bluuebunny/crossref_metadata_2025 huggingface dataset:
Challenges to computational quantum chemistry from contemporary advances in polyatomic molecular electronic spectroscopy
Authors: Kasha Michael , Parthenopoulos Dimitri , Dellinger Barry
Published: January 2005

Molecular electronic spectroscopy featuring intramolecular proton transfer and twisted intramolecular charge transfer poses a whole new range of problems for computational quantum chemistry. The development of the four‐level laser based on the intramolecular proton‐transfer focuses on the subtleties of the interaction of the singlet and triplet electronic state manifolds of the two different tautomeric species. Examples are given of the sensitive variation of proton‐transfer fluorescence with chemical substitution. A competing excitation channel is shown to exist when internal molecular torsion couples with sudden polarization to yield a twisted intramolecular charge transfer configuration. In such systems, three competing fluorescences can be observed. Several electronic puzzles will be presented that can provide fertile territory for quantum chemical computations. © 1993 John Wiley & Sons, Inc.
Theoretical treatment of electron capture processes for closed‐ and open‐ shell systems
Authors: Bacchus‐Montabonel M. C. , Amezian K.
Published: January 2005

A full theoretical treatment of electron capture processes using ab initio configuration interaction methods within a semiclassical collisional formalism has been developed. An application to closed‐ and open‐shell systems is presented on the example of oxygen‐ion/He collisions. © 1993 John Wiley & Sons, Inc.
The molecular simulation approach to complex hydrodynamics
Authors: Mareschal M. , Kestemont E. , Mansour M. Malek
Published: January 2005

We present here some recent results obtained in the direct simulation of nonequilibrium fluids. First, the transition to convection is studied by means of a hard‐disk model with the appropriate boundary conditions. The quantitative agreement between molecular dynamics and Navier–Stokes hydrodynamics is an impressive argument in favor of the hydrodynamic modeling down to atomic scales, even in nonequilibrium states. Next, the Rayleigh–Brillouin spectrum of a hard‐sphere fluid subjected to a temperature gradient is presented. The simulation is used as a quantitative test of the “Landau–Lifshitz” hypothesis concerning the amplitude of the fluctuating hydrodynamic fluxes, extended to nonequilibrium. Finally, a short description of shock waves in a dilute system is given and the discrepancy between the molecular simulations and the continuum approach is further discussed. We conclude by some remarks concerning the perspectives in nonequilibrium fluids offered by the development of large‐scale computers. © 1993 John Wiley & Sons, Inc.
Ab initio studies of crystal field effects. VII. Structure of 2,3‐diketopiperazine using a 13‐molecule cluster, a calculation involving 1092 basis functions
Authors: Peeters Anik , Van Alsenoy C. , Lenstra A. T. H. , Geise H. J.
Published: January 2005

The structure of 2,3‐diketopiperazine in a crystal phase (P21/c), modeled by a 13‐molecule cluster surrounded by point charges, was completely optimized using standard gradient procedures. The (ab initio) MIA approach was used to perform the SCF step, which using a 4‐21G basis set, involves calculations using 1092 basis functions. Results are in very good agreement with experimental (X‐ray) data, in contrast to a previous study in which a model was used consisting of only point charges. © 1993 John Wiley & Sons, Inc.
The structure of n‐fold negatively charged C60 (n = 1, 2,…,6)
Authors: Hutter Jürg , Peter Lüthi Hans
Published: January 2005

Restricted Hartree–Fock calculations were performed to optimize the structure of C60 and its negatively charged ions. The length of the short bond was found to increase lineraly with the charge, whereas the longer bond steadily decreases, resulting in a difference of only 0.0266 Å between the two bond types. Icosahedral symmetry was retained in all calculations. The state with the highest spin multiplicity is calculated to be the lowest in energy for all ions. © 1993 John Wiley & Sons, Inc.
Cluster‐size convergence of some physical parameters of bare (Nin) and CH3‐Chemisorbed (CH3Nin) nickel (111) clusters: An ab initio study
Authors: Bureau Christophe , Defranceschi Mireille , Delhalle Joseph
Published: January 2005

Hartree–Fock calculations of the molecular geometry and electronic properties (ionization potentials) of bare Nin clusters (n = 3, 7, 13, and 25) and nickel clusters bearing a chemisorbed CH3 radical are reported. A basis of double‐zeta quality on light atoms including carbon and a model core potential along with double‐zeta plus polarization auxiliary basis set for nickel atoms have been used. Computed properties serve to assess cluster‐size convergence when nickel atoms core electrons are described with [1s–3d] pseudopotentials. In spite of their simplicity, results with these potentials are in good agreement with previously reported theoretical and experimental data on seven‐atom clusters. This suggests that the cluster‐model approach to surface bonding using simple pseudoopotentials can lead to meaningful results and tractable computationl times when applied to larger chemisorbed organic molecules. Thus, this approach may be adequate to help find out direct experimental evidence of the interfacial bonding for grafted structures obtained under cathodic electropolymerization of acrylonitrile on nickel surfaces. © 1993 John Wiley & Sons Inc.
Polarization correction of the electrostatic potential for aromatic compounds: Study of the nucleophilic attack
Authors: Dive G. , Dehareng D.
Published: January 2005

