bluuebunny/crossref_metadata_2025_split huggingface dataset:
HF–CC model for atoms and molecules
Authors: Clementi E. , Corongiu G. 
Published: November 2002

AbstractThe Hartree–Fock–Clementi–Corongiu method (HF–CC) is revisited, aiming at an unified formulation for post‐HF energy computations in atomic and molecular systems. For atomic systems new parameterizations of the HF–CC functional are proposed for the computation of atoms. The previous HF–CC molecular functional (Clementi, E.; Corongiu, G. Theochem 2001, 543, 39), revisited and recalibrated with a new optimization of the parameters, is tested with a sample of 131 molecules, including radicals, H‐bond, and van der Waals systems. The atomization energy is decomposed into “HF classic” energy (the sum of the HF nuclear electron, HF kinetic, and HF Coulomb energies), “HF exchange” energy, and correlation energy; the latter is computed with a scaling functional with atomic, covalent, ionic, and van der Waals contributions. For the sample of 131 molecules, the computed HF–CC atomization energies have an average standard deviation of 1.89 kcal/mol. The atomic and molecular components of the correlation energy are decomposed into nuclear electron, kinetic, Coulomb, and exchange contributions; these decompositions characterize the HF–CC model and are used to explain the origin of the chemical bond. Computations on van der Waals systems show the validity of the HF–CC method also for long‐range weak interactions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Hydrogen‐bonding effects on electronic g‐tensors of semiquinone anion radicals: Relativistic density functional investigation
Authors: Neyman Konstantin M. , Ganyushin Dmitri I. , Rinkevičius Žilvinas , Rösch Notker 
Published: November 2002

AbstractWe report results of systematic g‐tensor calculations of hydrogen‐bonded complexes of two benchmark semiquinone anion radicals, 1,4‐benzoquinone and tetramethyl‐1,4‐benzoquinone (duroquinone), with water and methanol molecules. The calculations have been carried out with the help of a recently developed g‐tensor module that is based on a relativistic density functional method that takes spin–orbit interaction self‐consistently into account. We demonstrate the applicability of this new computational scheme to describe quantitatively delicate effects of hydrogen bonding on electronic g‐tensor values. Also, we explored general trends of how g‐tensors depend on the structure and stoichiometry of hydrogen‐bonded semiquinone complexes. Complexes exhibiting one hydrogen bond per oxygen atom of the quinones with a linear arrangement of the CO … H moieties were shown to feature g‐shifts induced by these hydrogen bonds that are in close agreement with measured electron paramagnetic resonance data. Based on deviations of calculated and measured g‐components, we classify all other model complexes studied as less probable under the experimental conditions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Electronic excitations and first hyperpolarizability of 2‐methyl‐4‐nitroaniline clusters
Authors: Guillaume Maxime , Botek Edith , Champagne Benoît , Castet Frédéric , Ducasse Laurent 
Published: November 2002

AbstractThe crystal packing effects upon the static and dynamic first hyperpolarizability of 2‐methyl‐4‐nitroaniline (MNA) clusters have been addressed at the TDHF/AM1 level. Due to electrostatic interactions, in 1‐D arrays of MNA unit cells extending along the a crystallographic axis, the effective hyperpolarizability strongly increases whereas stacking unit cells along the b or c axes leads to a reduction of these quantities. These effects are magnified in the case of frequency‐dependent quantities. The first hyperpolarizability variations can mostly be related to bathochromatic and hypsochromatic shifts as well as to changes in transition dipole moments of the low‐energy excited states that dominate the UV/visible absorption spectra. From calculations performed at the CIS/INDO/S level of approximation, these dominant excited states present an important intramolecular charge transfer character and bathochromatic (hypsochromatic) shifts are associated with increase (decrease) of the transition dipole moments. In addition, a large number of intermolecular charge‐transfer excited states have been detected. In the small MNA aggregates as well as in clusters extending only along the b or c crystallographic axes, these excited states possess small transition dipole moments. On the other hand, when stacking MNA unit cells along the a axis the electrostatic interactions induce a decrease of the excitation energies and an increase of the transition dipole moments of these intermolecular charge transfer states. As a consequence, they account for a nonnegligible contribution to the first hyperpolarizability. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Designer polynomials, discrete variable representations, and the Schrödinger equation
Authors: Weatherford Charles A. , Red Eddie , Wynn Albert 
Published: November 2002

