bluuebunny/crossref_metadata_2025_split huggingface dataset:
Selection rules in alternant systems
Authors: Živković Tomislav P. 
Published: January 2005

AbstractThe 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

AbstractIn 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

AbstractEnergies 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.
Multiphoton ionization of many‐electron atoms
Authors: L'Huillier Anne , Jönsson Lars , Wendin Göran 
Published: January 2005

AbstractWe 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

AbstractThe 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

AbstractA “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

AbstractThe 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

AbstractMolecular 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

AbstractA 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

AbstractThe 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

AbstractNeutral, 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

AbstractA 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

AbstractThe 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

AbstractBased 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

AbstractSpin‐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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractA 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.
Comparison of the Perdew–Wang exchange with the Dirac exchange
Authors: Csavinszky P. 
Published: January 2005

AbstractRecently, 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

AbstractWe 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

AbstractThe 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

AbstractWe 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

AbstractA 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

AbstractThe 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

AbstractIn 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.