important qualitative concept of energy‐weighted averages, a few simple rules are formulated that facilitate the analysis of the existence of surface states and enable the estimation of the effect of surface charge self‐consistency on the electronic structure of AB‐type crystal surfaces. Recent theoretical and experimental investigations of the (001) and (111) surfaces of refractory transition metal compounds TiC and TiN are discussed in detail. The problem of the stability of the polar (111) surface of TiC is mentioned.
Spin‐Independent alternant systems
Authors: Živković T. P.
Published: January 2005

Spin‐independent alternant systems are analyzed using the alternancy symmetry adapted (ASA) approach. This approach is based on an explicit construction of ASA operators that either commute (altemant) or anticommute (antialternant) with the particle‐hole symmetry operator K̂. The method yields an explicit construction and identification of all spin‐independent Hamiltonians that describe altemant systems (neutral and/or ionic) as well as an explicit identification of all linear properties characteristic to these systems. It also establishes the connection between the spin‐eigenstates and the particle‐hole symmetry.
The intramolecular conversion of monothioformic acid: An ab initio study
Authors: Toro‐Labbe A. , Cardenas C.
Published: January 2005

A complete description of the rotational isomerism of monothioformic acid is presented. Ab initio results concerning the barrier heights and the energy difference between the stable isomers are in good agreement with the experimental data. A reduced potential function deduced from our calculations has been used to characterize the kinetics and thermodynamics of the rotational isomerization. The reported ab initio values of equilibrium constants are in good agreement with the experimental ones. From the analysis of the available experimental data, we have found that the free energy and enthalpy behavior are quite similar and that no noticeable entropy contributions are involved in the internal rotation, in agreement with previous studies on unimolecular isomerization processes.
Self‐consistent reaction field calculations on the proton transfer in ammonia‐formic acid systems as a model for hydrogen bonding in amino acids in solution
Authors: Parra‐Mouchet Julia , Contreras Renato R. , Aizman Arie
Published: January 2005

The reaction field (RF) model of solvent effects, implemented within the SCF‐CNDO/2 scheme of calculation, has been applied to analyze the proton transfer in the NH3…HCOOH system in the presence of several polarizable media. The aim of such a study was to characterize the tatutomeric equilibrium between the neutral and zwiterionic forms of H‐bonded amino acids in aprotic solvents. Qualitative results concerning the energetics of this equilbrium show the stabilization of two different H‐bonded complexes, corresponding to two separate minima in the free energy surface. These well known double minima potentials are found to be dependent on both the intermolecular NO distance and the strength of the reaction field. The behavior of this model is qualitatively consistent with experimental observations of nitrogen‐substituted amino acids in solution: both show, for low values of the dielectric constant, tautomeric equilibria where the H‐bonded complexes appear to be more stable than the corresponding monomeric forms. The charge transfer process associated with the proton migration along the H‐bond is also discussed. It is found that the amount of charge transferred increases with the NO distance and with the RF strength, In order to test the general approach and compare it with previous work, calculations on the real monomeric systems glycine, β‐alanine, and γ‐amino butyric acid was also performed.
Use of invariants in bounding 2‐fermion reduced density matrix eigenvalues
Authors: Casida Mark E.
Published: January 2005

The known bounds on the eigenvalues of 2‐Fermion reduced density matrices may be improved by considering unitarily invariant properties of the associated natural geminals and of the system 1‐Fermion reduced density matrix. The polynomial invariants are particularly important, since they may be introduced systematically according to the degree of the polynomials.
Study of a possible reaction pathway in the polymorphic transformation of diacetamide
Authors: Bayard F. , Decoret C. , Royer J. , Smeyers Y. G. , Randez J. J.
Published: January 2005

A possible reaction path for the polymorphic transformation of trans‐trans solid diacetamide into its trans‐cis from is envisaged. A description of the two crystalline forms in the same crystallographic system is adopted, and the energy profile of the crystallographic reaction path is calculated by using an empirical 6‐exp. atom‐atom potential method and a 6‐13G ab initio procedure for the inter‐ and intramolecular contributions, respectively. Two barriers appear along the reaction path. The first corresponds to the destruction of the trans‐trans chains and the second to the formation of the trans‐cis dimers. These barriers, being lower than the acetyl rotation barier, are in agreement with the ability to isomerize diacetamide in solid state. The presence of a small energy minimum between these barriers would suggest the existence of an intermediate crystallographic form.
The energy of the homogeneous electron gas to second order
Authors: Handler George S.
Published: January 2005

