bluuebunny/crossref_metadata_2025_split huggingface dataset:
Measures of stochasticity for nonstationary initial preparations of coupled oscillators
Authors: Brickmann Jürgen , Hofmann Paul , Schmidt Peter C. 
Published: December 2004

AbstractThe contributions of different eigenstates of a nonlinearly coupled oscillator system to the expansion of a local wave packet are analyzed from an information theoretical point of view. Such a wave packet can be considered as a nonstationary vibrational state of an electronically excited manifold of a molecule after Franck–Condon type initial preparation. The distributions of these contributions are compared to their individual stochastic ideals using Ruch's concept of “mixing distances.” The stochastic ideals are constructed via a probability diffusion process between neighboring states of the original distributions, representing an initial preparation corresponding to a Hamiltonian with only irregular eigenstates. Gaussian minimum uncertainty wave packets as initial states in a two‐dimensional nonlinear oscillator system with classically regular and chaotic energy ranges are studied numerically. It is found that distance measures, partly reflecting the “mixing distance” of a distribution from its stochastic ideal, show a large fluctuation in the classical regular energy range and a small fluctuation in the range where most of the classical trajectories move chaotically. This indicates that for this type of initial preparation process the actual location of the initial state in space plays the dominant role for the dynamics in the low‐energy range while for wave packets starting near the dissociation energy of the model system this location becomes unimportant.
Ab initio quantum‐mechanical calculations on trifluoromethylamine
Authors: Heaton M. M. , Mills D. 
Published: December 2004

AbstractExtensive ab initio molecular‐orbital calculations were carried out on trifluoromethylamine (TFM) to elucidate changes in geometry and electronic structure upon fluorination. The calculations show that the decomposition of CF3NH2 is slightly endoenergetic, and the heats of atomization of CF3NH2 and CH3NH2 show decreased stability of the species upon fluorination. Characteristic of CF3NH2 is a highly polar, strong, short CN bond. More limited calculations were carried out on CF3OH and CH3OH, and the electronic structure of CF3OH is found to be generally similar to that of CF3NH2. The reduced basicity of the fluorinated amine cannot be ascribed to the inductive effect; the enhanced acidity of the fluorinated alcohol reflects the weakening of the OH bond. No evidence leads to a confirmation of the existence of nitrogen–fluorine hyperconjugation in the fluorinated amine.
An MC‐SCF procedure using an orthogonal transformation represented by a direct sum of two‐by‐two rotations
Authors: Igawa Akira 
Published: December 2004

AbstractN molecular orbitals are transformed to K(≦N) molecular orbitals, which are appropriate for an orbital optimization by two‐by‐two orthogonal rotations among themselves. This transformation is determined by using the second‐order structure of the variational energy surface. The optimum K/2 rotation angles are exactly determined. This helps to ensure the reliability of convergence. K/2 two‐by‐two rotations are repeated among these K transformed orbitals. Then, two‐electron molecular integrals are calculated by transformations among only 16 elements and by a permutation among K4/8 elements. Calculational time is considerably reduced compared with that of more usual transformations. Test calculations have been carried out on the ground state of CO. It shows that this procedure brings the calculation to the local region rapidly and reliably.
Usage de l'Algèbre de Lie su (n) dans l'Etude des Systèmes Quantiques à n Etats. V. Application aux Systèmes composés
Authors: Van Groenendael Augustin , Tillieu Jacques 
Published: December 2004

AbstractL'usage de l'algèbre de Lie su (n), proposé dans les articles précédents, est adapté a l'étude d'un système formé de deux sous‐systèmes, respectivement à nA et nB états. On montre comment diverses notions (opérateurs‐densités réduits, problèmes de corrélation, équations d'évolution globales et réduites) peuvent être précisées grâce au formalisme introduit.
First derivatives of correlated wave functions by a matrix‐oriented method: Preliminary application to molecular dipole and quadrupole moments
Authors: Jasien Paul G. , Dykstra Clifford E. 
Published: December 2004