The electrostatic potential (EP) and the polarization correction (PL) on the (EP) were calculated for four aromatic compounds: benzene, chlorobenzene, phenol, and benzoic amide, at the ab initio SCF level within two basis sets: 6‐31G** and MINI‐1. One calculation was performed using the polarized MINI‐1** basis set. The quantity total interaction energy (TEH) defined as −EP + PL can be used as an indicator of the nucleophilic attack preferential position. By reference to 6‐31G**, MINI‐1 provides very satisfactory results. Moreover, the MINI‐1** results are very similar to the MINI‐1 ones. It appears that −EP does not provide a reliable tool to study the nucleophilic attack susceptibility, whereas TEH seems to be very well adapted for this kind of approach. © 1993 John Wiley & Sons, Inc.
Preliminary theoretical study of perfluorodimethyl ether and its protonated form
Authors: Fontaine Michèle , Delhalle Joseph , Defranceschi Mireille , Lécayon Gérard , Boissel Jacques
Published: January 2005

3‐21G RHF calculations on (CF3)2O and (CF3)2OH+ molecules are carried out to assess the changes induced in the molecular properties of ether links in poly (perfluoro ethers) interacting with acid sites at contact surfaces. Geometry of the species, vibrational frequencies, proton affinity, and energetics of the (CF3)2OH+ fragmentation provide a preliminary basis to understand problems raised by lubricant degradation. © 1993 John Wiley & Sons, Inc.
Ab initio study of the electronic properties of potential antagonists of the glycine receptor: 1. Transition state of the 2‐pyridone · H2O/2‐hydroxypyridine · H2O tautomeric equilibrium
Authors: Michel Andrég. , Trudel Yolaine , Dion Claude
Published: January 2005

Glycine receptor substrates are molecules potentially involved in tautomerism phenomena. This study is expected to provide information that might help understand their intrinsic reactivities; such physicochemical data would allow us to establish predictive models in a search for better antagonists. Ab initio molecular orbital studies, using 3‐21G and 6‐31G* basis sets, are reported for the tautomeric equilibrium of 2‐pyridone · H2O/2‐hydroxypyridine · H2O. The geometry of the transition state has also been optimized. The results show the important effect of the water molecule through the formation of hydrogen bonds. This system will be used as a prototype for the design of antagonists of the glycine receptor, a potential site for the action of new antiepileptic drugs and compounds preventing ischemic brain damage. © 1993 John Wiley & Sons, Inc.
Theoretical evaluation of atomic charges to be integrated into conformational analyses of neutral lipids
Authors: Culot Christine , Dory Magdalena , Durant Francois , Vercauteren Daniel P.
Published: January 2005

Conformational analyses of large molecules as fatty acids and triglycerides are usually amenable by molecular mechanics. A correct evaluation of the electrostatic energy term is thus crucial in determining reliable results. In this contribution, we have considered the most abundant fatty acids in biomembranes, i.e., lauric, stearic, oleic, and elaidic acid, and the corresponding triglycerides, i.e., trilaurin, tristearin, triolein, and trielaidin, and estimated the Mulliken and potential‐derived charges both at the semiempirical AM1 and ab initio HF MO STO‐3G level. Atomic charges obtained by the Mulliken population analysis do not take into account the full geometry of the molecule. On the contrary, the change of conformation, due to different chains length or the presence of a trans or cis double bond, greatly influences the repartition of the potential‐derived charges. A systematic comparative analysis shows that charges calculated by AM 1 are not suitable because as they do not reproduce potential‐derived charges obtained by ab initio. © 1993 John Wiley & Sons, Inc.
Ab initio rotational constants of interstellar species: Cyanoacetylene hydrogenated derivatives
Authors: Arnau Arturo , Silla Estanislao , Tuñón Iñaki
Published: January 2005

By means of ab initio calculations, the rotational constants and dipole moments of HnC3N (n = 1, 3, 5, 7, and 9) species have been calculated at the HF / 6‐31G* level of theory. Selected cases have been also calculated at the MP2/6‐31G* level and the influence of calculation level on rotational constant values is briefly discussed. Some of these species were discovered in the interstellar medium, while others have still not been detected there, although their existence is very probable. The results given here could help in their detection. © 1993 John Wiley & Sons, Inc.
Harmonic oscillator tensors. II. angular momentum expressions of matrix elements of vibrational operators
Authors: Palting Pancracio
Published: January 2005

The spatial symmetries of the harmonic oscillator and the recently found irreducible tensors constructed from the associated annihilation and creation operators are exploited to obtain new expressions for the elements of the matrix representatives of several examples of vibrational operators. Since all vibrational operators are expressible in terms of the irreducible tensors, their matrix elements reflect the angular momentum symmetry inherent in them, for the results derived here are in terms of the Clebsch–Gordan coefficients and the isoscalar factors that arise from the couplinig rule of the irreducible tensors. Familiarity with the mathematical properties of these quantities derived from the elementary theory of angular momentum facilitates the evaluation of many vibrational operators that may be of importance in the study of potentials in this basis. In particular, it is shown that the nonvanishing of matrix elements is governed by a law of conservation of angular momentum along the axis of quantization of the nondegenerate harmonic oscillator. © 1993 John Wiley & Sons, Inc.
Polystyrene models. III. modeling backbone/side‐group interactions by an ab initio study of 2,4‐diphenylpentane
Authors: Lagowski J. B. , Vancso G. J.
Published: January 2005