The energy of an infinite, homogeneous electron gas is examined by second order perturbation theory using a Hartee‐Fock rather than a noninteracting particle unperturbed state. The second order energy still diverges for small promotions k, albert than as ln|ln k| rather than as In k.
Comparison of the Perdew–Wang exchange with the Dirac exchange
Authors: Csavinszky P.
Published: January 2005

Recently, Perdew and Wang presented an expression for the exchange energy of a many‐electron system. This functional has been adapted in the present work in variational density functional calculations of the total atomic binding energy of the Ne atom. For this atom similar calculations have been done earlier using the Dirac expression for the exchange energy. Three different total energy functionals have been considered: (1) the Thomas‐Fermi‐Perdew‐Wang functional, (2) the above functional augmented by (1/9) of the original Weizsäcker correction to the Thomas‐Fermi kinetic energy, and (3) the same functional as in 1 augmented by (1/5.3) of the original Weizsäcker correction to the Thomas‐Fermi kinetic energy. The electron (number) density of the Ne atom has been constructed from hydrogen‐like one‐electron wave functions containing three variational parameters. Each of three total energy functionals have been minimized with respect to these parameters. Comparison of the Perdew‐Wang and Dirac exchange energies shows that, in each of the three model pairs, the Perdew‐Wang exchange energy is larger in magnitude than the Dirac exchange energy. It is also found that the exchange energy obtained by 2, ‐12.31 a.u., is in excellent agreement with the value of ‐12.22 a.u. calculated by Perdew and Wang with the Clementi and Roetti near‐Hartree‐Fock density.
Correlation effects at a local level and CI partitions: 23S State of helium
Authors: Largo‐Cabrerizo A. , Largo‐Cabrerizo J.
Published: January 2005

We have extended the study of the description of local correlation effects provided by the different partitions of a configuration interaction (CI) wave function to the first excited state of the helium atom. Our analysis makes use of the local mean values of different moments of the interelectronic distance computed with the CI partitions as well as with Hartree‐Fock and Hylleraas‐type wave functions.
The mechanism of bacteriorhodopsin functioning. I. The light‐induced proton throw‐over by retinal
Authors: Krasnogolovets V. V. , Protsenko N. A. , Tomchuk P. M. , Guriev V. S.
Published: January 2005

The path of proton motion in protein bacteriorhodopsin is constructed: Tyr 43 → Tyr 26 → Asp 212 → Tyr 83 → Tyr 57 → Tyr 79 → Glu 204 → Tyr 64 → Glu 74. From the position of quantum theory the light‐induced isomerization of retinal, the process carrying out the proton and throw‐over from the inlet H‐bond chain (Tyr 43 → Tyr 26 → Lys 216) to the outlet one (Lys 216 → Asp 212 → … → Glu 74) are investigated. The kinetics of the transition of the excitation from the π‐electron subsystem to the vibrational one is studied; the function of the nonequilibrial excitation distribution of the collective intramolecular retinal vibrations is found. The mechanism of deprotonation of Schiff base (transmission of proton to the outlet channel) is considered. The correspondence between the theoretically estimated states and spectroscopically identified forms of bacteriorhodosin (initial form bR, intermediates J, K) is proposed.
An examination of perturbation–variational theory and scaling at fifth order
Authors: Cullen John M. , Zerner Michael C.
Published: January 2005

We examine various perturbation–variational approximations for molecular electronic energy when the fifth‐order perturbational energies are available. Such theories require very little additional computation once the sequence of perturbation energies are known yet supply a bound even when the peturbation sequence is poorly convergent. We choose for computational examples results obtained very rapidly from a zeroth order wave function consisting of doubly occupied localized bonds and examine polarization within these bonds, delocalization, and bond breaking. In general, we find that the fifth‐order energy itself, and especially the [2, 1] Padé approximant on this sequence, are especially accurate in estimating the total energy and more accurate than any variational scheme when the zeroth order localized wave function is a good description of the electronic structure. The variational results, however, are nearly as accurate, and a [1, 0] Padé on the sequence of variational results is remarkably robust, even in those cases where the perturbation sequence is poorly defined.

