bluuebunny/crossref_metadata_2025_split huggingface dataset:
Recent computational developments with the self‐consistent electron pairs method and application to the stability of glycine conformers
Authors: Dykstra Clifford E. , Chiles Richard A. , Garrett Michael D. 
Published: December 2004

AbstractSignificant improvements in calculational efficiency and capability with the self‐consistent electron pairs (SCEP) method has resulted from several new computational developments. A new procedure for constructing the important internal Coulomb and exchange operators has substantially reduced the preiteration time. A general scheme for utilizing molecular symmetry has been used to advantage in reducing the number of pair functions and external operators that must be found explicitly at each iteration. A projection operator tool has been implemented and found to be quite effective at minimizing the number of iterations required at some point on a potential energy surface when an SCEP wavefunction exists for some nearby point. These and other improvements in the program construction have yielded sizable reductions in time for some representative test cases, including water and a potential energy curve for formaldehyde. The new SCEP program also performs low‐order perturbation theory treatments and coupled electron pair approximation (CEPA) calculations using the same operator approach. The usefulness of the approach is demonstrated by very large scale calculations on the stability of the two interstellar glycine conformers. These calculations involve the variational treatment of 82,205 symmetry‐adapted singly and doubly substituted configurations involving 225 internal electron pairs.
A reassessment of some restricted Hartree–Fock limit molecular energies and an investigation of the applicability of Ermler and Kern's procedure for their estimation
Authors: George Philip , Trachtman Mendel , Bock Charles W. 
Published: December 2004

AbstractNew estimates of Hartree–Fock limit energies (ERHF) for selected AH and AHn hydrides, diatomic and linear polyatomic molecules have been made utilizing ESCF values recently reported in the literature for HF, N2, CO, NH3, and CH4 which are very close to the respective limits. These new values have been used to investigate the applicability of Ermler and Kern's procedure for estimating ERHF: i.e., a factor f is first evaluated from data for reference molecules, where f = ERHF/ESCF, which is then used with ESCF values for other molecules to obtain their ERHF values. f has been evaluated for three groups of reference moleculesHF, H2O, NH3, CH4, N2, and CO; CH4, C2H2, C2H4, and C2H6; and C2H2, HCN, and N2utilizing ESCF data in the literature for many Gaussian‐type orbital (GTO) basis sets together with some new values calculated at the (9,5,1) to (13,8,2) levels. Trends in the variation of f within each group of reference molecules from one basis set to another, and the trends in f from one group of reference molecules to another, are discussed in detail. To minimize the influence of these effects in an ERHF estimate it is recommended that the f value should be derived from reference molecules which possess a similar combination of structural features, i.e., bonded hydrogen, single, double, or triple bonds, and the number of lone‐pair electrons. Further calculations show that an f value based on data for closed‐shell molecules is not applicable to open‐shell species.
On computation of the topological resonance energy
Authors: Mohar B. , Trinajstić N. 
Published: December 2004

AbstractThe topological resonance energy (TRE) model was introduced independently by the Zagreb Group (in 1975) and by Aihara (in 1976). Several practical obstacles arise in the computation of TRE. Ways to surmount them are pointed out. Descriptions of algorithms, optimizations, and the efficient computer program are presented. Possible numerical instability is discussed.
Ab initio studies of structural features not easily amenable to experiment. 17. The molecular structures of some strained cyclic hydrocarbons and estimates of their strain energies
Authors: Van Alsenoy C. , Scarsdale J. N. , Schäfer Lothar 
Published: December 2004

