bluuebunny/crossref_metadata_2025 huggingface dataset:
Applicability of multi‐reference many‐body perturbation theory to the determination of potential energy surfaces: A model study
Authors: Zarrabian S. , Paldus J.
Published: January 2005

The recently written CI‐based multi‐reference many‐body perturbation theory (MR‐MBPT) program package is exploited to study a simple ab initio minimum basis set model involving four hydrogen atoms in a rectangular configuration. This model was examined earlier by several authors using both coupled cluster (CC) and finite‐order MBPT approaches. Here we present the MR‐MBPT results up to the 50th order and examine the effect of various shifting techniques on the convergence behavior of this approach. It is shown that in contrast with CC methods, both single and MR finite‐order MBPT potential energy calculations are plagued with convergency and intruder state problems, which can be particularly severe when the latter approach is employed for non‐degenerate situations.
Approximating quasi‐particle density functional calculations of small active clusters: Strong electron correlation effects
Authors: Beznosjuk S. A. , Minaev B. F. , Dajanov R. D. , Muldakhmetov Z. M.
Published: January 2005

A new constructive approach for deriving a quantum field chemistry (QFC) is proposed. As a matter of fact, the approach is a direct application of the concept of spontaneously broken symmetry of a free‐electron‐field vacuum to the exact definition of a condensed‐state chemical microstructure. The main idea is to identify the properly modified ground states of the vacuum with ground states of some compact quasi‐molecular systems condensed in a set of “kink”‐bounded molecular {vα} subspaces. Phase transitions of the electron vacuum are as usually expressed in terms of quantum order parameter set {ϕα1(x) = [ρ(x)]1/2 exp(iλα)}, which defines a single electron densities ρα1(x)=nα(x)/Nα (here Nα=∫nα(x) dx) in exact molecule ground states ρα1(x)=|α1(x)|2. The order parameters are obtained by the self‐consistent procedure of minimizing the ground‐state energy Eα=Nαϵα−1(N) for each open molecular “compacton” with respect to the number of electrons Nα and an average single‐electron energy ϵα−1(Nα). Account is taken of topological definitions of the various molecular constituents: atoms, atomic functional groups, molecules, and their clusters. The stability of diverse clusters is investigated by the method of approximating quasi‐particle density functional (AQDF). It gives particular attention to the description of peculiar intermolecular clusters (Mn) composed with single‐atom molecules (M1=A). Such clusters may be used to simulate some active centers that bear the responsibility for strong effects of nonlinearity and dissipation in condensed states. Some results of AQDF‐calculation of small active Rh‐clusters (n=2, 3, 4) are taken under consideration to illustrate that such clusters resemble neither fragments of bulk solids nor molecules in a gas.
The virial theorem scaling model for estimating the charge sensitivities of hydrogens in molecules
Authors: Streszewski Marcin , Nalewajski Roman F.
Published: January 2005

A simple variational model of hydrogens in molecules is presented, using the virial theorem (Fock) scaling of the wave function to account for the orbital relaxation due to a change in the number of electrons. The resulting interpolative formulas for the energy allow realistic predictions of the spin‐nonpolarized or spin‐polarized hardness (electron repulsion) parameters and other sensitivities of the hydrogen systems in molecules, including the realistic bare nuclei limit (N → 0) data.
The intermolecular vibrations of the bifurcated water dimer: An Ab initio study
Authors: Muguet F. F. , Robinson G. W. , Bassez‐Muguet M.‐P.
Published: January 2005

The intermolecular modes of the bifurcated water dimer are determined at the HF level using an extended basis set. In these computations, the donor libration frequency is found to be real and the bifurcated structure does not collapse toward the linear dimer. This result is contrary to all previous ab initio computations, which have predicted a Hessian with one negative eigenvalue. A good representation of other intermolecular modes, such as the libration of the acceptor, also requires an extended basis set. An interesting infrared active transition is predicted around 444 cm−1. This transition, which corresponds to the donor wag, is found in the low‐temperature spectrum of water in a N2 matrix.
String model for the rate constant of nonadiabatic solvation in the hydration reaction of carbon dioxide
Authors: Tachibana Akitomo , Fueno Hiroyuki , Tanaka Eishi , Murashima Masatoshi , Koizumi Masahiko , Yamabe Tokio
Published: January 2005

Perturbation theory for the rate constant of nonadiabatic solvation is presented. Miller's treatment of the RRKM theory is used for the unperturbed rate constant, and thereby the “string model” of chemical reaction is used for the description of the perturbation of solvation. The reaction path under the influence of the external force field is defined as the intrinsic reaction coordinate (IRC) that is treated as a string. The string is thrown in the external static force field that acts as a nonadiabatic source of perturbation. As an application of the present treatment, the effect of the external point charge for hydration reaction of CO2 is calculated as a preliminary model for catalytic activity of the carbonic anhydrase in the tissues and lung capillary vessels.
Aromaticity measured by Kekulé structures
Authors: Hall George G.
Published: January 2005

The total energies, in the Hückel theory, of a large number of polyhex molecules and some containing a four‐membered ring have been calculated and analyzed. The results confirm that within a set of isomers the number of Kekulé structures is the dominant variable. It is proposed that this term should be recognized as the aromatic term. The remaining contributions to the energy can be attributed to local features of the molecule. Comparison with the Hess–Schaad analysis of the same energies indicates that this treatment is simpler, more complete, and closer to the original concept of aromaticity.
Modified virtual orbitals (MVO) in limited CI calculations
Authors: Wasilewski J.
Published: January 2005