We also examine several scaling techniques, or partitionings of the Hamiltonian. Although these scaling techniques do accelerate convergence of the perturbation sequence, none that we examine give better results, than the [2, 1] padé, which is independent of any scaling.
A comparison of different many‐body perturbation theory calculations of the ground state of SiS
Authors: Lima Eneida G. , Canuto Sylvio
Published: January 2005

A comparison of different many‐body perturbation theory (MBPT) calculations of the ground state rotational and vibrational constants of SiS is made. The calculations are performed up to the complete fourth‐order MBPT level, and in all cases two basis sets are utilized. The results of the third‐order and some incomplete fourth‐order calculations are in good agreement, but the complete fourth‐order is among the worst as compared with the experimental data. Analysis of the different contributions to the calculated correlation eneriges points towards the necessity of including even higher‐order terms of the(MBPT) expansion.
The quasiparticle concept in vibrational–electronic problems in molecules. I. Partitioning of the vibrational–electronic Hamiltonian
Authors: Hubač Ivan , Svrček Michal
Published: January 2005

The partitioning of the vibrational–electronic Hamiltonian is presented. This partitioning is based on a new quasiparticle transformation that is constructed in such a way that the adiabatic approximation is included into the unperturbed Hamiltonian; nonadiabacity, anharmonicity, and electron correlation are treated as perturbations. We also present the second quantization treatment for bosons. The many body perturbation theory expansion for the vibrational–electronic Hamiltonian is suggested. A comparison of this approach is made with gradient techniques.
Ab‐initio calculation of resonance states of H2O
Authors: Iwaki Hiroaki , Noro Takeshi , Ohno Kimio
Published: January 2005

In a dissociation attachment experiment of water, three peaks were observed at 7,9, and 12 eV. The origin of the third peak has been believed to be 2B2. However, the calculated energy of this state is 0.6 eV higher than the experimental value. This discrepancy is quite large compared with the case of the lower two peaks. In this study we propose new candidates for resonant states responsible for the third peak. The configurations considered are (3a1)−1(3pa1)2, (3a1)−1(3pb1)2, (3a1)−1(3pb2)2, (3a1)−1(3pa1)1(3pb1)1, (3a1)−1(3pb2)1(3pa1)1, and (3a1)−1(3pb2)1(3pb1)1 which have the parent state (3a1)−1(3pa1)1, (3a1)−1(3pb1)1, or (3a1)−1(3pb2)1. The energy levels arising from these configurations are calculated by a method of configuration interaction. A Few resonance states, which could be responsible for the third peak, are found. New decay process of these states are proposed.
The generalized padé approximants and chemical graph theory
Authors: Cioslowski J.
Published: January 2005

The theory of the Baker–Gammel approximants is applied to the spectral density function associated with the adjacency matrix. As a result, various upper and lower bounds to topological invariants are obtained in a systematic manner. In particular, the (N, M)‐type lower bound to the total π‐electron energy of benzenoid hydrocarbons is derived for the first time.
Eigenlevel calculations for screened coulomb potential systems via an integration‐free algorithm: Space pruning technique
Authors: Baykara N. A. , Demiralp M.
Published: January 2005

In this work, an algebraic method is developed to determine the eigenlevels of radially screened Coulomb potential systems. The method is mainly based on the concept of space pruning. The basis functions are combined in a way such that the resulting approximate eigenfunction and its several images under certain positive integer powers of the system's Hamiltonian (H) remain in the domain of H. Certain structural parameters are introduced into the basis functions to imitate some known behaviors of the true eigenfunctions of the system.