The Structure and the energy of the stable conformations of the two possible stereochemical configurations of 2,4‐diphenylpentane are obtained using the ab initio molecular orbital theory. The objective was to mimic the possible structures and determine the corresponding energies of the dyads of syndiotactic and isotactic polystyrene and, consequently, to study the interactions between the phenyl groups. The results of complete geometry optimization showed significant changes in geometrical parameters compared with those expected from the ideal hydrocarbon structure. The steric strain is most pronounced in some of the gauche conformations where large (approximately 40°) distortions of the backbone torsional angles and/or simultaneous phenyl group rotations in the range of 30°–40° away from its global minimum position may occur. In addition to the discussion of the geometrical parameters, the corresponding dipole moments are also calculated and differences related to the various structures and discussed. © 1993 John Wiley & Sons. Inc.
Transport of excitation energy in a doped molecular aggregate. III. numerical investigation of exciton hopping with various exciton‐depleting processes
Authors: Datta Sambhu N. , Prabhu N. V.
Published: January 2005

A brute‐force numerical investigation has been carried out on the hopping of excitons in a three‐dimensional molecular aggregate. Possibilities of vibronic decay, rapid chemical reactions of saturated species, radiative decay of overpopulated molecules, and cooperative chemical reactions involving saturated exciton populations on traps of two different types have been considered. Investigation have been performed with two types of initial distribution of excitons—facial and random—and for 10,000 or, sometimes, for 20,000 time steps each of duration 1ps. Several interesting observations have been made from this computer experiment: (1) The total number of occurrences of fast reactions depends upon the initial distribution of excitons. (2) It decreases if other exciton depleting processes are at work. (3) It also depends on the pattern of placement of traps. (4) The location of impurities also affects the rate of occurrence of these reactions. Thus, more reactions occur when the excitons are initially concentrated on one face and traps are suitably located on the path of flow of these excitons. A random initial distribution tends to equilibrate the excitons quickly over all the lattice points, thus giving rise to fewer reactions. (5) The number of reactions need not necessarily increase with the number of reaction centers; in fact, it decreases as more centers are added when the supply of excitons is severely limited. (6) A Complicated dynamics results when different types of additional processes, viz., enhanced fluorescence, radiative emissions, and cooperative chemical reactions are simultaneously allowed. The cooperative process has been clearly found to dominate. A first‐order rate constant of about 108 s‐1 has been calculated for the occurrence of the cooperative process. This rate is affected when other nonconserving processes are switched on. Observations (1), (4), and (5) are the most important conclusions of our work. They lie outside the scope of traditional models such as the random walk model, the diffusion model, and the lattice model for the migration of excitons in a molecular aggregate. © 1993 John Wiley & Sons, Inc.
Complex scaling and Lyapunov converters
Authors: Kumičák Juraj , Brändas Erkki J.
Published: January 2005

It is proved that for a certain class of potentials dilation operators used in the method of complex scaling (CSM) represent a special case of Lyapunov converters. The time evolution of quantum systems is investigated from the viewpoint of this result, with special emphasis on the time evolution of resonant functions. © 1993 John Wiley & Sons, Inc.
On the positivity condition in the nonunitary transformation theory of irreversibility
Authors: Brändas E. J. , Antoniou I. E.
Published: January 2005

The Misra–Prigogine–Courbage Nonunitary Transformation Theory of Irreversibility is revisited in connection with the role played by the fundamental positivity preserving condition of the associated semigroup evolution. Comparisons between various viewpoints referring to general context of non‐Markovian formulations are given. Some recent criticism of the Misra–Prigogine–Courbage theory regarding attempts to derive irreversibility and the approach to equilibrium without any approximations and/or loss of information is corrected. © 1993 John Wiley & Sons, Inc.
Generalized spectral decompositions of mixing dynamical systems
Authors: Antoniou I. , Tasaki S.
Published: January 2005

We introduce a method for the explicit computation of the eigenvalue problem of the evolution operator of mixing dynamical systems. The method is based on the subdynamics decomposition of the Brussels–Austin groups directed by Professor I. Prigogine. We apply the method to three different representatives of mixing systems, namely, the Renyi maps, baker's transformations, and the Friedrichs model. The obtained spectral decompositions acquire meaning in suitable rigged Hilbert spaces that we construct explicitly for the three models. The resulting spectral decompositions show explicitly the intrinsic irreversibility of baker's transformations and Friedrichs model and the intrinsically probabilistic characters of the Renyi maps and baker's transformations. The dynamical properties are reflected in the spectrum because the eigenvalues are the powers of the Lyapunov times for the Renyi and baker systems and include the lifetimes for the Friedrichs model. © 1993 John Wiley & Sons, Inc.
Amplitude squared squeezed states in the Fock–Bargmann space
Authors: Martín M. , Sandoval L. , Rivas‐silva J. F. , Palma A.
Published: January 2005