AbstractThe general procedure for constructing a set of orthonormal polynomials is given for an arbitrary positive definite weight function, w(x), in the interval [a, b]. The Lanczos method is used to generate the three‐term recursion relation, which is then used to produce the polynomial coefficients. A discrete variable representation (DVR) is constructed from Gaussian nodes and weights that result from the three‐term recursion relation. These are termed “designer polynomials” and the associated “designer DVRs.” It will be shown by construction that every such set of “synthetic polynomials” carries an associated DVR. The term “designer” derives from the fact that the interval [a, b] and the weight function w(x) are arbitrary (except that w(x) must be positive definite on [a, b] and must have continuous derivatives except at a finite number of isolated discontinuities) and may be adapted to the physical problem of interest. The difficulties of applying a DVR to a “bare” Coulomb problem will be illustrated on a “toy” model in one dimension (1‐D hydrogen atom). A solution for the 1‐D Coulomb problem will be given, thereby motivating the need for designer DVRs. In doing so, a new set of polynomials is defined with a weight function w(x) = |x|kexp(−α|x|), (such that k = −1, 0, +1, +2, …) between the symmetrical limits [−∞, +∞]. These are called “synthetic Cartesian exponential polynomials (SCEP).” These polynomials are then used in a spectral and pseudospectral (DVR) representation to solve the 1‐D hydrogen atom problem. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Influence of many‐body interactions on resistance of a grain boundary with respect to a sliding shift
Authors: Dorfman Simon , Fuks David , Malbouisson Luiz A. C. , Mundim Kleber C. , Ellis Donald E. 
Published: November 2002

AbstractWe performed nonempirical simulations of the properties of the tungsten Σ3(111) grain boundary (GB) with a boron atom and demonstrate the influence of many‐body interactions on the resistance of the GB with respect to sliding. We also studied the propagation of relaxations in the vicinity of the GB. The many‐body interatomic potentials (IP) used in these simulations were obtained with the recursion procedure from ab initio total energy calculations. At each step of the slip process, the equilibrium positions of the atoms near GB were calculated with the generalized simulated annealing technique. It was demonstrated how the sliding shift influences the penetration of the elastic field inside the grain. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Structure and relative energies of the conformers of n‐butyl cyanide and 5‐hexynenitrile
Authors: Atticks Karissa A. , Bohn Robert K. , Michels H. Harvey 
Published: November 2002

AbstractThe microwave spectrum of n‐butyl cyanide and 5‐hexynenitrile (5‐cyano‐1‐pentyne) reveal the presence of three conformers that have been characterized. Each of these molecules can exist in anti–anti (AA), anti–gauche (AG), gauche–anti (GA), gauche–gauche (GG trans), and gauche–gauche′ (GG′ cis) conformations. The first three conformers have been observed experimentally and characterized in both compounds. In previous studies, we examined the microwave spectrum of the related n‐butyl acetylene molecule and observed and characterized the mono‐13C‐isotopomers of the AA and GA forms of n‐butyl acetylene as well as the AG form, albeit at lower intensity. The GG conformers, in both cis and trans arrangements have not been identified to date for these molecules. The structures and energies of the five possible conformers of each molecule have been determined through quantum mechanical calculations at the HF and MP2 levels of theory, using both double‐zeta [6‐31G(d,p)] and triple‐zeta [6‐311+G(d,p)] basis sets. The calculated energies of the five possible conformers of each molecule are close, and a change in the relative stability order of these conformers is found at different levels of theory. For n‐butyl cyanide and n‐butyl acetylene, the GA form lies lowest in energy, both experimentally and at the MP2 level of theory. For 5‐hexynenitrile, the GG trans form is predicted to lie lowest in energy. The predicted conformer stability order is in agreement with a model of a net attractive electrostatic interaction between the π‐electron densities and the closest hydrogen on the next‐nearest‐neighbor carbon. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Substituent effects on the electronic structure and pKa of benzoic acid
Authors: Hollingsworth Chad A. , Seybold Paul G. , Hadad Christopher M. 
Published: November 2002