A modification of virtual orbitals seems to be an inexpensive way of calculating a large part of the correlation energy. Three types of MVOS have been compared in this paper, on the basis of limited valence‐shell CI calculations for some lower states of the H2O molecule: A — orthogonalized eigenfunctions of exchange operators; B — eigenfunctions of one combined exchange operator; C — eigenfunctions of the Fock operator for a highly ionized system. For small virtual spaces (20% of available orbitals used), approach C gives 70% of EcorrCI‐SD, being superior to the two other approaches. When using only 50% of MVOS, all the three types of approaches result in calculating 90% of EcorrCI‐SD, but the ionic orbitals give a better description of the state separations.
Single‐excitations‐adapted molecular orbitals for a simplified description of electronic excited states
Authors: Cioslowski J.
Published: January 2005

An orbital transformation that minimizes the sum of the squares of the off‐diagonal Hamiltonian elements between the singly excited configurations is described. The resulting single‐excitations‐adapted molecular orbitals (SEAMOS) are found to be very suitable for the description of electronic excited states. The test calculations on the singlet and triplet excited states of the N2, CO, and H2CO molecules are presented.
A calculation of the isotropic and anisotropic spectral moments of the dipole oscillator strength distribution of N2
Authors: Diercksen G. H. F. , Oddershede J. , Paidarova I. , Sabin J. R.
Published: January 2005

We calculate a range of isotropic and anisotropic spectral moments of the dipole oscillator strength distribution for N2 in the random phase approximation. The internuclear dependence of, in particular, S∥(k) is nonnegligible and vibrationally averaged moments are reported. The results are compared to other calculations and to experiment when available, and we conclude that the scheme gives reliable results. Based on our results, we predict a 10% anisotropy in the stopping power of oriented N2 molecules.
MNDO calculations on borazine derivatives. The substitution of one [HNBH] fragment for one [HCCH] fragment in benzene to form the azaborines and the nature of the cyclotrimer of the 1,2‐isomer
Authors: Massey Scott T. , Zoellner Robert W.
Published: January 2005

The three azaborine isomers with the formula C4H6BN, 1,2‐, 1,4‐, and 1,3‐azaborine (I, II, and III), have been examined using MNDO (modified neglect of diatomic overlap) calculations. The most stable azaborine was I (heat of formation ‐8.147 kcal/mol), followed by II (+11.60 kcal/mol) and III (+16.64 kcal/mol). Qualitatively, although the π‐ and π*‐orbitals calculated for the azaborines exhibited an ordering similar to that in benzene and borazine, the HOMO/LUMO energy differences (9.27, 9.68, and 8.44 eV, respectively) were smaller than was the difference calculated for borazine (12.81 eV), but of the same magnitude as the difference for benzene (9.76 eV). With the exception of borazine, each molecule had a π‐orbital for the HOMO and a π*‐orbital for the LUMO; borazine's LUMO was a π*‐orbital. The calculated shapes and atomic contributions for the π‐and π*‐orbitals of the azaborines were best described as “hybrids” of the π‐ and π*‐orbitals of benzene and borazine. As was observed for the π‐ and π*‐orbitals of borazine, the azaborines exhibited increased orbital density at the nitrogen atom in the π‐bonding orbitals and at boron in the π‐antibonding orbitals, as would be predicted from electronegativity considerations. Although I and II exhibited significant double‐ and single‐bond localization, all of the ring bonds in III were delocalized. The delocalization in III was not uniform but, rather, resembled two inequivalent fused allyl systems. The cyclotrimer (IV) of 1,2‐azaborine (heat of formation ‐44.07 kcal/mol), based purely on thermodynamic considerations, was predicted to form spontaneously from three monomer molecules with the concurrent loss of three molecules of dihydrogen. The cyclotrimers that could theoretically be produced from 1,2‐azaborine without the loss of dihydrogen (IVc and IVt) were each calculated to be less stable (heats of formation +24.45, and +33.29 kcal/mol, respectively) than was the experimentally observed IV. The carbon molecules triphenylene (TP) and cis‐ and trans‐4a,4b,8a,8b,12a,12b‐ hexahydrotriphenylene (TPc and TPt) (heats of formation +76.79, +101.6, and +103.1 kcal/mol, respectively) were each calculated to be less stable than were the azaborine cyclotrimer analogs, as was observed in comparisons of benzene with the azaborines and borazine.
Approximate density matrices and Husimi functions using the maximum entropy formulation with constraints
Authors: Morrison Robert C. , Parr Robert G.
Published: January 2005

The entropy of an electronic system is defined in terms of the Husimi function, a nonnegative distribution function in phase space. The Husimi function is calculated by maximizing the entropy subject to the constraints that the Husimi function give a Gaussian convolution of the desity when integrating over the momentum coordinates and that its second moment with respect to momentum give a sum of Gaussian convolutions of the density and the kinetic energy density. The result is compared with the Wigner function. Equations are given for calculating the density matrix from the Husimi function. The resulting equation for the exchange energy requires a difficult numerical integration. An alternate method is used to obtain the density matrix from an approximate partially collapsed Husimi matrix that gives the maximum entropy Husimi function as its diagonal. The results are exact for the harmonic oscillator ground state. Exchange energies calculated for H and the He isoelectronic series through C+4 show slight improvements over those calculated using a maximum entropy Wigner function.
Theoretical treatment of chemisorption using an Ab initio SCF Green‐matrix formalism
Authors: Fleck O. , Ladik J.
Published: January 2005