AbstractThe molecular structures of tricyclohexane, norbornane, quadricyclane, and cubane were completely refined by standard ab initio force relaxation on the 4‐21G level. The results can be used as a basis to interpret some contradictory experimental reports found in the literature and to establish some hitherto unobserved structural trends involving the CC and CH bond distances of the cases studied. Group Delta;E values for C(H)3(C), C(H)2(C)2, and C(H)(C)3 (Benson's notation) derived from the total energies of the completely relaxed 4‐21G geometries of a number of unstrained hydrocarbons are also listed. The values are used to estimate the strain energies of the systems studied and of the optimized 4‐21G geometries of cyclopropane, cyclobutane, cyclohexane, and bicyclo (2.1.0)pentane. Cooperative effects in the strain energies are discussed.
An Ab initio study on the conformations of protonated, neutral, and deprotonated amidine
Authors: Zielinski Theresa Julia , Peterson Michael R. , Csizmadia Imre G. , Rein Robert 
Published: December 2004

AbstractAb initio SCF molecular orbital calculations have been performed to ascertain the conformational preferences of protonated, neutral, and deprotonated amidine [HC(NH)NH2], using the 3‐21G split valence basis set. The states of eight stable species, eight transition states, and four higher‐order saddle points have been determined by complete geometry optimization utilizing analytic energy gradient techniques. Protonation at the amidine NH is preferred over the –NH2 site by 37.1 kcal/mol. Neutral amidine has rotational barriers of 9.6 and 11.7 kcal/mol for the HNCN cis and trans isomers, respectively, while all the stable HC(NH2)2+ and HC(NH)2− species possess torsional barriers larger than 23 kcal/mol. There is, however, essentially free C—N single‐bond rotation in HC(NH)NH3+, the calculated barriers being 0.7 and 1.8 kcal/mol for the cis and trans HNCN isomers, respectively.
Computer generation of nuclear spin species and nuclear spin statistical weights
Authors: Balasubramanian K. 
Published: December 2004

AbstractThis article develops computer programs for computer generation of nuclear spin species and nuclear spin statistical weights of rovibronic levels. The programs developed here generate nuclear spin species and statistical weights from the group structures known as generalized character cycle indices (GCCIs) which are computed easily from the character table of the PI group of the molecule under consideration. Procedures are illustrated with examples.
The conformation of malformin A
Authors: Hall David , Lyons Paul John , Pavitt Nicola , Trezise John A. 
Published: December 2004

AbstractThe conformation of the cyclic pentapeptide malformin A has been deduced by systematically generating possible models for the molecule, and minimizing the conformation energy of each. Only one of the low‐energy solutions is fully consistent with the reported CD and NMR spectra, and this is the proposed model. The disulfide ring linking adjacent cystine residues is highly strained, as has been predicted from the SS vibration frequency. The conformation of the backbone, but not that of the disulfide ring, is similar to a model previously proposed.
Graph theoretical classification and coding of chemical reactions with a linear mechanism
Authors: Bonchev D. , Kamenski D. , Temkin O. N. 
Published: December 2004

AbstractLinear mechanisms of catalytic and noncatalytic chemical reactions which are theoretically feasible have been classified and coded using a detailed procedure for the unique numbering of cycles, edges, and vertices in the kinetic graphs. The following classification criteria are used in a hierarchical order: number of cycles and vertices, mutual connectivity of the cycles, manner of linking any pair of cycles, number of elements linking two cycles, mutual position of two cycles joined to a third one, orientation of edges, and presence of pendant vertices. All the types and classes of mechanisms are presented for reactions having up to five and four routes, respectively.
A quantum mechanical test of diophantine methods
Authors: Burdick S. A. 
Published: December 2004

AbstractA blend of Haselgrove's method and the biased selection method for evaluating multidimensional integrals was tested. The results were mixed. The error estimate varied from being proportional to 1/N when N was less than ca. 60,000 to being proportional to 1/√N when N was greater than 60,000. Also, for N greater than 60,000, the error estimate was one‐half the error estimate given by biased selection alone. These numbers should be compared with the 10,000 points used to find an optimum set of Haselgrove's parameters. It is reasonable to expect that if 100,000 points were used in the optimization of Haselgrove's parameters that the above results would be found with 60,000 replaced by 600,000.
Spherically symmetric axial Gaussian lobe 3d and 4f orbitals
Authors: Shillady Donald D. , Talley David B. 
Published: December 2004