Squeezed states are normally studied in configuration space or using the Glauber formalism for the coherent states. Here we show that the use of the Fock–Bargmann space can lead to more general solutions, especially in the case of squeezed states associated with the square of field amplitude. The relations that we obtain are completely general and several well‐known, approximate results can be obtained as the limit cases of our approach.© 1993 John Wiley & Sons, Inc.
The hydrogen atom in a semi‐infinite space limited by a hyperboloidal boundary
Authors: Ley‐Koo E. , Mateos‐Cortés S.
Published: January 2005

The hydrogen atom in a semi‐infinite space limited by a hyperboloidal boundary, with the nucleus at a focus, is investigated as a model of an atom on the surface of a solid. The energy eigenvalues, hyperfine structure, and electric dipole moment of the system are evaluated for different focal distances and eccentricities of the boundary. It is shown that the system tends to become infinitely degenerate at the ionization threshold as the boundary closes in approaching the nucleus. This model includes as special cases the corresponding models in the literature with plane and paraboloidal boundaries. © 1993 John Wiley & Sons, Inc.
Related upper and lower bounds to atomic binding energies
Authors: Tai C. C. , Vatsya S. R. , Pritchard H. O.
Published: January 2005

The method proposed by Singh for the calculations of lower bounds to atomic binding energies has been generalized to encompass upper bounds as well. The result is a pair of related matrix eigenvalue problems, constructed from similar sets of basic matrix elements, with the solution of one yielding the tower bound, and of the other, the upper bound. The upper bounds are identical to those calculated by the Rayleigh–Ritz method, which can be useful when the inversion of the normalization matrix is ill‐conditioned. The lower bounds are comparable with the best available in the literature. © 1993 John Wiley & Sons, Inc.
Analytical calculation of the electrostatic interaction energy within the CNDO framework
Authors: Dehareng D. , Dive G. , Ghuysen J. M.
Published: January 2005

This work analyses the adequacy of an analytical electrostatic energy formulation within the CNDO framework to predict the stable conformations of large molecular complexes. Comparisons are made with abinitio results for small systems such as water‐formamide, methanol‐water‐imidazole, or guanine‐cytosine and with AM1 results for two large systems: a molecular tweezer + the 9‐methyladenine complex and a model active site of the α‐chymotrypsin and its ligand complex. This approach is efficient in providing reliable confromers for large molecular systems in a very fast way. © 1993 John Wiley & Sons, Inc.
Optimally convergent determinantal expansion of many‐electron wave functions
Authors: Thephilou A. K. , March N. H.
Published: January 2005

It is shown that the many‐electron wave function can be expanded optimally in terms of a set of slater determinants that reduces considerbly the number of determinats differing by a single‐particle wave function. The method is based on a variational procedure. The first step of the present method is the Brueckner optimization. The other determinants of the expansion are obtained by successive optimizations performed under additional constraints. The method is illustrated by specifically three‐and four particle example. Finally, it is concluded that the eigenstates of a many‐ particle system can be approximated by states that neglect single‐particle excitations. The Zero‐order approximation of the present scheme is the Hartree–Fock approximation. © 1993 John Wiley & Sons, Inc.
A numerical integration scheme for the evaluation of correlation energy functionals
Authors: Fortunelli Alessandro , Salvetti Oriano
Published: January 2005

A numerical integration scheme is presented for three‐dimensional integrals occurring in electronic structure calculations, concentrating attention on the evaluation of the correlation energy through a density‐functional expression. The scheme is based on the choice of density‐based weight functions that naturally partition the space into “atomic” volumes (in which the integration is performed in terms of spherical coordinates) and “diatomic” volumes (in which the integration is performed in terms of confocal elliptical coordinates). Such a choice is justified on the basis of the analytical behavior of the integrand. The attainable accuracy and the required computational effort within the proposed scheme are discussed in detail in a test application on the C60 molecule in the symmetrical configuration. Finally, a comparison with previously proposed schemes is presented. © 1993 John Wiley & Sons, Inc.
Decay dynamics of a metastable state in a time‐varying electric field: A three‐level prototypical system
Authors: Adhikari S. , Bhattacharyya S. P. , Bhattacharyya D. M.
Published: January 2005

In a numerical experiment, the dynamics of natural decay of a metastable state is shown to be strongly affected by the presence of an external time‐varying electric field of high intensity. The metastable state, depending upon its level width, is shown to be stabilized by a near‐resonant electric field. The applied field may even cause population transfer to a neighboring bound level that is not directly populated by the natural decay process. The effects of continuous and pulsed perturbations are both analyzed numerically, and perturbation, theoretically. © 1993 John Wiley & Sons, Inc.
Computation of many‐center exchange integrals over Slater orbitals up to 4d by means of optimized Gaussian expansions
Authors: Montagnani Raffaele , Salvetti Oriano
Published: January 2005

The previously described optimization of small Gaussian expansions, to be employed for the computation of many‐center two‐electron integrals over Slater basis sets, has been extended, with some refinements, up 4d orbitals. © 1993 John Wiley & Sons, Inc.
Ab initio potential energy function for the dynamics of the fluoronium ion
Authors: Gianturco F. A. , Buonomo E. , Semprini E. , Stefani F. , Palma Amadeo
Published: January 2005