We present a treatment of chemisorption that uses the Green matrix of the surface and the admolecule, as well as a cluster calculation of a small surface cluster. Since full HF‐SCF‐band structures with nonlocal exchange are available now for larger systems for a couple of years, it is possible to treat the substrate surface and the admolecule on equal footing. This offers a natural way to solve the embedding problem. We have applied the formalism to the adsorption of H2 on beryllium and show that in contrast to cluster calculations there appears a binding, which is experimentally verified.
The influence of the solvent on the conformational energy differences due to the anomeric effect
Authors: Montagnani Raffaele , Tomasi Jacopo
Published: January 2005

The solvation free energy ΔGsol of molecules exhibiting the anomeric effect is computed in an approach that considers a continuous distribution for the solvent. A partition of ΔGsol into separately evaluated contributions confirms that changes in the energy of the systems due to changes in conformation of the solute are ruled by the electrostatic contribution. A comparison with the “exact” values indicates that the approximate expression for the electrostatic contribution to ΔGsol are not accurate enough to permit a proper modeling of the solvent influence on the anomeric effect. The systems are composed of methanediol, methoxymethanol, dimethoxymethane, and 2‐methoxytetrahydropyran in carbon tetrachloride, chloroform, acetone, and water. The calculations have been performed at the SCF level with the STO‐3G and 4–31Gbasis sets.
On some relations to wave functions of one‐dimensional Schrödinger equation
Authors: Bakuradze R. Sh. , Stankevich I. V.
Published: January 2005

A model problem connected with the investigation of a 2n one‐dimensional Fermion system in a nonhomogeneous well with infinitely high walls is considered in this paper. The one‐particle density ρ(x) is supposed to be known. We show that in terms of ρ(x) and the potential v(x) without using the Schrödinger equation there may be found wave functions and one‐particle energy levels appropriate to the density ρ(x). An integro‐differential equation satisfied by the functions v(x) and ρ(x) is obtained. The problem of satisfying N, v‐representability by a nonnegative function ρ(x) is reduced to the problem of finding conditions for the existence of a solution of a certain equation.
Midgap levels of photoexcited conductive polymers. II. Detailed analysis of trans‐polyacetylene
Authors: Tanaka Kazuyoshi , Yoshizawa Kazunari , Takeuchi Tomonari , Yamabe Tokio
Published: January 2005

We have studied in more detail aspects of the soliton‐antisoliton pair in a trans‐polyacetylene chain based on our previous study of the midgap levels associated with the photogenerated oppositely charged soliton‐antisoliton pair in conductive polymers employing the concept of the molecular orbital interaction (Part I of this study, this issue). The intersoliton distance has been estimated to be about 10 Å from the Pariser–Parr–Pople method. We have found that the energy gap between the midgap levels is estimated to be 0.45 eV, being significantly related to an additional photoinduced absorption.
Ionization potentials, electron affinities, electronegativities, and hardnesses of fractional charged atoms from the density‐functional theory
Authors: Guo Yufei , Whitehead M. A.
Published: January 2005

The electron‐correlation and self‐interaction corrected generalized exchange local‐spin‐density functional theory with the Gopinathan, Whitehead, and Bogdanovic Fermi‐hole parameters has been employed to give self‐consistent field calculations for the quark atoms, the first‐ and second‐order positive ions, and the first‐ and second‐order negative ions of the quark atoms with fractional nuclear charges \documentclass{article}\pagestyle{empty}\begin{document}$ Z = N \pm \frac{1}{3} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ Z = N \pm \frac{2}{3} $\end{document}. A special technique to obtain the converged second‐order negative ions is discussed. The first and second ionization potentials and electron affinities are calculated by the differences of the total energies between the ionized and nonionized systems and compared with the empirical inter‐extrapolation results. The agreement between the present calculations and the inter‐extrapolated results is excellent for the ionization potentials and reasonably good for the electron affinities of the quark atoms. Finally, the calculated ionization potentials and electron affinities are used in obtaining the electronegativities and hardnesses for these quark atoms.
Further investigation of the spatial generalization of Kato's theorem by a variational density‐functional approach
Authors: Csavinszky P. , Kubaska T.
Published: January 2005

In a recent work, Csvinszky formulated a variational density‐functional approach for the investigation of the spatial generalization of Kato's theorem by March. In the previous work, a 10‐electron model system (filled K and L shells) was considered. In the present work, the investigation is extended to two other cases: a 28‐electron model system (filled K, L, and M shells) and a 60‐electron model system (filled K, L, M, and N shells). As in the previous work, several expressions are adapted for the kinetic‐energy functional of the electrons and the intriguing question of identifying the “best” kinetic‐energy functional is raised and discussed.
Ag vibrational levels of cyclobutadiene on a new potential energy surface
Authors: Čársky Petr , Downing John W. , Michl Josef
Published: January 2005