AbstractA direct, matrix‐oriented procedure in the context of the self‐consistent electron pairs (SCEP) method has been implemented for the evaluation of first derivatives of the energy of a variational configuration interaction wavefunction. This has been applied in calculating dipole and quadrupole moments of the correlated charge distributions of several small molecules. As already established, the analytical differentiation being carried out yields properties slightly different than those calculated by integration with the one‐electron density. The matrix‐oriented analytical differentiation is computationally competitive with construction of the one‐electron density.
Quantum‐chemical study of the relation between electronic structure and pA2 in a series of 5‐substituted tryptamines
Authors: Gomez‐Jeria J. S. , Morales‐Lagos D. , Rodriguez‐Gatica J. I. , Saavedra‐Aguilar J. C. 
Published: December 2004

AbstractWe have analyzed the dependence of the serotonin receptor binding affinity on the electronic and steric reactivity indexes for a group of 5‐substituted tryptamines. The approaches employed are a new nonempirical Quantitative Structure—Activity relationship approach and multiple regression analyses. The results suggest that the variation of the receptor binding affinity in 5‐substituted tryptamines is related to the variation of the net charge of two atoms and to the steric bulk of the N‐substituent. A receptor model is proposed.
Bond length alternation in cyclic polyenes. VI. Coupled cluster approach with wannier orbital basis
Authors: Takahashi M. , Paldus J. 
Published: December 2004

AbstractThe role of electron correlation effects on the bond‐length alternation in linear metalliclike systems, as modeled by cyclic polyenes CNHN, N = 2n = 4v + 2, v = 1,2,…, is examined using the coupled cluster approach in the localized Wannier basis formalism. A recently developed approximate coupled pair approach which accounts for connected quadruply excited clusters is employed together with various truncation schemes for the localized doubly‐excited cluster components. It is found that for the physical value of the coupling constant, the electron correlation has only a very slight effect on the potential energy curves, yielding almost the same values for both the magnitude of the bond‐length alternation and for the stabilization energy relative to the symmetric equidistant structures as the restricted Hartree‐Fock theory. This is in contrast to a strongly correlated region where the correlation effects stabilize the undistorted non‐alternating structures. Different mechanisms of the bond‐length alternation or Peierl's distortion as implied by a simple Hückel Hamiltonian and by the Pariser‐Parr‐Pople Hamiltonian models are also pointed out.
Symmetry aspects of diatomics‐in‐molecules (DIM) calculations: Construction of spin‐adapted bases
Authors: Vojtík Jan 
Published: December 2004

AbstractA new procedure for construction of spin‐adapted DIM bases is described. The procedure is based on the direct diagonalization approach, is quite general, and seems to be especially suited for computer implementation. The structure of the bases obtained by the procedure appears to be favorable for performing symmetry operations necessary in setting up the Hamiltonian matrices in semiempirical applications of the DIM method.
Inner projection with and without perturbation theory: The anharmonic oscillator revisited and the quadratic Zeeman effect in ground‐state hydrogen
Authors: Cizek J. , Vrscay E. R. 
Published: December 2004

AbstractThis paper serves a twofold purpose. First, Löwdin's inner projection in both nonperturbative and perturbative forms is applied to the quartic anharmonic oscillator. Inner projection with perturbation theory yields rational approximations to Brillouin–Wigner‐type perturbation expansions. These lower bounds are compared with [N − 1, N] Padé approximants to the Rayleigh–Schrödinger perturbation series for this problem. These Padés are also expressible as the even convergents, w2N, of a Stieltjes‐type continued fraction. The latter representation has certain advantages with respect to its Padé counterpart. Inner projection without perturbation theory provides significantly better results than the perturbative version. The application of inner projection techniques to a perturbed hydrogen atom is not straightforward. The usual problems associated with the continuum spectrum of hydrogen are present. By means of a nonunitary “tilting” transformation associated with the Lie group SO(4, 2), these problems may be bypassed. In the SO(4, 2)‐reformulated eigenvalue problem, a reinterpretation of the basic variables, as developed by Silverstone and Moats, yields a new Hamiltonian that permits direct use of the inner projection method. This method has been applied to the ground state of the hydrogen atom in a magnetic field, using both four‐ and eight‐dimensional basis manifolds. This represents the first application of inner projection to this problem.
Cation–ligand interactions: Reproduction of extended basis set Ab initio SCF computations by the SIBFA 2 additive procedure
Authors: Gresh N. , Pullman A. , Claverie P. 
Published: December 2004