AbstractThe axial Gaussian lobe orbital (AGLO) representations of 3d and 4f orbitals proposed by LeRouzo and Silvi have been angularly optimized to ensure spherical symmetry of filled 3d and 4f shells. The functions have been tested on the hydrogen atom in the presence of high quality s and p basis sets and found to provide excellent minimal Gaussian representations of polarization functions. Exact orbital degeneracy is not obtained within each shell, however. Tabulated values are given to allow arbitrary scaling of the 3d and 4f lobe mimic orbitals.
Koopmans' defects in metal group VIII carbonyls—a many‐body approach
Authors: Böhm Michael C. , Gleiter Rolf 
Published: December 2004

AbstractThe ionization potentials of iron pentacarbonyl (1), ethylene–iron tetracarbonyl (2), cobalt tetracarbonylhydride (3), and nickel tetracarbonyl (4) have been calculated using a Green's function perturbation method based on the INDO approximation. It is shown that the deviations from Koopmans' theorem are largest in the Co complex, while the smallest reorganization energies of strongly localized MOs with predominant metal 3d character are found in the Fe carbonyls 1 and 2. The calculated Koopmans' defects are analyzed by an investigation of the relaxation terms of the self‐energy part and are compared with previous INDO results for Cr, Mn, and Fe tricarbonyl derivatives. Additionally, orbital energies, bond indices, and net charges for the ground states of 1–4 are calculated and compared with experimental data.
MINDO/3 Calculation of carbocation heats of formation
Authors: Harris J. Milton , Shafer Steven G. , Worley S. D. 
Published: December 2004

AbstractHeats of formation calculated by MINDO/3 are reported for 42 carbocations for which experimental heats of formation have been published. Errors associated with these calculations can be large, with an overall range of ±13 kcal/mol. Correction of systematic errors in the MINDO/3 calculations by means of hydrocarbon models and isodesmic relationships results in a reduction in the range of errors to ±8 kcal/mol. Comparison with experimental heats of reaction of hydride transfer equilibria minimizes experimental errors and gives an average absolute error of 2 kcal/mol with a range of ±3 kcal/mol.
Studies in hydrogen bonding: Association within small clusters of water, methanol, and ammonia molecules using Jorgensen's intermolecular pair potentials
Authors: Brink George , Glasser Leslie 
Published: December 2004

AbstractThe geometries of the dimer, trimer, and tetramer hydrogen‐bonded clusters of water, methanol, and ammonia molecules have been derived using previously published intermolecular pair potentials containing constants optimized from ab initio calculations. The lowest energy forms for the dimers of all three types of molecules have an open structure, while the trimers and tetramers have cyclic structures. The results are compared with those previously described using another empirical potential, EPEN.
A semiempirical formulation for the study of molecular interactions
Authors: Fraga Serafin 
Published: December 2004

AbstractThe coefficients of a 1/R expansion (containing long‐range electrostatic interaction, medium‐range electrostatic‐induced dipole attraction, short‐range dispersive attraction, and very short overlap interaction terms) have been determined, within a semiempirical approach, from the results of a priori calculations. The formulation has been applied to the study of the stable conformations of benzene complexes.
Spin–orbit and dispersion energy effects in XeF
Authors: Krauss M. , Stevens W. J. , Julienne P. S. 
Published: December 2004

AbstractSpin–orbit and dispersion energy contributions to the energy curves of XeF are examined. A rapid variation in the spin–orbit coupling with internuclear separation is found for both the ground and excited states. This result can explain the experimentally observed ordering of the ionic excited states when the spin–orbit perturbation couples 2σ and 2π energy curves obtained by both all‐electron and effective core potential (ECP) calculations at the first‐order configuration interaction (FOCI) level of accuracy. Damped dispersion energy contributions to the ground‐state energy curve are shown to be comparable to the charge transfer contribution. The energy curve for XeF is in reasonable agreement with experimental results and a calculation of the analogous XeCl curve confirms the qualitative correctness of the calculation. The energy curves and transition moments were then applied to two problems related to the efficiency of the XeF laser. Photodissociation of the X state provides a means of removing a bottlenecked vibrational level but a calculation of the radiative transition probability between the X and A states finds the cross section is too small to yield rates competitive with collisional deactivation. The bottlenecked state may also be removed by electron dissociative attachment but the calculated energy curves for the X states of XeF and XeF− do not cross at a low energy indicating a small cross section.
Strain energies of some bridgehead olefins as calculated with the MM2 force field
Authors: Warner Philip M. , Peacock Stephen 
Published: December 2004