GVB/[5s3p1d/3s1p] energies were calculated for 31 geometries of cyclobutadiene in the D2h point group. These geometries differed in the values of the symmetrized internal coordinates for two CC stretching and one CCH bending modes. The data points were fitted to the expansions of in powers of . Variational calculations provided the following energies of the lowest Ag vibrational levels (with respect to the vibrational ground state): 4.4; 1161.2; 1162.3; 1304.0; 1322.8; 1920.3; and 1991.0 cm−1.
Harmonic oscillator tensors. I. The nondegenerate case
Authors: Palting Pancracio
Published: January 2005

It is shown that the Heisenberg Lie algebra of the nondegenerate harmonic oscillator leads to a basis {J+, J0, J−} of LASU(2). The Hamiltonian of the system is proportional to J0, and the basis elements give rise to irreducible tensors in the associative enveloping algebra of the Heisenberg Lie algebra. The construction of these irreducible tensors is studied with special attention being paid to the case in which they act upon a single vector space spanned by the harmonic oscillator basis functions. A tensor coupling rule is developed, and useful application is made of it in the calculation of general expressions for vibrational operators and their matrix elements. Throughout, the value of the additional algebraic quantum numbers (l, m) is emphasized.
MCSCF/MCLR Studies of potential energy surfaces, spectra, and properties of the X1A1 and a3B2 states of ozone
Authors: Nordfors David , Ågren Hans , Jensen Hans Jørgen Aa.
Published: January 2005

Potential energy surfaces, properties, and spectra of singlet (X1A1) and triplet (a3B2) ozone are investigated by means of MCSCF/MCLR analytical response theory calculations. MCSCF analytical gradients and Hessians are used to locate equilibrium and transition‐state structures and to obtain associated vibrational and rotational constants, infrared intensities, and dipole moments. By means of MC linear response functions, electronic excitation energies, and oscillator strengths, static and dynamic polarizabilities as well as dispersion (C6) coefficients are obtained. Good agreement is achieved, in some cases within experimental error margins, for properties where experimental data are known. A very low IR intensity for triplet ozone is predicted.
Formulation of N‐ and v‐representable density functional theory. IV. Non‐Born–Oppenheimer approach
Authors: Kryachko Eugene S. , Ludeña Eduardo V. , Mujica Vladimiro
Published: January 2005

Local‐scaling transformations are employed in order to formulate a desity functional theory where both electrons and nuclei are treated quantum mechanically. Because of the properties of these transformations, the ensuing version of density functional theory is N‐ and v‐ representable. In particular, we study the effect of these transformations on both densities and generating wave functions, and as a result, a functional for the energy expressed in terms of the electronic and nuclear densities is constructed. Plausible physical approximations for simplifying this electronic‐nuclear functional are considered, and upon variation of the fuctional with respect to the electronic and nuclear densities, a system of coupled Euler–Lagrange equations is obtained.
Analytic form of Thomas–Fermi–Dirac dielectric function for III–V compound semiconductors
Authors: Gurumurugan K. , Chandramohan D.
Published: January 2005

The nonlinear Thomas–Fermi–Dirac equation is solved variationally for positive and negative point charges in various III–V compound semiconductors. The parameters are presented, and the results are compared with those obtained numerically.
The prism algorithm for two‐electron integrals
Authors: Gill Peter M. W. , Pople John A.
Published: January 2005

A new approach to the evaluation of two‐electron repulsion integrals over contracted Gaussian basis functions is developed. The new scheme encompasses 20 distinct, but interrelated, paths from simple shell‐quartet parameters to the target integrals, and, for any given integral class, the path requiring the fewest floating‐point operations (FLOPS) is that used. Both theoretical (FLOP counting) and practical (CPU timing) measures indicate that the method represents a substantial improvement over the HGP algorithm.
Spin‐orbit (core) and core potential integrals
Authors: Pitzer Russell M. , Winter Nicholas W.
Published: January 2005

Molecular integral formulas and corresponding computational algorithms are developed for the relativistic spin‐orbit and core potential operators that are obtained from atomic relativistic calculations by means of the effective core potential procedure. Much use is made of earlier work on core potential integrals by McMurchie and Davidson. The resulting computer code has been made part of the ARGOS (Argonne, Ohio State) program from the C⋅OLUMBUS suite of programs, which computes the needed integrals over symmetry‐adapted combinations of generally contacted Gaussian atomic orbitals.
Optimized Gaussian basis sets for use with relativistic effective (core) potentials: LiAr
Authors: Wallace Nora M. , Blaudeau Jean P. , Pitzer Russell M.
Published: January 2005

Balanced atomic basis sets, at the double‐zeta level with d functions, have been developed for the elements LiAr within the relativistic effective core potential procedure. The number of primitive functions, their orbital exponents, and the number of contractions were chosen for use in both Hartree–Fock and correlated calculations. Spin‐orbit splittings have been obtained using the approximate operator corresponding to relativistic effective potentials and are compared with experimental values.
Exact and approximate solutions to the one‐center McMurchie–Davidson tree‐search problem
Authors: Johnson Benny G. , Gill Peter M. W. , Pople John A.
Published: January 2005