AbstractIn the framework of the additive SIBFA 2 procedure, the intermolecular interaction energy is computed as a sum of five terms: ΔE = EMTP + Erep + Epol + ECT + Edisp. In order to assess the accuracy of the procedure to compute cation–ligand interactions, the interaction of alkali (Na+, K+) and alkaline‐earth (Mg2+, Ca2+) cations with two representative ligands H2O and HCOO− has been studied and the results compared with those of ab initio SCF extended basis set computations. The additive procedure reproduces very satisfactorily the results of ab initio computations as concerns the numerical values of the interaction energies and the equilibrium cation–ligand distances, as well as the evolution of the energy components. A detailed study of these components at different distances helps, in particular, to delineate the relative weights of the charge‐transfer and polarization contributions within the second‐order energy.
Guanine and adenine‐amino acids interactions: An ab initio study
Authors: Sapse A. M. , Jain Duli C. 
Published: December 2004

AbstractAb initio calculations using an STO‐3G Gaussian basis set are performed in order to obtain the binding energy to the 06 and N7 of guanine of such amino acid models as the guanidinium ion for arginine, the ammonium ion for lysine, the methanol for serine, and the formamide for glutamic acid. The binding of formamide to adenine is also investigated. The charged ions exhibit a much higher binding energy to the bases, as expected, than the uncharged molecules. The order of binding strength is NH > guanidinium+ > formamide > methanol, and for formamide, guanine > adenine.
Method of local‐scaling transformations and density‐functional theory in quantum chemistry
Authors: Petkov Ivan Zh. , Stoitsov Mario V. , Kryachko Eugene S. 
Published: December 2004

AbstractThe method of local‐scaling transformations is developed with a view to applications of the nonrelativistic quantum mechanical variational principle in quantum chemistry. Some properties of local‐scaling transformations are studied. The method presented links local‐scaling transformations and one‐electron densities via the ordinary differential equation that provides a proof of the theorem on the existence of the energy density functional within any given orbit and its universality in a sense of the general theory of ordinary differential equations, and allows the construction of this functional explicitly.
[2 + 2] Cycloadditions. A concerted pathway for acetylene plus ethylene
Authors: Andes Hess B. , Schaad L. J. , Reinhoudt D. N 
Published: December 2004

AbstractThe [2s + 2a] cycloaddition of ethylene and acetylene has been studied. A transition structure of C2 symmetry was located on the potential surface. Activation energies for the process are also reported.
The conformation of o‐benzosemiquinone radical and its assignment of the proton hyperfine coupling constants by using the indo and the molecular geometry adjusting methods
Authors: Shinagawa Yasuko , Shinagawa Y. 
Published: December 2004

AbstractThe minimal energy conformations of o‐benzosemiquinone anion radical were calculated for several cases of assignments by using the INDO method and the molecular geometry adjusting method. In order to know the effect of lithium ion in the solvent, the minimal energy conformations of the system of LiOH and o‐benzosemiquinone anion radical were calculated. The calculations of the minimal energy conformations of this radical in t‐butyl alcohol, alkaline aqueous ethanol, alkaline water, neutral methanol, and acetonitrile were carried out. The total energies of the minimal energy conformations in the assignment |A3| > |A4| were lower than those in the assignment |A3| < |A4|.
Comments on the davydov hamiltonian for quasi‐one‐dimensional molecular chains
Authors: Tuszyński J. A. 
Published: December 2004

AbstractThe almost lossless transfer of energy and electronic charge along quasi‐one‐dimensional molecular substances such as peptide chains (H  N  C  O)n found an explanation in a model proposed by Davydov. The Davydov model is based on the dipole—dipole interactions between neighboring peptide groups and on the fact that the internal CO vibrations are coupled to the elastic deformations of the chain. The Davydov Hamiltonian is written in the position‐space representation and, on making a continuum transformation, leads to a nonlinear Schrödinger equation whose solutions are solitons. A Davydov soliton is a coupled pair of an exciton and a lattice deformation. In this paper, the Davydov Hamiltonian is transformed to the reciprocal lattice and its equivalent, second‐quantized Hamiltonian is investigated. Some important observations are made about the coupling constants, their dispersion relations, and the equation of motion for the ladder operators. Our procedure is free of semiclassical approximations but, instead, assumes the onset of Bose condensation. The resultant nonlinear Schrödinger equation is similar to that of Davydov, but a more complete set of solutions is found.
Correlation hole and physical properties: A model calculation
Authors: Cohen Leon , Lee Chongmoon 
Published: December 2004