AbstractThe effects of substituents on the pKas of a set of 16 substituted benzoic acids have been examined using density functional theory [B3LYP/6‐311G(d, p)] calculations. A variety of quantum chemical parameters were examined as indicators for the variations observed in the experimental pKas, including the Löwdin, Mulliken, AIM, and natural population analysis charges (QL, QM, QA, and Qn) on atoms of the dissociating carboxylic acid group, the energy difference (ΔEprot) between the carboxylic acid and its conjugate base, and the Wiberg bond index. Several of these calculated quantities yielded excellent correlations with the experimental pKa: ΔHprot, r2 = 0.958; QL(H), r2 = 0.963; Qn(CO2H), r2 = 0.969; QL(CO), r2 = 0.970; and QL(CO2H), r2 = 0.978. The best correlation observed with the pKa, as expected, was from the empirical Hammett constants designed for this purpose (r2 = 0.999). © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Quadricyclane radical cation isomerization reactions: A theoretical study
Authors: Larsson Per‐Erik , Salhi‐Benachenhou Nessima , Dong Xicheng , Lunell Sten 
Published: November 2002

AbstractOn the C7H8·+ potential energy surface studied in this work, the electrocyclic isomerization of the quadricyclane (Q·+) to the norbornadiene (N·+) radical cations has attracted much attention. Our interest, however, concerns alternative rearrangement reactions on this surface. Using quantum chemical methods, B3LYP/6‐311+G(d,p), MP2/6‐31G(d,p), and single‐point CCSD(T)/6‐311+G(d,p), a skeletal rearrangement path leading from Q·+ to the bicyclo[3,2,0]hepta‐2,6‐diene radical cation (BHD·+) is found. The barrier for this path is 4.9 kcal/mol higher at the CCSD(T)/6‐311+G(d,p)//B3LYP/6‐311+G(d,p) level than the Q·+ to N·+ barrier. Moreover, this rearrangement is found to proceed through an intermediate, bicyclo[2.2.1]hepta‐2‐ene cation (BHE·+), with a conversion barrier to BHD·+ of 8.1 kcal/mol while the barrier for conversion back to Q·+ is 12.8 kcal/mol, computed with CCSD(T)/6‐311+G(d,p)//B3LYP/6‐311+G(d,p). Interestingly, the 1H hyperfine coupling constants computed for this intermediate structure show an excellent agreement with the experiment by Williams and coworkers. This ESR spectrum, observed after irradiation of N and Q loaded CF3CCl3 matrices, was assigned to BHD·+; our calculations, however, show that it is BHE·+ that gives rise to the observed spectrum. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Treatment of shape and Auger resonances using the dilated electron propagator
Authors: Mishra Manoj K. , Venkatnathan Arun 
Published: November 2002

AbstractAn overview of the dilated electron propagator method based on operators defined on biorthogonal orbitals from bivariationally obtained complex scaled SCF procedure is presented and discussed. Results from applications to atomic and molecular electron attachment and detachment resonances using different decouplings of the dilated electron propagator are analyzed to adduce the role of correlation and relaxation effects in the formation and decay of shape and Auger resonances. Feynman–Dyson amplitudes of the dilated electron propagator are examined to try and unravel the mechanistic underpinnings of these resonances. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
S‐matrix expansions in view of complex dilation theories
Authors: Alferova T. , Elander N. 
Published: November 2002