The various structures and geometries of the cation FH (fluoronium ion) have been investigated over a broad range of relative orientations between the proton and the HF molecule using ab initio, correlated wave functions and focusing on the lowest‐energy channel that can lead to the production of vibrationally excited HF in the collisions with near‐thermal H+ atoms. Various quality controls have been carried out on the accuracy of the target molecule description and a reliable, overall structure of the vibrational–rotational potential energy surface is obtained, for the first time extensively enough, to carry out scattering calculations in the direct excitation channels for the rotovibrational modes. © 1993 John Wiley & Sons, Inc.
Least‐motion stepwise [2 + 2] ketene cycloadition pathways predict the more hindered cis‐cycloadducts
Authors: Firestone Raymond A.
Published: January 2005

Ketene cycloadditions give predominantly the more hindered product. This has been adduced as evidence for concerted antarafacial cycloaddition in accord with the Woodward–Hoffmann rules. However, stepwise mechanisms, heretofore discounted, are now shown to account for the products in a simple least‐motion manner that requires only that rotation in the intermediates be not greatly faster than cyclization. This requirement is amply supported by the literature. © 1994 John Wiley & Sons, Inc.
Net signs of molecular graphs: Dependence of molecular structure
Authors: Gutman Ivan , Lee Shyi‐Long , Luo Yeung‐Long , Yeh Yeong‐Nan
Published: January 2005

The concept of internal connectivity associates to each eigenvector of the molecular graph a graph with signed edges. We show that the respective edge signs can be deduced without knowing the graph eigenvectors. The dependence of the edge signs on molecular structure is elucidated. The sign of an edge whose end vertices are νr and νs is found to be the result of separate interactions through all paths connecting the vertices νr and νs. The analysis of the edge signs is particularly simple in the case of acyclic systems. © 1994 John Wiley & Sons, Inc.
Electron‐pair distributions and chemical bonding
Authors: Wang Jiahu , Smith Vedene H.
Published: January 2005

The electron‐pair (intracule and extracule) densities of the first‐row hydrides may be understood on the basis of the different chemical‐bonding schemes in this series. Thus, the LiH pair densities show a strongly ionic nature and may be fairly well described by Li+H−. The CH‐pair densities, on the other hand, may be approximated by the promolecular superposition of a carbon and a hydrogen atom with an accumulation of pair density in the internuclear region signifying covalency. In the case of FH, its pair densities show a predominately ionic structure and are closer to those of F−than to those of the promolecular superposition of a fluorine and a hydrogen atom. The slight deformation of longitudinal pair densities observed in FH is largely due to the presence of the H+. © 1994 John Wiley & Sons, Inc.
Local hardness revisited: Definition and the spin‐polarized Kohn–Sham formulation of density functional theory
Authors: Garza Jorge , Robles Juvencio
Published: January 2005

The definition of local hardness is clarified within the Kohn–Sham formulation of the density functional theory. Exact equations for the local hardness of open‐ and closed‐shell electronic systems are derived. It is found that the Kohn–Sham independent particle kinetic energy does not contribute to local hardness. The apparent contradiction with previous work of Parr et al. is cleared up when the local hardness definition is revised. © 1994 John Wiley & Sons, Inc.
On the space of eigenvectors of molecular quantum mechanics
Authors: Wulfman Carl
Published: January 2005

As the representative point on an electronic energy surface moves along any closed level curve, the corresponding electronic eigenvectors behave as does the normal to a Moebius strip when its foot is moved around the strip. This introduces a topology into the space of eigenvectors that is not envisaged in the definition of Hilbert space, but is fundamental to molecular quantum mechanics. Salient properties of these “non‐Hilbertian” vector spaces are established. © 1994 John Wiley & Sons, Inc.
Generalized bond orders
Authors: Randić Milan , Guo Xiaofeng
Published: January 2005

We introduce generalized bond orders defined in terms of weighted Kekule valence structures. The weights were determined by the contributions of linearly independent and minimal conjugated circuits in individual Kekule valence structure. When special values for the contributions of conjugated circuits of different size are assumed, one obtains quantities that show considerable similarity to the Pauling and the Clar's bond orders. Pauling bond orders are obtained when one assumes that all conjugated circuits make equal contribution to bond orders. © 1994 John Wiley & Sons, Inc.
Approximately N‐representable density functional density matrices
Authors: Soirat A. , Flocco M. , Massa L.
Published: January 2005

For the first time, we obtain practical density matrices approximately N‐representable by correlated‐determinant wave functions, which are functionals of the electron density and entirely defined by information obtainable from the X‐ray coherent diffraction experiment. © 1994 John Wiley & Sons, Inc.
Protonation of fused aromatic systems: Ab initio study of some model Wheland intermediates
Authors: Eckert‐maksić M. , Maksić Z. B. , Klessinger M.
Published: January 2005

Structure and properties of Wheland's intermediates are studied in some model systems. The effect of the annelated small ring is simulated by bending two vicinal CH bonds in benzene toward each other. The extent of bending describes the ring‐size effect of a small fused fragment molecule. The electrophilic substituent is represented by a proton. Calculations at the HF/6‐31G* and MP2/HF/6–31G* levels of sophistication confirm the Mills–Nixon (MN) hypothesis. It is conclusively shown that ß‐complexes are more stable than are α‐protonated species. The results are interpreted in terms of the ground‐state density distribution dictated by rehybridization and of the transition structure π‐electron redistribution triggered by protonation. Their effects are additive to a large extent as far as CC bond distances are concerned. Energetic properties are determined by the interplay of two characteristic π‐electron localization patterns, one caused by the initial angular strain and the accompanying rehybridization related to the ground state and the other one occurring during formation of the proton σ‐complex. This interplay destabilizes α‐intermediates relative to ß‐intermediates. © 1994 John Wiley & Sons, Inc.
A comparative basis‐set study of NeH+ using coupled‐cluster techniques
Authors: Pendergast Phil , Heck John M. , Hayes Edward F.
Published: January 2005