AbstractThe correlation hole of Coulson and Nielson and its extension to momentum space by Banyard and Reed is studied by using an exactly solvable model. For this model all relevant quantities pertaining to the correlation hole have been calculated exactly. We use this model to study the relationship between the fit to the correlation hole for an approximate wave function and the closeness of the approximate energies to the exact ones. We show that, although in general the better the fit the closer are the approximate physical quantities to the exact ones, there are exceptions where that is not the case. Also, we present a convenient method for the calculation of the two particle distribution in momentum space and generalize the concept of the correlation hole by defining it in the pseudophase space of position and momentum.
Synchronism in the Diels‐Alder reaction
Authors: Burke Luke A. 
Published: December 2004

AbstractDefined as the formation of two new bonds of equal length at the same time, synchronism has been shown to be not possible for the Diels‐Alder reaction when the products are not of strict Cs symmetry. This has been done in two steps. First, the SCF‐MO transition‐state structures for the cyclization reactions of ethylene with cyclopentadiene and furan have been calculated. The activation energies (and heat of reaction) with the STO‐3G, 3‐21G, and 4‐31G basis sets are as follows: with cyclopentadiene 35.0 (−80.7), 30.0 (−32.0), 38.4 (−19.9) kcal/mol; with furan 33.4 (−66.4), 34.6 (−16.5), 42.1 (−8.8) kcal/mol. Then, using a bond‐order‐bond‐length relationship, a value for the tendency toward asynchronism has been calculated for substituted reactants where the two new bonds are arbitrarily held equal. This tendency has been shown in all cases which break strict Cs symmetry. Further, inspection of the atomic‐orbital coefficients showed that the substituents cannot be considered as mere perturbations on the pi system in many cases.
Conformational behavior of azabiphenyls
Authors: Barone Vincenzo , Lelj Francesco , Russo Nino 
Published: December 2004

AbstractThe conformational behavior of biphenyl‐like compounds constituted by benzene and/or azabenzenes has been investigated at the STO‐3G Hartree‐Fock level. Although the STO‐3G basis set probably overstimates conjugative attraction, it seems able to provide reliable general trends. In this connection the analysis of over 30 molecules shows that compounds with the same ortho substituents (including nitrogen lone pairs) have very similar conformational behavior, irrespective of their composition. Furthermore, the comparison of different isomers shows that repulsive interactions decrease in the order hydrogen–hydrogen > lone pair – lone pair > hydrogen lone pair. Conjugative, electrostatic, and steric interactions have also been expressed by well known semiempirical functions, thus allowing a better analysis of the overall torsional potential.
Classical and quantum mechanical calculations of conformational probability density distributions. Two‐rotor molecules
Authors: Smeyers Yves G. , Hernández‐Laguna A. 
Published: December 2004

AbstractIn order to study the dynamical structure of a two‐rotor molecule, such as acetone, as a function of temperature, conformational probability density distributions are computed by using three different approaches: the so‐called current approach, the classical approach, and the quantum mechanical oscillator approach. It is found that the three procedures yield comparable results, at least at normal temperature (25°C), although the current and, especially, the classical approaches give rise to too sharp distributions when compared with the quantum mechanical results. Owing to its simplicity, the current approach may be used advantageously, and it is easily extendible to many‐rotor systems. Finally, it is verified that deuteration does not affect appreciably the conformation.
Thermal motion from monte carlo simulations of aqueous ionic solutions
Authors: Marchese Francis T. , Beveridge David L. 
Published: December 2004