AbstractThe MM2 force field has been used to calculate the strain energies of a representative series of bridgehead alkenes, and the results compared to similar previous calculations using the MM1 force field. The new results roughly parallel the old ones, although the strain energy of a given compound is normally higher with MM2 than MM1. These differences are largely negated, however, when the OS (olefinic strain) value is considered.
Orbital topology. III. Orbital mapping of unsymmetrical molecules. A survey of the thermal ring opening of isoelectronically substituted cyclobutenes and benzocyclobutenes
Authors: Kelsey Donald R. 
Published: December 2004

AbstractOrbital mapping analysis based on CNDO/2 molecular orbitals has been used to survey the thermal ring‐opening isomerizations of cyclobutenes and benzocyclobutenes. Isoelectronic substitutions within the molecular framework of cyclobutene (e.g., CH2 replaced by CH−, OH+, NH, NH2+) result in ground‐state orbital correlations via both conrotatory and disrotatory pathways in several cases, in contrast to the parent hydrocarbon conrotatory stereochemistry. The results substantiate the heteroatom effects previously revealed by orbital mapping for the disrotatory thermal isomerizations of isoelectronic Dewar benzenes. Qualitative patterns, such as nodal shifts in the butadiene π orbital, are discussed in relation to the mapping correlations. The isoelectronic benzocyclobutenes give ground‐state orbital correlations via conrotatory pathways only, which suggests that delocalization may reduce the heteroatom perturbation.
The structure and stability of the acetylene dication
Authors: Pople John A. , Frisch Michael J. , Raghavachari Krishnan , Schleyer Paul V. R. 
Published: December 2004

AbstractThe dication C2H has been investigated by ab initio molecular orbital theory. It is found to have a linear (D∞h), structure with a triplet (3σ−g) ground state. Deprotonation to C2H+ is exothermic by 9.8 kcal/mol, but this process is hindered by a large barrier of 65 kcal/mol.
Acceleration of convergence in Monte Carlo simulations of aqueous solutions using the metropolis algorithm. Hydrophobic hydration of methane
Authors: Kincaid Robert H. , Scheraga Harold A. 
Published: December 2004

AbstractConvergence problems encountered in the computer simulations of aqueous solutions are discussed. Solute–solvent radial distribution functions are shown to converge very poorly when the standard Metropolis Monte Carlo procedure is applied. To overcome this difficulty, several modifications are made in the Metropolis method. Optimum maximum step sizes are determined for simulations of liquid water. A scheme is employed for preferential sampling of both the solvent and the solute molecules. To test these modifications, a simulation is carried out for pure liquid water, treating a single water molecule as a “solute.” The convergence of the radial distribution functions is found to be accelerated significantly. A further test is made by simulating an aqueous solution of methane, consisting of one methane molecule (using the EPEN/2 potential for methane–water interactions) and 124 water molecules (using the MCY potential for water–water interactions). Again, the convergence of solute–solvent radial distribution functions is found to be accelerated. The computation of partial molar thermodynamic quantities, however, still suffers from convergence difficulties. This problem is discussed in detail. The EPEN/2 potential is found to yield structural and thermodynamic features of hydrophobic hydration that are consistent with available experimental and theoretical results for aqueous solutions of methane.
Substituent effects on the charge density in the formyl group
Authors: Eisenstein M. , Hirshfeld F. L. 
Published: December 2004