Unrestricted Hartree‐Fock, coupled‐cluster calculations are reported for the ground state of NeH+ using atomic basis sets of increasing size and accuracy for both Ne and H. The goal is to determine the basis set and coupled‐cluster level of calculation needed to obtain a NeH+ potential energy curve of known accuracy. Here, it is shown that calculations using a quintuple zeta basis at the coupled‐cluster singles and doubles level with noniterative triples, CCSD(T), predict a Ne—H bond dissociation energy that is within about 0.01 eV of the exact Born–Oppenheimer molecular electronic structure result. Spectroscopic constants determined using the Simons–Parr–Finlan procedure are found to be in very good agreement with the experimental results. Calculations at the augmented quadruple zeta level for the two lowest triplet excited states of the NeH+ species are presented. Both of these states separate into ground‐state Ne+ and H(1s). The resulting potential curves predict stable minima at the SCF, CCSD, and CCSD(T) levels with dissociation energies of about 0.07 eV. Spectroscopic constants from the potential curves and dissociation constants are reported. © 1994 John Wiley & Sons, Inc.
Electronic structure calculations of 1,3‐dipolar cycloadditions using density functional and Hartree–Fock methods
Authors: Sosa Carlos , Andzelm Jan , Lee Chengteh , Blake James F. , Chenard B. L. , Butler Todd W.
Published: January 2005

Local spin density (LSD) methods were used to study the concerted 1,3‐dipolar cycloadditions for fulminic acid plus acetylene, fulminic acid plus ethylene, and nitrone plus ethylene. Cartesian Gaussian double‐zeta split‐valence basis sets augmented with one set of polarization functions (DZVP) were used for the LSD calculations. The LSD calculations were performed with the LSD exchange functional (Dirac) and with the Vosko, Wilk, and Nusair correlation energy functional (VWN). Nonlocal spin‐density corrections (NLSD) were estimated with the exchange functional of Becke and the correlation energy functional of Perdew (VWN + BP) and Becke, and the correlation energy functional of Lee, Yang, and Parr (B‐LYP). Vibrational frequencies were computed at the VWN and B‐LYP levels by numerical differentiation of the analytical first derivatives of the energy. Each of these reactions was examined using Hartree–Fock and Møller–Plesset perturbation theory for comparison. Geometry optimizations were carried out at the Hartree–Fock level with the 6‐311G(d,p) basis set, and correlation energies were computed up to the MP4SDTQ/6‐311G(d,p) level of theory. For the reactions of fulminic acid plus acetylene, fulminic acid plus ethylene, and nitrone plus ethylene, our best estimated density functional barrier heights are 7.8 ± 1.5, 8.9 ± 0.3, and 11.05 ± 1.9 kcal/mol, respectively. These results are in reasonable agreement with the correlated wave‐function calculations and experimental estimates. © 1994 John Wiley & Sons, Inc.
A dimensionally scaled generalization of constrained search energy density functionals
Authors: Valone Steven M.
Published: January 2005

A dimensionally scaled generalization of constrained search density functional theory allows access to some simple model problems. These might be valuable for testing and perhaps improving conventional density functionals. One specific model problem is solved: When extrapolated to infinite numbers of spatial dimensions, the energy density functional for spherically symmetric, two‐electron systems can be calculated to arbitrary accuracy. © 1994 John Wiley & Sons, Inc.
Equilibrium geometry and electronic structure of benzylidene, arylethylidene, and heterocyclic arylidene malononitriles
Authors: Moustafa H. , El‐Taher S. , Hamed Maher M. , Hilal R.
Published: January 2005

The equilibrium geometry of some benzylidene, arylethylidene, and heterocyclic arylidene malononitriles has been calculated within the framework of the MNDO–MO formalism. Various structural factors are analyzed and discussed in terms of localized and delocalized MOS and π‐interaction between methylene malononitrile (MMN) and aryl moieties. The possibility of charge transfer (CT) from aryl to MMN moieties has been examined. The presence of a heterocyclic ring introduces a small perturbation into the MMN nuclear frame. p‐Substituents have a pronounced effect on the magnitude and direction of the dipole moment. The variation of ionization potentials and bond order with the Hammet σ p are examined and a straight‐line relationship is obtained. The correlation between physiological activity and quantum mechanical properties is analyzed in terms of heat of formation, ionization potential, dipole moment, and charge density on the aryl ring. © 1994 John Wiley & Sons, Inc.
Electronic structure of DNA dimer units, d(AG) · d, (CT), d(TG) · d(CA), · d(GT), and d(GA), in A and B conformations by DV–Xα cluster calculations
Authors: Shinoda Takako , Shima Nobuyuki , Tsukada Masaru
Published: January 2005