The main purpose of this work is not to reproduce the results for screened Coulomb potential systems but to develop a method which does not necessitate integration at any stage and can later be adapted to atomic and molecular systems as easily as possible.
Graph theory of molecular orbitals beyond tight‐binding approximation
Authors: Zhou Zhongxiang
Published: January 2005

The graph theory of molecular orbitals including second neighbor interactions (η) is considered here. Graphical methods of getting the characteristic polynomial for the π system of a conjugated molecule are given. It is shown that the characteristic polynomial can be factorized if there are symmetries in the molecule.
Inclusion of relativistic effects into ZDO methods. III. A. Quasi‐relativistic INDO/1 version
Authors: Boča Roman
Published: January 2005

The quasi‐relativistic CNDO/1 method for molecular orbital calculations has been extended to the INDO/1 version where all monocentric repulsion integrals are taken into account. This version has been applied to calculate molecular geometries of MXn‐type molecules, with the central atom belonging to from the second to seventh period. Properties of hypothetical superheavy elements (Z = 110–117) have been predicted. Transition metal complexes of the MCl‐type (M = Ni, Pd, Pt, and ePt) have been investigated from the point of view of spin‐state and configuration stability.
Theoretical investigation of the Na+ + H2 system. III. Trajectory calculations of vibrationally inelastic collision processes
Authors: Zuhrt Christian , Jeziorek Danuta , Żurawski Bronislav
Published: January 2005

On the basis of an analytical potential energy surface for the electronic ground state of the Na+ + H2 system reported recently, extensive trajectory calculations have been performed to study the collision dynamics of vibrationally inelastic processes at total energies up to ∼3 eV. Special attention is given to the relative efficiacy of translational and rotational energy, respectively, in promoting vibrational energy transfer. Vibrational transitions are found to be substantially enhanced by initial molecular rotation. Furthermore, the applicability of simple models is discussed.
New numerical approaches to the solution of the N‐well Schrödinger equation
Authors: Wierzbicki Andrzej , Bowman Joel M.
Published: January 2005

Two new numerical methods for solving the Schrödinger equation for an N‐well periodic potential are presented. One is a diagonalization method based on Floquet–Bloch formalism, and the other is a renormalized Numerov–Cooley method with periodic boundary conditions. The numerical superiority of these methods over the standard diagonalization technique is demonstrated. The methods are illustrated by applications to internal rotation.
Hydrogen bonding in vitamin B6
Authors: Plant J. S. , Greenwood H. H.
Published: January 2005

Double‐well potential energy surfaces for hydrogen bonding in crystalline vitamin B6 have been associated with molecular environmental effects. New calculations, involving improved representations of a fragmentation model, include the introduction of a second pyridoxinium chloride system within the model in a “dimer‐like” configuration. The new results confirm the double‐well potential and the prediction of the experimentally observed position for the proton as being due essentially to environmental effects.

Atomic difference maps are presented for the charge density distributions, which reflect the nature of the bonding as it depends on the proton position. Mulliken populations are examined particularly in relation to the “intermolecular” transfer of electrons.
The energy of a molecule as a sum of terms related to its nuclei when the nuclear motion is quantum mechanically described*
Authors: García‐Sucre M.
Published: January 2005

We show that even when the nuclear motion is quantum mechanically described, the energy of a molecular state can be exactly expressed as a sum of terms, each one related to a nuclei of the molecule. This is a generalization of a similar result obtained by Politzer and Parr for the case in which the nuclei are in fixed positions. In fact, the nuclear motion is exactly taken into account through a modified screening function derivative evaluated on each nucleus of the molecule. We discuss how the screening function derivatives corresponding to the fixed nuclei case relate to those which take into account the nuclear motion. A simple relation between these two arises in the so‐called crude Börn–Oppenheimer approximation.
Projection operators and Clebsch–Gordan coefficients for projective representations
Authors: Altmann S. L.
Published: January 2005

Previous discussions of the bases and projection operators for projective representations are analyzed and alternatives are proposed. Detailed proofs are provided for a result which is often assumed or proved within unacceptable limitations, namely that the form of the projection operators which is standard for vector representations is also valid for unitary projective representations. These proofs provide necessary conditions for this result, and they are constructed in terms of the definition given for the bases of projective representations. The calculation of Clebsch–Gordan coefficients for projective representations by means of the projection operators is discussed. Whereas in the method of Dirl the work is carried out entirely in terms of the matrix representations, and the symmetrization of the bases has to be considered in a second step, all the work of this paper is conducted starting from the symmetrization of the bases of the projective representations, so that those two steps are carried out simultaneously.
Spectroscopic and structural features of small thiocarbonyl molecules in low‐lying excited states: Further applications of a variant of the orthogonal gradient method
Authors: Das Kalyan K. , Mukherjee D. , Bhattacharyya S. P.
Published: January 2005