We have attempted to optimize the cost (the total number of floating‐point operations required) of using the McMurchie–Davidson RNLMj recurrence relation. Rigorous solutions of the tree‐search problem inherent in the cost minimization are given for total angular momentum L ≤ 7. For L ≥ 8, the rigorous search algorithm is prohibitively expensive, and we propose an approximate algorithm that generates highly optimized trees. Cost comparisons demonstrate that the present scheme is consistently superior to two others currently in use.
Portable tools and applications for parallel computers
Authors: Harrison Robert J.
Published: January 2005

A distributed‐memory model is adopted for the development of parallel ab initio codes with an emphasis on portability as well as efficiency. A highly portable message‐passing toolkit (inspired by the Argonne ACRF PARMACS) is described. Examples and results of its use are presented.
The application of cyclic spin permutations to the theory of strongly correlated electron systems
Authors: Cheranovskii V. O.
Published: January 2005

The problem of the reduction of some strong correlated electron models to spin models is considered. Effective spin Hamiltonians for Hubbard and Emery modles with a strong repulsive potential are obtained.
Conformational preferences of 6‐furfuryl amino purine and 6‐benzyl amino purine
Authors: Tewari Ravindra
Published: January 2005

Conformational preferences of N6‐furfurylamino purine (kinetin) and N6‐benzyl amino purine (BAP) have been investigated theoretically by the quantum chemical perturbative configuration interaction using localized orbitals method. The predicted most stable conformations for these molecules are quite similar. The N6 substituents in both these molecules are oriented toward N(1) and away from the imidazole moiety of the purine. The furfuryl ring in kinetin as well as the aromatic benzene ring in BAP are not coplanar with the purine ring. Comparison of these results with the preferred conformation of another compound N6‐(Δ2‐isopentenyl) adenine reveals striking similarity in the orientations of the N6 substituents in these cytokinin‐active plant‐growth‐stimulating substances.
Bethe logarithm and Lamb shift for the hydrogen molecular ion
Authors: Bukowski Robert , Jeziorski Bogumił , Moszyński Robert , Kołos Włodzimierz
Published: January 2005

A variational method is used to evaluate Bethe logarithm for the ground electronic state of the hydrogen molecular ion. The trial function employed in the calculations analytically represents the singular behavior of the exact solution of the variational problem for high virtual photon energies. This ensures that the accuracy of six significant figures can be obtained with basis sets not exceeding 70 functions. For the equilibrium internuclear separation of 2.0 bohr, the calculated value of the Bethe logarithm, equal to 2.31936, lies outside the bounds 2.35 and 2.56, representing the best previous estimation of this quantity. The accurate values of the Bethe logarithm for internuclear separations ranging from 0.6 to 6.0 bohr are tabulated and used to calculate the radiative corrections to the rovibrational energy levels of H2+, HD+, and D2+. The influence of these corrections on the transition and dissociation energies is also examined.
Calculation of time autocorrelation and spectral functions using locally expanded potentials
Authors: Cesar Amary , Ågren Hans
Published: January 2005

We present theory for autocorrelation and spectral functions and for the description of molecular motions on a multidimensional potential energy surface. The basic idea is to express the autocorrelation functions by stepwise series expansions of the molecular surface using only local information. This approach thus presents one way to avoid the dimensionality catastrophe that is associated with dynamic studies using globally calculated and fitted polyatomic energy surfaces. The autocorrelation function is obtained by solving the quantum equations of motions in a set of locally defined internal coordinates uncoupled to rotation and translation. A Fourier transform of the autocorrelation function gives a time‐evolved interpretation of the formation of the vibronic spectrum. Locally expanded and quadratically approximated Hamiltonians are assumed throughout the reaction path. The formalism implements in a direct fashion the information obtained from molecular gradient and Hessian techniques, so that the buildup of the spectrum can be followed along with the reaction walks on the molecular hypersurface. The method applies to unbound or quasi‐bound as well as to bound potentials. In the latter case, the calculation of the autocorrelation function also provides a cost‐effective way to perform Franck–Condon analyses, since, in contrast to conventional Franck–Condon methods, it avoids sum‐over‐state and basisset procedures completely. A set of 3N – 6 Cartesian coordinates are analyzed for the purpose of translational and rotational‐free walks on a locally and quadratically expanded multidimensional potential energy surface without recoursing to the standard Eckart conditions and/or the necessity of performing supplementary transformations on the gradient vector and the Hessian matrix. Demonstrations of the formalism are given by calculations of time autocorrelation and spectral functions using locally expanded potentials corresponding to N2 2IIu and O2 2IIg photoionization.
Theoretical study on the reaction mechanism of the alcoholysis of isocyanates
Authors: Tang Ming‐Sheng , Fu Xiao‐Yuan
Published: January 2005

In this paper, the reactions of HNCO or CH3NCO with methanol have been studied using the ab initio MO method. The geometries of the reactants and products have been optimized by the energy gradient method. The calculated results are in accordance with experiment. Using Powell's method, by the minimization of the Euclidean norm σ of the gradient, the structures of the transition state (TS) for the two reactions were obtained. The structures have been further confirmed as TS by finding that there is one and only one negative eigenvalue for their force constant matrix. The imaginary vibration mode corresponding to the TS was also discussed. The calculated activation barriers of these two reactions are 96.02 and 95.13 kJ/mol, respectively. It can be concluded that the alcoholysis reaction of isocyanate is a nucleophilic addition reaction with methanol as an electronic donor and isocyanate as an acceptor and that the hydroxyl hydrogen of CH3OH plays an important role in the reaction.
The core electron shake phenomenon
Authors: Ågren Hans , Carravetta Vincenzo
Published: January 2005