AbstractThe statistical pattern recognition procedure of Marchese and Beveridge for the analysis of Monte Carlo simulations of aqueous ionic solutions [J. Am. Chem. Soc. 106, 3713 (1984)] has been extended to include average hydrogen positions as well as oxygen positions. In addition, thermal ellipsoids have been calculated for each atom and displayed graphically. Application of this procedure to the analysis of a dilute aqueous solution of Zn++ reveals an octahedral arrangement of water molecules within the ion's first hydration shell. The thermal ellipsoids show that most of the water motion is manifest in the hydrogen atoms.
The calculation of ligand‐field multiplet states from multiple‐scattering Xα wave functions
Authors: Daul C. , Goursot A. 
Published: December 2004

AbstractA method is outlined for the calculation of the multiplet ligand‐field states of transition metal complexes. The procedure involves the use of MS‐Xα wave functions, in connection with irreducible tensor operators, and allows the calculation of the elements of the many‐electron CI matrices. Comparison of the calculated and experimental multiplet state energies of CrF, CrCl, and MnF allows one to conclude that the method is useful for the prediction of ligand‐field spectra of transition‐metal complexes.
Theoretical interpretation of infrared spectra of the ClH stretching vibration in the gaseous (Ch3)2O · · · HCl complex
Authors: Wójcik Marek J. 
Published: December 2004

AbstractThe paper presents a theoretical interpretation of the fine structure of infrared spectra of the ClH stretching region in the gaseous (CH3)2O ··· HCl complex. The idea of Millen and Schrems is adopted, in which the spectrum represents a vibrational progression of the hydrogen‐bond stretching mode, and each subband is formed by the progression of the hydrogen‐bond bending mode. The rotational structure is present in such defined fine bands. Theoretical bands are calculated for three different temperatures and for the complex with DCl using two coupling parameters evaluated from the bandwidth. Results show that the most intense peak is associated with the Oσ ← Oσ transition in the stretching O ··· Cl vibration. They also suggest that the complex is nonplanar.
Core–Valence correlations and relativistic effects in heavy atoms for molecular applications
Authors: Durand Ph. 
Published: December 2004

AbstractTheoretical core effective potential methods are widely used in valence‐only electron molecular calculations. These methods, which imply the frozen‐core approximation, work well for the elements of the righthand side of the periodic table but are often unrealistic for metallic elements with highly polarizable cores. For these atoms one has to consider the polarization of the cores under the influence of the electric field created by the valence electrons. Moreover, relativistic corrections must be added for heavy atoms.Various theoretical approaches of core–valence interactions (polarization and core–valence correlations) will be reviewed, with a special emphasis on practical methods of calculation. The problem of handling the relativistic effects will mainly be discussed within the two‐component Pauli formalism. It will be shown that the Foldy–Wouthuysen transformation is not the unique way for deriving relativistic corrections and that the second‐order Dirac equation also provides a good starting point for obtaining relativistic corrections. Analytical exact results are given for the hydrogen atom. The accuracy of this approach is tested on many‐electron atoms and molecules.It is finally shown that the problem of the core‐valence separation is relevant to the general methodology of effective Hamiltonians that seems to provide the best promising way for filling the gap between the semiempirical and purely theoretical ab initio methods.
Interatomic interactions in density functional theory
Authors: Vela Alberto , Cedillo Andrés , Gázquez José L. 
Published: December 2004

AbstractThe concept of an atom in a molecule in the context of density functional theory is used to analyze different levels of approximation to the description of interatomic interactions. Such an approach strongly suggests the use of Kohn–Sham atomic densities as an alternative to Hartree–Fock atomic densities in the electron gas model of Gordon and Kim. The results for rare gases and ionic crystals show that both densities lead to similar results.
Multiple bonds and re(4f) binding energies of some rhenium halides
Authors: Katrib A. , El‐Issa B. D. , Ghodsian R. 
Published: December 2004

AbstractExperimental results on the bombardment of some rhenium trihalides by argon ions in a photoelectron spectrometer have yielded lower‐valency rhenium halides. The presence of the compounds have been made evident by the destabilization of the Re(4f) and the relative decrease in the halogen spectral line intensities. A theoretical approach to the energy of the Re(4f) states have been studied by the MS X‐α method. The molecular model used in the calculations is made up of rhenium polyhalides in D4h symmetry having the structure Re2X (n = 2, 4, 6). A discussion is also presented on the formation of multiple bonds as a result of the overlap between the dxy orbitals of rhenium.