Structural parameters of a set of five thiocarbonyl molecules in the lowest nπ* states are calculated by using a generalized orbital optimization algorithm (a variant of the orthogonal gradient method) in an INDO MCSCF framework. Transition energies, singlet‐triplet splittings, planar inversion barriers, and dipole moments in nπ* states of different spin multiplicities are reported. Predicted structural features agree reasonably well with available experimental or theoretical data. Some interesting trends are noted in the computed inversion barrier heights, singlet‐triplet splittings, and dipole moments in nπ* states.
Guessing the state of a quantum system from a limited set of observations
Authors: Eriksson K.‐E.
Published: January 2005

A quantum system is repeatedly prepared in the same way and then observed in measurement processes. The following problem is stated and solved: Given the results of a series of observations, what is the best guess for a density matrix describing the ensemble of prepared systems? The corresponding classical problem and its solution are obtained as a special case.
Determination of the potential from experimental data on energies and widths of quasi‐stationary levels
Authors: Fröman Nanny , Fröman Per Olof
Published: January 2005

Formulas for the energies and widths of shape resonances, i.e., quasi‐stationary levels for a quantal particle moving in a spherically symmetric potential, valid also close to the top of the potential barrier, have been published by Drukarev, Fröman, and Fröman. On the basis of these formulas, in the present paper the potential is explicitly expressed in terms of experimental data on the energies and widths of the quasi‐stationary states. The treatment is related to previous treatments by Wheeler and by Cole and Good, Jr., which are in turn related to the Rydberg‐Klein‐Rees method.
Hyperpolarizability approximations
Authors: Goscinski Osvaldo , Delhalle Joseph
Published: January 2005

The approximation of hyperpolarizabilities through the use of operator inequalities is sketched. Previously applied methods to ordinary polarizabilities are extended. They lead to expressions involving moments of oscillator strength distributions related through sum rules to ground state properties. Systematic applications of new formulae are suggested.
On particle–antiparticle mirror symmetry
Authors: Jarlskog C.
Published: January 2005

After a brief review of the historical development of the concept “antiparticle,” some recent theoretical results on CP violation are reviewed.
Correlation of mutagenicity of 1,1‐dimethyl‐3‐(X‐phenyl)‐triazenes with molecular orbital energies and hydrophobicity
Authors: Shusterman Alan J. , Johnson A. Sherwood , Hansch Corwin
Published: January 2005

A set of 18 aryltriazenes and the antitumor drug DTIC, whose mutagenicity has been determined in the Ames test, has been studied using octanol‐water partition coefficients (P) as a measure of relative hydrophobicity and MNDO molecular orbital energies and electron distributions to account for variation in their electronic characteristics. A good structure‐activity relationship was found using log P in combination with either εHOMO, or qHOMO, where the latter is defined as the HOMO electron density on N1 of the triazene. The dependence of mutagenicity on hydrophobicity is similar to what had been found earlier by Venger, Hansch, Hatheway, and Amrein [4] for the aryltriazenes alone using σ+ as a descriptor of electronic characteristics. A consistent role for electronic factors is found using σ+, εHOMO and qHOMO, namely, increased mutagenicity is associated with structural variations that increase the electron donating ability of the triazenes.
Perturbative virtual SCF CI treatment for energy levels of coupled oscillator systems
Authors: Hidalgo A. , Zúñiga J. , Bastida A. , Requena A.
Published: January 2005

A perturbative SCF CI treatment to obtain energy levels of coupled oscillator systems is proposed. The method uses the virtual SCF basis set, and the SCF equations are solved by means of a perturbative treatment that provides the diagonal matrix elements involved in the CI calculation. The off‐diagonal matrix elements are calculated using a commutation relationship derived from exact quantum theorems. Numerical results for several systems are obtained and compared with those from others SCF, SCF CI, and variational treatments.
Sur la theorie des equations de Hartree–Fock dependent du temps. V. Etude d'un modele spherique a 2 electrons. Calcul du tenseur d'hyperpolarisabilite d'ordre 3
Authors: van Groenendael F. Smet et A.
Published: January 2005