We review theory and computational aspects for analysis of the core electron shake phenomenon.
Molecular integrals of Breit interaction over Laguerre Gaussian‐type functions
Authors: Fujimura Naoki , Matsuoka Osamu
Published: January 2005

The Breit interaction consists of two interactions of the magnetic and the retardation terms. The molecular integrals involving the former interaction are shown to be reduced to the familiar integrals of the Coulomb repulsion. The integrals involving the latter interaction are formulated over the Laguerre Gaussian‐type functions and some advantages of the formulation are discussed. Bayman's gradient formula of the solid spherical harmonic is generalized to include the gradient of the homogeneous solid spherical harmonic.
On the evaluation of overlap integrals with exponential‐type basis functions
Authors: Homeier Herbert H. H. , Steinborn E. Otto
Published: January 2005

The numerical properties of a one‐dimensional integral representation [H.P. Trivedi and E. O. Steinborn, Phys. Rev. A 27, 670 (1983)] for the overlap integral of a two‐center product of certain exponential‐type orbitals (ETOs) the B functions [E. Filter and E. O. Steinborn, Phys. Rev. A 18, 1 (1978)], are examined. B functions possess a very simple Fourier transform that results in relatively simple general formulas for molecular integrals derivable using the Fourier transform method. In addition, molecular integrals for other ETOs, like the more common Slater‐type orbitals, can be written as finite linear combinations of integrals with B functions because these functions span the space of ETOs. The integrand of the integral representation mentioned above shows peculiarities requiring special quadrature methods, especially in the case of highly asymmetric charge distributions. It is shown that good results can be obtained using Möbiustransformation‐based quadrature rules well suited for the numerical integration of functions possessing a sharp peak at or near one boundary of integration [H. H. H. Homeier and E. O. Steinborn, J. Comput. Phys. 87, 61 (1990)]. The method based on Möbius‐type quadrature is compared to several other methods — based on other quadrature rules, finite sums, and infinite series representations together with convergence acceleration — to evaluate overlap integrals with B functions. The numerical results indicate that the new quadrature schemes are more efficient than are the other methods.
Dynamic stability of silacarbonyl ylide
Authors: Tachibana Akitomo , Fueno Hiroyuki , Okazaki Iwao , Yamabe Tokio
Published: January 2005

The structure of silacarbonyl ylide in the singlet gound state is obtained by ab initio quantum chemical calculations. This proves the recent experimental observation of silacarbonyl ylide by the photolysis of oxasilitane [W. Ando, K. Hagiwara, and A. Sekiguchi, Organometallics 6, 2270 (1987)]. The dynamic stability of the silacarbonyl ylide is studied by the TCSCF/3‐21G(*) method. The second‐order Jahn‐Teller effect is examined in connection with the active role of frontier orbitals. Configuration analysis is used to clarify characteristic electronic structure of the silylene‐carbonyl interaction.
Structure of Mn2(CO)10 and Mn2(CO)9: Insights from ab initio calculations
Authors: Veillard Alain , Rohmer Marie‐Madeleine
Published: January 2005

Optimization of the Mn–Mn distance in Mn2(CO)10 with various basis sets of at least doublezeta quality results in Mn–Mn bond lengths in the range of 3.07–3.15 Å, 0.2–0.25 Å longer than the experimental value of 2.895 Å. Incrementing the basis set with diffuse p functions (exponent 0.0332) on the carbon atoms improves the calculated bond length to a value of 2.876 Å at the CI level, as a consequence of a charge transfer between each Mn atom and the equatorial carbonyls of the other Mn atom. For Mn2(CO)9 four structures have been studied at the SCF and CI levels with assumed geometries. The structure with a symmetric bridging carbonyl turns to be much higher in energy at the SCF level. The two structures which are purely metal–metal bonded [corresponding to the departure of an axial or equatorial carbonyl from Mn2(CO)10] are nearly degenerate in energy and more stable than the structure with a semibridging carbonyl by 5 kcal/mol at the SCF level and 10–11 kcal/mol at the CI level (seemingly at variance with the conclusions of matrix experiments that favor the semibridging structure).
Relativistic corrections in the LCGTO–local‐spin‐density method. I. Atomic bases for transition metals
Authors: Wilkin A. , Salahub D. R.
Published: January 2005

Compact orbital GTO basis sets optimized with a nonrelativistic Hamiltonian, then decontracted, are utilized in an SCF treatment with a quasi‐relativistic scalar Hamiltonian including the mass–velocity and one‐electron Darwin operators. Ionization and excitation energies, orbital energies, and radial mean values obtained from different expansion patterns have been tested in atomic calculations for Ag and generalized for Cu and Au atoms. The one‐electron spin–orbit operator is also used in an SCF treatment. Spin–orbit coupling energies are calculated for Cu, Ag, and Au atoms.
Determination of ab initio polarizabilities of polymers: Application to polyethylene and polysilane
Authors: Champagne Benoit , André Jean‐Marie
Published: January 2005