AbstractIn this article, we use the complex dilation formalism to compute S‐matrix residues for a number of complex Schrödinger‐type eigenvalues in a limited spectral region. These residues are then used as expansion parameters for the corresponding S‐matrix. The aim of this work is to find numerically stable algorithms that can be applied to problems based on realistic only numerically represented potentials. Two formalisms are described. In the first, we compute the Jost functions ℱ+(k) and ℱ−(k) as well as the derivative dℱ/dk numerically at the eigenvalue kν. Using the second method, we generalize a formula described by Newton [(Scattering Theory of Waves and Particles; McGraw‐Hill; New York, 1966) for bound states to resonant eigenvalues. Here, the S‐matrix residue is obtained by integrating the resonant eigenfunction in the complex coordinate space. Our formalism is tested on an exactly solvable problem (Bargmann, V. Phys Rev 1949, 75, 301–303) and a classic complex dilation potential (Bain, R. A. et al. J Phys B Atom Mol Phys 1974, 7, 2189–2202). © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Classical trajectory‐based approaches to solving the quantum Liouville equation
Authors: Donoso Arnaldo , Martens Craig C. 
Published: November 2002

AbstractThe time‐dependent quantum mechanics of heavy particles moving on a single potential energy surface can often be represented surprisingly well by the evolution of classical trajectory ensembles. However, manifestly quantum mechanical phenomena—such as transitions between coupled electronic states, electronic coherence and its decay, or quantum mechanical tunneling—require fundamental modification of the purely classical motion. We introduced and developed an approach to this problem that is based on solving the quantum Liouville equation using ensembles of classical trajectories. In this article, we describe the general approach and its application to the problems of nonadiabatic dynamics, coherent multistate electronic–nuclear dynamics, and tunneling through potential barriers. When viewed from the trajectory ensemble perspective, quantum effects arise as a breakdown of the statistical independence of the trajectories in the ensemble and a nonlocal entanglement of their collective evolution. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Symmetry anomaly in disubstituted benzenes
Authors: André J.‐M. , Brédas J. L. , Coropceanu V. , Silbey R. J. 
Published: November 2002

AbstractA striking feature in disubstituted benzenes is that the well‐known degeneracy scheme of the π levels of the unperturbed benzene ring is conserved when donor and acceptor groups of the same strength are introduced in para positions (antisymmetrical substitution). This degeneracy is lifted when substituting with two identical donor or acceptor groups (symmetrical substitution). In the antisymmetrical magnified image substitution, even though the perturbation affects only one of the two degenerate π levels, a compensation effect has the result that the degeneracy of the π levels is not only conserved but their energies remain unchanged; thus, the perturbation only affects the energy of the lowest occupied π level and that of the highest unoccupied π level. The same feature is observed for other even annulenes. This symmetry anomaly is analyzed by various methods. The connection with Berry's phase problem is also discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Electron binding energies of nucleobases and nucleotides
Authors: Dolgounitcheva O. , Zakrzewski V. G. , Ortiz J. V. 
Published: November 2002

AbstractElectron propagator theory is applied to the calculation and interpretation of electron binding energies of molecular fragments found in nucleic acids. This theory provides a systematic approach to correlation problems in quantum chemistry while retaining a one‐electron picture of electronic structure. Photoelectron spectra for the purine and pyrimidine bases found in nucleic acids have been assigned with the aid of calculations that employ an electron propagator approximation that has been developed for the study of large molecules. This method, the partial third‐order approximation (P3), has been shown to generate reliable electron binding energies for closed‐shell molecules and anions. Correlation corrections to canonical, Hartree–Fock orbital energies are necessary for assignment of cationic states. Electron detachment energies for H2PO also have been assigned successfully with the P3 method. These results indicate that the P3 method is a suitable procedure for predicting electron binding energies of nucleotide anions. Correlation effects are important in determining the ordering of final states with holes localized on the phosphate or the base regions in the nucleotide. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Using 1,3‐butadiene and 1,3,5‐hexatriene to model the cis‐trans isomerization of retinal, the chromophore in the visual pigment rhodopsin
Authors: Blomgren Fredrik , Larsson Sven 
Published: November 2002