In order to apply the TDHF equations in the matrix form we established previously, we conceive a model for which the static Hartree–Fock equations are rigorously solved; so we study problems only linked to the TDHF method, independent of any further approximation (for example, of LCAO type). This model is made of a spherical box containing electrons subject to a particular potential. We solve, numerically, the TDHF equations at different orders, and we calculate the third order hyperpolarizability tensor. We then compare our results with the ones given by a variation‐perturbation method.
Shell model calculations: An efficient new algorithm
Authors: Novoselsky Akiva , Katriel Jacob , Valliègres Michel , Chen Jin‐Quan , Gilmore Robert
Published: January 2005

We describe an efficient new algorithm which extends the range of feasible shell model calculations. This algorithm is applicable to single shell and multiple shell configurations, where two or more quantum numbers (e.g., L and S) are required to label the states within each shell. The algorithm proceeds by factoring the shell model Hilbert space into a product of subspaces, one for each angular momentum. N‐particle wave functions are built up recursively from N – 1 particle wave functions. Three kinds of N – 1‐ to N‐particle coefficients are required to carry out the construction of N‐particle electron (or fermion) states from N – 1 particle states. These are (1) coefficients of fractional parentage (CFPs) within a single shell, (2) outerproduct isoscalar factors (OISFs) within a single angular momentum subspace, and (3) innerproduct isoscalar factors (IISFs) which describe how multishell states within the complementary angular momentum subspaces are combined to form totally antisymmetric wave functions. All three types of N – 1‐ to N‐particle coefficients are generated recursively using a single powerful and efficient matrix diagonalization algorithm. Matrix elements of single particle creation and annihilation operators are expressed in terms of single particle CFPs, OISFs, and IISFs. We also describe an efficient algorithm for computing matrix elements of products of creation and anihilation operators by inserting and summing over complete sets of intermediate states. This is the Feynman‐like sum over path overlaps procedure. Timing benchmarks are presented comparing the new Drexel University shell model (DUSM) code with a state of the art shell model code.
Ab‐Initio MRD‐CI calculations for breaking a chemical bond in a molecule in a crystal or other solid environment. II.H3C—NO2 decomposition of nitromethane in a nitromethane crystal with voids
Authors: Roszak Szczepan , Keegstra Phillip B. , O'neal Douglas W. , Hariharan P. C. , Kaufman Joyce J.
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 crystal or other solid environment. Our technique involves solving a quantum chemical ab‐initio SCF explicitly for a system of 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 carried out for breaking a bond in the reference molecule. This method is completely general. The space treated explicitly quantum chemically and the surrounding space can have voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc. We previously applied this procedure successfully to the H3CNO2 bond dissociation of nitromethane in a nitromethane crystal with extensive testing of the number of molecules that have to be included explicitly in the SCF and how many molecules have to be represented by more distant multipoles. The results indicated that it took more energy to dissociate the H3CNO2 bond when the nitromethane molecule was in the crystal than it did to dissociate that bond in the free nitromethane molecule. In this present study we have investigated the effect of voids (both in the nitromethane molecules treated explicitly in the SCF and those in the environment represented by multipoles) on the calculated H3CNO2 bond dissociation energies.
A quantal approximation to the study of the collisional resonances in the NA–N2 system
Authors: Campos‐Martinez J. , Delgado‐Barrio G. , Villarreal P. , Miret‐Artes S.
Published: January 2005

Experimental works suggest the existence of an oscillating complex in the collisional quenching of Na with N2 molecules. Until now, however, few efforts have been dedicated to the study of these possible complexes. In this work, a diabatic and diabatic‐configuration interaction method are proposed in order to investigate the existence of these quasibound levels. The results and their comparison with other available data are presented and discussed.
Quantum mechanical calculations on vibronic activity in the MCD spectra of carbonyl compounds
Authors: Marconi Giancarlo
Published: January 2005

The magnetooptical properties (B terms) vibronically induced have been calculated for a series of carbonyl compounds in the region of the first absorption band. The rules deduced experimentally for the signs and sizes of the B terms induced by vibrations of different symmetry are generally confirmed by these calculations.