An ab initio method for calculating the longitudinal linear polarizability of polymeric chains is described. This method is equivalent to an uncoupled Hartree–Fock scheme. It is applied to polyethylene and polysilane in minimal STO‐3G and extended 4‐31G basis sets. The study describes important techniques for solving the difficulties met in actual calculations: band reordering of the band structures, calculation of analytical derivatives of the energy bands ϵn(k) and LCAO coefficients cnp(k), and errors caused by the improper lattice sum truncations of the Hartree–Fock matrix.
The electronic transition energies of four alternative C60 molecules
Authors: László I. , Udvardi L.
Published: January 2005

The low‐lying allowed electronic transition energies of four alternative structures to the C60 cage molecule are examined. The bond turning transformation of Stone and Wales is used to generate new C60 structures with the symmetries C2v, D2h, and D6h. We have found that the C60 is indeed a truncated icosahedron as originally proposed, but the production of other alternative structures in specific experimental conditions is not excluded.
Effect of conjugated length on the calculated nonlinear second‐order optical susceptibilities of some organic molecules
Authors: Gao X. L. , Feng J. K. , Sun C. C.
Published: January 2005

Calculations of the nonlinear second‐order optical susceptibilities (βijk) for disubstituted conjugated organic molecules are reported. The calculations are performed using the semiempirical INDO/CI method combined with a sum‐over‐states expression for βijk. The calculated results show that the second‐order static susceptibility (β0) varies as the 2.6 power of the length of the conjugated π‐electron system. Furthermore, it is found that the amount of substituent charge transfer from ground state to first excited state decreases with inccreasing chain length. A limiting value of β0 with the length of the conjugated system is discussed.
Exact nonlinear optical coefficients of quantum cell models with interacting electrons
Authors: Soos Z. G. , McWilliams P. C. M. , Hayden G. W.
Published: January 2005

Diagrammatic valence‐bond (DVB) methods yield exact dynamic nonlinear optical (NLO) coefficients for π‐electron models of conjugated molecules and polymers. We contrast exact coefficients with mean‐field or Hückel results and discuss several qualitative differences. The length and alternation dependencies of NLO spectra for Pariser–Parr–Pople (PPP) models provide a unified treatment of even‐parity states and two‐photon processes in polyenes, polydiacetylenes, and polysilanes. We emphasize relationships between one‐ and two‐photon spectra, correlation effects involving both parallel and transverse NLO coefficients, and the nature of correlated states in polar conjugated molecules and in polymers with centrosymmetric backbones.
A dispersion formula for the second dynamic hyperpolarizability as exemplified by helium
Authors: Bishop David M. , Pipin Janusz
Published: January 2005

A simple one‐parameter formula:
equation image
is introduced for the description of the dispersion in the second dynamic hyperpolarizability (γ), where ω = ω1 + ω2 + ω3 and ω = ω + ω + ω + ω. This formula is appropriate for both atoms and the macroscopic hyperpolarizabilities of molecules (as defined with respect to laboratory axes). Its suitability is tested for helium for five different nonlinear optical processes (including, for the first time, the ac Kerr effect). A comparison is made between the values of 𝒜 found by fitting ab initio γ values and those derived from a formula proposed by Hellwarth et al.
Theoretical study on solvation effects in chemical reactions: A vibrational coupling model
Authors: Tachikawa Hiroto , Lunell Sten , Törnkvist Christer , Lund Anders
Published: January 2005

A vibrational coupling model to treat the solvation effects in chemical reaction rate calculations is proposed and applied to the intramolecular hydrogen transfer reaction CH3O· → ·CH2OH in the condensed phase. The effect of solvation is taken into account in two ways: (1) the solvent effect on the activation energy of the reaction is simulated by including 39 surrounding water molecules, represented by fractional charges at the assumed atomic positions, in the potential energy surface calculation; and (2) the vibrational couplings between the 10 nearest solvent molecules and the molecules constituting the reaction system are explicitly included in a vibrational frequency calculation. RRKM theory with Miller's tunneling correction included is employed to calculate the rate constants. The effect of solvation causes a significant change in the chemical reaction rate, mainly through a lowering of the activation energy. The vibrational coupling causes a slight increase of the rate constant in the tunneling region by perturbing the vibrational frequencies of the reactant and transition states, which appear in the rate‐constant expression, but has little effect at higher temperatures.
Small‐angle electron scattering by formaldehyde and ketene: Effects of electron correlation and chemical binding
Authors: Sasaki Yorihiko , Takeuchi Hiroshi , Konaka Shigehiro , Kimura Masao
Published: January 2005

The total (elastic plus inelastic) intensities of 51 keV electrons scattered by H2CO and H2CCO have been measured over a range of K = (4π/λ) sin(θ/2) = 1–9.5 Å−1 and compared with the theoretical intensities calculated with SCF and CI wave functions. Significant discrepancies are found between the experimental intensities and the theoretical ones based on the SCF wave functions. Most of the chemical binding and electron correlation effects observed in the total scattered intensities are reproduced by the theoretical intensities based on the CI wave functions calculated with the basis set including polarization functions on all atoms. © 1992 John Wiley & Sons, Inc.
An accurate lower‐bound calculation of the lowest triplet states of he and its isoelectronic series
Authors: Kleindienst H. , De Groot U.
Published: January 2005