AbstractThe short polyenes 1,3‐butadiene and 1,3,5‐hexatriene are used to model the cis‐trans isomerization of the protonated Schiff base of retinal (PSBR) in rhodopsin (Rh). We employed the complete active space self‐consistent field (CASSCF) method for calculation of the potential energy surfaces (PESs) in C2 symmetry. In the calculations, the central bond was twisted from 0 to 180° in the first singly excited singlet state (Sse), i.e., the state dominated by a configuration with one electron excited from HOMO to LUMO. It was found that the PES of 1,3‐butadiene has a maximum whereas the PES of 1,3,5‐hexatriene has a minimum for a twist angle of 90°. This is explained by a shift in border of single and double bonds in the Sse state. The first step in the cis‐trans isomerization of PSBR, which is the formation of the C6C7 (see Scheme 1 for numbering) twisted PSBR in the first excited singlet state (S1), inside the protein binding pocket of the visual pigment Rh is modeled using crystal coordinates and the calculations performed on 1,3‐butadiene and 1,3,5‐hexatriene. More specifically, a plausible approximate structure is calculated in a geometric way for the C6C7 90° twisted PSBR, which fits into the protein binding pocket in the best possible way. It has been shown earlier that PSBR has an energy minimum for this angle in S1. The CASSCF method was used to investigate the wave function of the calculated structure of PSBR. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Nonsymmetrical perturbation theory for improving coupled‐cluster wave functions
Authors: Surján Péter R. , Szabados Ágnes , Szekeres Zsolt 
Published: November 2002

AbstractA general nonsymmetrical formulation of multireference perturbation theory (PT) is used to correct the energy and wave function obtained from limited coupled‐cluster (CC) calculations. Following the spirit of CC theory, we use the Hartree–Fock wave function in bras while the CC wave function is used as the ket reference. The resulting PT is compared to the symmetrical (Hermitean) formulation and to the systematic CC(n)PT(m) theory of Hirata et al. Numerical results up to the fifth order in PT are obtained in a determinantal formalism for a few sample molecules. The effect of partitioning is also studied. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Thermodynamics and kinetics of downhill protein folding investigated with a simple statistical mechanical model
Authors: Muñoz Victor 
Published: November 2002

AbstractThe possibility of downhill protein folding is one of the most intriguing predictions of the energy landscape approach. Finding examples of downhill folding has important practical implications because in the absence of a free energy barrier the mechanism of folding is amenable to experimental observation. Here, a simple statistical mechanical model of protein folding is used to analyze the thermodynamic and kinetic properties of downhill and two‐state folding. Folding free energy surfaces with and without a barrier separating denatured and native states can be generated with this model by simply modifying the curvature of the energy as a function of the order parameter, i.e., number of amino acids in incorrect conformation. Thermodynamic and kinetic analysis of surfaces with a barrier show the typical properties of two‐state folding: a first‐order equilibrium unfolding transition and exponential biphasic folding kinetics. In contrast, in downhill surfaces the unfolding process is noncooperative, and the apparent transition depends on the structural probe used to measure it. Therefore, this inherent behavior can be used as an experimental criterion for the identification of downhill folding. Surprisingly, the kinetics of downhill folding do not greatly deviate from exponential behavior unless the effective diffusion coefficient is assumed to decrese as folding progresses, exemplifying the increase in the roughness of the landscape. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Molecular relaxation and metalloenzyme active site modeling
Authors: Whittaker James W. 
Published: November 2002

AbstractMetalloenzymes represent a broad class of important biomolecules containing an essential metal ion cofactor in their catalytic active sites, forming biologic metal complexes that perform a wide range of important functions: activation of small molecules (O2, N2, H2, CO), atom transfer chemistry, and the control of oxidation equivalents. The structures of many metalloenzyme active sites have been defined by X‐ray crystallography, revealing transition metal ions in unique low‐symmetry environments. These bioinorganic complexes present significant challenges for computational studies aimed at going beyond crystal structures to develop a detailed understanding of the catalytic mechanisms. Considerable progress has been made in the theoretical characterization of these sites in recent years, supported by the availability of efficient computational tools, in particular density functional methods. However, the ultimate success of a theoretical model depends on a number of factors independent of the specific computational method used, including the quality of the initial structural data, the identification of important environmental perturbations and constraints, and experimental validation of theoretical predictions. We explore these issues in detail and illustrate the effects of molecular relaxation in calculations of two metalloenzymes, manganese superoxide dismutase and galactose oxidase. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002