AbstractSCF wavefunctions with similar extended Gaussian bases for the series HCOX with X = H, OH (syn and anti), NH2, CN, and F yield closely similar charge deformation density maps in the formyl region of all six molecules. The differences, measured by moments of partitioned atomic deformation densities, correlate almost linearly with the Hammett substituent parameters σI and σR of the several substituents X. However, systematic deviations, especially in the carbon fragment, suggest the need for modified values of the inductive parameters σI for the correlation of these molecular charge densities.
Restricted Hartree–Fock approximation. I. Techniques for the energy minimization
Authors: Rico J. Fernández , De La Vega J. M. Garcia , Alonso J. I. Fernández , Fantucci P. 
Published: December 2004

AbstractWe present a new procedure for direct minimization of the RHF energy, which presents advantages over SCF methods with respect to convergence rate and computational cost. In this procedure we combine several techniques with the aim of obtaining best directions and step lengths for the iterative search for a minimum of the energy. In this article we develop the theory. Therefore, we analyze the variational function; we present a short description of the minimization techniques and we discuss in detail the way in which they are to be used. The computational aspects of the procedure will be treated in the following article.
Dipole polarizability derivatives calculated by the Kirkwood method
Authors: Bishop David M. , Cartier Alain 
Published: December 2004

AbstractDerivatives, with respect to the symmetry coordinates, of the components of the dipole polarizabilities of HF, H2O, and CH4 are calculated using the Kirkwood method and a generalization of that method. The results for the bond‐stretching derivatives are in reasonable agreement with those found by finite field calculations; those for angle changes are of the correct sign, but their magnitudes are in poor agreement with the finite field ones. The results are also analyzed using a four‐parameter bond polarizability model.
Energetics and structure of the hydrated gaseous halide anions
Authors: Gowda B. Thimme , Benson Sidney W. 
Published: December 2004

AbstractEnergetics and geometries for the hydrated gaseous halide anions have been computed from a simple model in which the molecular dipole of water was composed of two parts, one due to a lone pair on oxygen (60%) and the rest to formal charges on the nuclei. The calculations were made for both the symmetric and nonsymmetric structures. A variety of structures were used to compute potential energies and distances with up to six water molecules. The total energy consisted of a sum of electrostatic, polarization, dispersion, and repulsion terms. Various sets of repulsive potential parameters, ranging from those determined from molecular beam experiments to those determined using experimental ion–water distances or energies, have been employed to compute repulsive interaction energies. It was found that the range parameters play a significant role in deciding the magnitudes of the distances and energies, as the latter are most sensitive to them. It was also shown that with a simple correlation scheme the consistency of the experimental energies and distances can be tested separately without using repulsive potential parameters from other sources. It also suggests that a range of parameters can be used to compute repulsion energies. Despite the fact that the model is greatly simplified, the agreement of both the predicted ion‐oxygen distances and energies with both experiment and other calculations is excellent. A detailed analysis of our calculation suggests that the negative ion clusters with one to three water molecules contain symmetric orientation of water molecules, while those with more than three may contain asymmetric orientations of water molecules or a mixture of both. From the log–log plots of hydration energies versus (R + radius of water molecule), we have proposed empirical expressions of the type ΔEn−1,n = 10·0x (R + 1.38)−y with both Pauling's and Ladd's radii for univalent ions with which stepwise hydration energies of the latter can be predicted if we know thier radii. The values predicted for the alkali cations are in excellent agreement with the experimental and theoretical values, indicating the consistency of the simple model.
Computer generation of the symmetry elements of nonrigid molecules
Authors: Balasubramanian K. 
Published: December 2004

AbstractAlgorithms and computer programs are developed for generating the symmetry elements of nonrigid molecules. These programs are based on the wreath product formalism for the symmetry groups of nonrigid molecules developed by the present author. Several examples are given to illustrate the procedures. Applications to weakly bound van der Waal complexes synthesized by supersonic beam expansion are also presented.