The electronic structure of d(AG) · d(CT), d(TG) · d(CA), d(AC) · d(GT), and d(TC) · d(GA), which are stacked base pairs in the DNA double helix, are elucidated for A and B conformations in detail by DV‐Xα cluster calculations. By using the S̃r values of the O2 (and O3) of each phosphate in the clusters together with those calculated in previous articles, it becomes possible to treat chemical reactivities of phosphates as recognition sites for all DNA fragments. Contour maps of the wave function of the HOMO and LUMO for bases differ subtly among the clusters and show the various fields around the P ion induced by the respective stacking constructed from four kinds of bases. © 1994 John Wiley & Sons, Inc.
Population analysis of pair densities: A link between quantum chemical and classical picture of chemical structure
Authors: Ponec Robert , Strnad Martin
Published: January 2005

The pairon population analysis based on the geminal expansion of pair densities is introduced and applied. As demonstrated by numerical data calculated for a series of simple molecules by the semiempirical MNDO method, the resulting populations provide a new simple means of visualizing the molecular structure. In addition to the reproduction of classical structural formula including the multiplicity of individual bonds, the resulting populations confirm the transferability of bond energies and also provide a simple interpretation of the concept of quantum chemical valence. © 1994 John Wiley & Sons, Inc.
Accurate computation of eigenfunctions for screened coulomb potentials
Authors: Núñez Marco A.
Published: January 2005

A numerical method is developed to obtain a sequence of functions converging to the eigenfunctions of the Schrödinger operator H = − ½ Δ + V(r) for V(r) = − Z/r + χ(r), where χ(r) is a continuous and bounded‐from‐below function for (r ∈ 0, ∞). The criterion of convergence in the convergence in the norm of the Hilbert space L2(0, ∞), which assures the accurate computation of the expected values for a symmetric operator, as we show. The method consists of solving the dirichlet problem inside a box of radius n by the Ritz method, whose convergence in the norm is proved using the compactness criterion. Using a physical argument, we show that the bounded states of the Dirichlet problem converge to those the unbounded system in the norm of L2(0, ∞) as n grows. The method is applied to the potentials V(r) = − Z/r + ari (i ≥ 0) and V(r) = − Z/r + a/(1 + rλ); in each case, we show the numerical convergence of eigenfunctions, energies, and density moments. © 1994 John Wiley & Sons, Inc.
Improved local density functional approach for atomic systems
Authors: Ghosh Swapan K. , Deb B. M.
Published: January 2005

Through a new local density approximation to the kinetic energy density functional introduced by us recently, a simple Thomas–Fermi‐like scheme for the direct calculation of electron density in atoms is proposed. The calculated density is nonsingular at the nucleus and the energy values are in very good agreement with the corresponding Hartree–Fock results for atoms. © 1994 John Wiley & Sons, Inc.
Calculating excitation energies with the help of cumulants
Authors: Schork Tom , Fulde Peter
Published: January 2005

Recently, it has been shown that the ground‐state energy of a quantum many‐body system can be written in terms of cumulants. In this article, we show that the energies of excited states can be expressed similarly. These representations are suitable for various approximation schemes, e.g., projection techniques. The explicit use of cumulants ensures size consistency in all approximations. The theory is then applied to the computation of the energy bands of a semiconductor with a diamond structure. As we focus especially on the effects of electron correlations, we consider a simplified model and rederive results obtained previously by the variational local ansatz method. © 1994 John Wiley & Sons, Inc.
The band gap of alternant 1D π‐electron systems
Authors: Tyutyulkov N. , Dietz F. , Klein D. J. , Seitz W. A. , Schmalz T. G.
Published: January 2005

Within the framework of the AMO–EHF approximation for the Hubbard model, it is shown that the band gap ΔE of an arbitrary regular alternant 1D π‐system (polymer), having a singlet ground state, is different from zero, i.e., the ground state is a dielectric one. At least one of the three components of the energy gap—the topological, Δtop; the geometrical, Δgeom; or the correlation, Δcorr (combined via the equation ΔE = {Δ + (Δtop + Δgeom))2}1/2—is different from zero. Numerical results for several 1D alternant polymers with conjugated π‐systems are given. © 1994 John Wiley & Sons, Inc.
Linear response calculation of potential energy curves of BH
Authors: Jaszuński Michał
Published: January 2005

The multiconfiguration time‐dependent Hartree–Fock (MC TDHF) approximation is applied to study the excited states of the BH molecule. The potential energy curves and spectroscopic properties of the excited states are analyzed. The MC TDHF results are compared to similar MC SCF calculations for individual excited states. © 1994 John Wiley & Sons, Inc.
Electrostatic potential and electric field mapping of some sweeteners of the suosan series: A search for the structure–activity relationship
Authors: Santhosh C. , Mishra P. C.
Published: January 2005

Electrostatic potential and electric field maps of some suosan series of sweeteners have been studied. Geometries optimized using the MNDO molecular orbital method and charge distributions obtained by the Mulliken population analysis were employed in the calculations. Sweetness potencies of the molecules were compared with electrostatic potentials and electric field strengths near the terminal COOH group. The observed sweetness potencies of the compounds and calculated electric fields near the carbonyl oxygen of the COOH group are found to be strongly correlated. © 1994 John Wiley & Sons, Inc.
Some remarks concerning spherulitic growth
Authors: Gadomski A. , Łuczka J.
Published: January 2005