Accurate lower bounds for the nonrelativistic lowest triplet states of 4He and ∞He and the He isoelectronic series up to Z = 10 were calculated by the method of variance minimization using wave functions with logarithmic terms. Furthermore, the method of Lehmann/Goerisch was extended to self‐adjoint operators with an essential spectrum. Both methods were compared for 4He. © 1992 John Wiley & Sons, Inc.
Conformational studies of (S)idazoxan and (S)methoxyidazoxan using AM1 and PM3 semiempirical molecular orbital methods
Authors: Hariharan Sivaram , Shelver William H.
Published: January 2005

Semiempirical calculations of the α‐2 receptor antagonist, Idazoxan, its methoxy derivative, and their respective cations and anions revealed the most stable conformers were significantly affected by electrostatic interactions. These studies showed the importance of the intramolecular electrostatic interactions between the oxygen of the benzodioxane ring and the nitrogens of the imidazoline ring in determining the global minima of these molecules. The NH bond of the imidazoline ring was attracted to the dioxane oxygen and caused the imidazoline ring to rotate to a position allowing close proximity of these groups. Favorable interactions between the oxygens and the imidazoline stabilized the axial conformer, whereas unfavorable interactions increased the stability of the equatorial isomer. AM1 and PM3 calculations sometimes predicted different global minimum conformers, indicating the need for caution in reliance on either method, particularly when hydrogen bonds play an important role in determining the structure. © 1992 John Wiley & Sons, Inc.
Conformational study of Met‐enkephalin in its zwitterionic form
Authors: Perez Juan J. , Loew Gilda H. , Villar Hugo O.
Published: January 2005

An extensive exploration of Met‐enkephalin in its zwitterionic form has been carried out, in order to characterize the different low‐energy conformational domains accessible to this pentapeptide. The study builds on previous studies carried out in our lab for the neutral molecule, which provided the initial geometries from which the conformational space of the charged molecule could be scanned. The initial conformations were subjected to a series of high‐ and lowtemperature molecular dynamics simulations. Snapshots along each trajectory were taken, minimized, and used as starting points in further MD trajectories until no lower‐energy conformers could be characterized. The CHARMm force field was used throughout the study for this purpose. The same search strategy was used in these studies simulating two different environmental conditions, a distance‐dependent dielectric ϵ = r and a high constant dielectric ϵ = 80. In the low dielectric environment, the formation of the salt bridge dominates the structure. In the high dielectric environment, the screening of the electrostatic interactions results in weaker intramolecular interactions. In both cases, the Gly2–Gly3 β‐turn‐type structures are preferred over the Gly3–Phe4 turns, in marked contrast to what is found for the neutral molecule. The lowest‐energy structures from both environmental conditions were reoptimized in the presence of a cluster of explicit water molecules. Reoptimization of the structures considering explicit water structures did not result in significant conformational changes for the structures characterized with the ε = r or ε = 80 environments.
Do intelligent configuration search techniques outperform random search for large molecules?
Authors: Judson R. S. , Colvin M. E. , Meza J. C. , Huffer A. , Gutierrez D.
Published: January 2005

We compare three global configuration search methods on a scalable model problem to measure relative performance over a range of molecule sizes. Our model problem is a 2‐D polymer composed of atoms connected by rigid rods in which all pairs of atoms interact via Lennard–Jones potentials. The global minimum energy can be calculated analytically. The search methods are all hybrids combining a global sampling algorithm with a local refinement technique. The sampling methods are simulated annealing (SA), genetic algorithms (GA), and random search. Each of these uses a conjugate gradient (CG) routine to perform the local refinement. Both GA and SA perform progressively better relative to random search as the molecule size increases. We also test two other local refinement techniques in addition to CG, coupled to random search as the global method. These are simplex followed by CG and simplex followed by block‐truncated Newton. For small problems, the addition of the intermediate simplex step improved the performance of the overall hybrid method. © 1992 John Wiley & Sons, Inc.
Investigating the possibility of simultaneously finding an electron‐hole and an electron‐pair in a molecule: Delocalization, competition of ionic vs. covalent character, and related effects in push–pull ethylenes
Authors: Karafiloglou Padeleimon , Marcos Enrique Sánchez
Published: January 2005

This work examines the possibility of finding an electron‐hole and an electron‐pair simultaneously in a π‐system substituted by an electron‐donating (NH2) and/or electron‐withdrawing (NO2) group. The contributions of various ionic [(+), ( ↑ ↓ )] structures are calculated from ab initio SCF–CI wave functions, using a recently developed general multielectron population analysis. The molecules studied are two monosubstituted ethylenes, aminoethylene and nitroethylene, and a disubstituted ethylene, the 2‐nitroethenamine (push–pull ethylene) in its two configurational forms. The influence of the NH2 and/or NO2 group in delocalization and ionic (vs. covalent) character of the CC double bond are investigated, along with examining the experimental chemist formalism of electron‐pair “displacements” in several resonance structures. Analysis of mutual dependence of an electron‐hole and an electron‐pair, at short and long distances, leads to the conclusion that a push–pull π‐system can stimulate the simultaneous existence of an electronhole and an electron‐pair even for nonvicinal positions. The relationship between the electronpair distributions and contributions of the corresponding structures are also examined; the effects of electronic correlation are analyzed as well. © 1992 John Wiley & Sons, Inc.