A new approach to the description of the formation of spherulites in a polymer solution is proposed. It is based on an analytical scheme that takes into account the mass conservation law as a fundamental evolution equation. Three physically interesting cases are considered, both which, on deterministic and stochastic levels, can reflect an asymptotic behavior characteristic for spherulites, namely, R(t) ∝︁ t, where R(t) is a radius of the spherulite measured at instant t. A few examples of systems similar to that under study are mentioned. © 1994 John Wiley & Sons, Inc.
Calculations of the electronic structure of various aperiodic polymers by an elongation method
Authors: Imamura Akira , Aoki Yuriko , Nishimoto Kazushi , Kurihara Youji , Nagao Atsuhiro
Published: January 2005

An elongation method, which had already been proposed to calculate the electronic structure of aperiodic polymer efficiently, was applied to atactic polypropylene, with the approximation level of the extended Hückel method. The atomic populations thus calculated were found to be dependent sensitively on the tacticity of the polymer. Next, this elongation method was applied to the π‐electron stacking system composed of ethylene and butadiene, etc. Excellent agreement was found between the elongation method and the usual extended Hückel calculations. Thus, the elongation method can confidently be applied to π‐electron molecular crystal systems. Finally, the local density of states of polyacetylene with both cis–transoid and trans–cisoid structures was calculated. We found that the site with large local density of state in the valence band of a polymer chain gives large electron transfer to the site with large local density of state in the conduction band of another polymer chain. © 1994 John Wiley & Sons, Inc.
Effective crystal field approach to the binding energy calculation of alkaline metals
Authors: Koleżzyńnski A. , Ptak W. S. , Tkacz‐śmiech Katarzyna
Published: January 2005

A method of description of a crystalline solid is presented and discussed in the light of the variational method. The proposed method refers to the muffin‐tin cellular method but it is formulated in the language of electron density, without referring to wave‐function treatment. Model calculations of p‐dependent binding energy for alkaline metals have been performed. The results show that the proper adjustment of the effective charge of atomic core zeff parametrizing the electron density in the cell allows one to reproduce exact values of binding energy at the equilibrium distance. The numerical results prove that the dependence between the charge zeff and the radius of the cell RMT may be represented by the functión zeff = (21)1/2(1 + B exp(‐ARMT)), where A and B are numerical parameters changing with high regularity throughout the period and I denotes the first ionization energy. © 1994 John Wiley & Sons, Inc.
Electronic structure and properties of hexagonal wurtzite‐type SiC
Authors: Ruiz Eliseo , Alvarez Santiago , Alemany Pere
Published: January 2005

The periodic ab initio Hartree–Fock linear combination of atomic orbitals program CRYSTAL has been used to investigate the electronic structure of the wurtzite‐type phase of SiC. Binding energy, lattice parameters (a,c), and the internal cordinate have been optimized. Inclusion of polarization functions, which has been shown to be necessary in the description of related systems, has been used to compute the bulk modulus and its pressure derivative and the central zone A1 transverse optical phonon frequency. The electronic structure of SiC is analyzed using density of states projections and electron density maps. Results are compared with those obtained for the cubic zinc blende‐type phase. © 1994 John Wiley & Sons, Inc.
The onset of classical chaos in atom–surface scattering
Authors: Guantes R. , Borondo F. , Jaffé Charles , Miret‐artés S.
Published: January 2005

The relationship between surface rainbows and the onset of classical chaos in atom–surface scattering is discussed. The principal focus is the onset of chaos as a function of experimentally controllable parameters. The correspondence between the classical trapping and quantum mechanical selective adsorption resonances is briefly discussed. The connection with the more mathematical work of Grebogi and co‐workers is also considered. © 1994 John Wiley & Sons, Inc.
Theoretical study of many‐body effects in the photoelectron spectra of unsaturated hydrocarbons
Authors: Lisini A. , Fronzoni G. , Decleva P.
Published: January 2005

A theoretical investigation on the core and valence photoelectron spectra of unsaturated hydrocarbons of increasing complexity, ethylene, trans‐butadiene, and trans‐hexatriene, has been performed in order to analyze the role of the most important many‐body effects on the spectral features and the evolution of these effects along the series. The capability of various configuration interaction (CI) schemes to interpret both the core and the valence ionization satellite structures has been pointed out. The 3h‐2v(π)‐1p and 2h–1p CI schemes employed, together with an accurate choice of the basis set and the explicit inclusion of the relaxation, has proven to be very useful tools for this kind of study, being sufficiently accurate to explain the most important correlation features and extendible, at the same time, to longer chain systems. © 1994 John Wiley & Sons, Inc.
Investigation of the 1E states in cyclic polyenes
Authors: Bracken P. , Čížek J.
Published: January 2005

An analysis withing the one‐dimensional Hubbard model of the 1E type state in cyclic polyenes is reported. In terms of the resonance integral β, a small β analysis and asymptotic large β analysis have been carried out. Energy as a function of β has been calculated numerically, and a second state with 1E symmetry has been found. Some missing information about the higher momentum state of this symmetry is provided. © 1995 John Wiley & Sons, Inc.