bluuebunny/crossref_metadata_2025_split huggingface dataset:
Calculation of transition density matrices by nonunitary orbital transformations
Authors: Malmqvist Per Åke 
Published: January 2005

AbstractAn efficient scheme for calculating one‐ and two‐electron transition density matrices for two wave functions is described. The method applies to CAS (complete active space) wave functions and certain multireference CI expansions. The orbital sets of the two wave functions are not assumed to be equal. They are transformed to a biorthonormal basis, and the corresponding transformation of the CI coefficients is carried out directly, using the one‐electron coupling coefficients.
Butadiene and benzene π‐electron SCF ground states in the bond orbital resonance theory approach
Authors: Živković Tomislav P. 
Published: January 2005

AbstractButadiene and benzene SCF ground states are analyzed using the bond orbital resonance theory (BORT) approach. The results obtained are compared with the analogous description of these states within the VB approach. It is shown that the structural weights of BORT Kekulé structures are much larger than the structural weights of the corresponding VB Kekulé structures. For example, in the butadiene case the structural weight of the BORT Kekulé structure is over 0.941, while the structural weight of the VB Kekulé structure is only 0.221. In addition, in the BORT approach one has to consider a much smaller number of resonance structures than in the VB approach. This suggests that the description of conjugated systems should be more successful in the BORT than in the VB basis, and that formal Kekulé structures should be interpreted in the BORT and not in the VB sense.
An intermolecular potential function for the Fe(II)–H2O system from ab initio calculations
Authors: Gonzàlez‐Lafont A. , Lluch J. M. , Oliva A. , Bertrán J. 
Published: January 2005

AbstractAn intermolecular potential for the Fe(II)–H2O system has been determined from ab initio calculations which have been obtained with Huzinaga's MINI‐2 basis set. Interaction energies for more than 100 points of the potential energy surface were fitted to an analytical function that contains 11 adjustable parameters. The goodness of the fitting and its applicability to the study of Fe(H2O) clusters and to Monte Carlo simulations are discussed.
Transferability of atomic descriptions in molecules
Authors: Rico J. F́ernandez , Alvarez‐Collado J. R. , Paniagua M. , Ĺopez R. 
Published: January 2005

AbstractIn a previous paper we proposed a new electrostatic description of the molecular electronic density in terms of charges, dipoles, quadrupoles, octupoles, etc. In this paper we apply this analysis to some first members of alkanic, ethylenic, acetylenic, alcoholic, aldehyde, amine, and nitrile series. Results show the clear transferability of the atomic descriptions for chemically similar atoms as well as the remarkable sensitivity of the electronic density versus changes in the basis sets, pointed out in our previous work. We discuss the calculation of molecular potentials from these atomic descriptions (and present an example) and some practical applications.
Energy expressions for atomic configurations in the L–S coupling scheme
Authors: Dinesha K. V. , Iyer Suman B. , Vishveshwara Saraswathi 
Published: January 2005

AbstractThe matrix elements of the spin‐free Hamiltonian between two atomic configuration state functions (CSF'S) in the L–S coupling scheme are expressed in terms of the atomic integrals Fk's and Gk's. Using these general expressions, the matrix elements have been obtained for all the atomic configurations with three valence electrons that have not been solved so far by earlier methods. The scope for applying this new approach to obtain the Auger line energies and the promotion energies of metals that involve more than two partially filled shells is indicated. The energy expressions for some of the relevant configurations are tabulated.
On the construction of the effective pair correlation function with the fixed zero value at the points where two electron positions coincide
Authors: Gritsenko O. V. , Bagaturyants A. A. , Zhidomirov G. M. 
Published: January 2005

AbstractThe effective pair correlation function g̃(N)(r1;r2) accounting for the contribution of the kinetic energy functional to the electron correlation energy has been constructed. This function possesses the asymptotic property g̃(N)(r1;r2) = 0 at r1 = r2 and provides an efficient formalism for a model description of the electron correlation in many‐electron systems.
Reaction rates for O + HD → OH + D and O + HD → OD + H
Authors: Garrett Bruce C. , Truhlar Donald G. 
Published: January 2005

AbstractWe have calculated reaction rates for the reactions O + HD → OH + D and O + DH → OD + H using improved canonical variational transition state theory and least‐action ground‐state transmission coefficients with an ab initio potential energy surface. The kinetic isotope effects are in good agreement with experiment. The optimized tunneling paths and properties of the variational transition states and the rate enhancement for vibrationally excited reactants are also presented and compared with those for the isotopically unsubstituted reaction O + H2 → OH + H. The thermal reactions at low and room temperature are predicted to occur by tunneling at extended configurations, i.e., to initiate early on the reaction path and to avoid the saddle point regions. Tunneling also dominates the low and room temperature reactions for excited vibrational states, but in these cases the results are not as sensitive to the nature of the tunneling path. Overbarrier mechanisms dominate for both thermal and excited‐vibrational state reactions for T > 600 K. For the excited‐state reaction (with initial vibrational quantum number n > 0) a transition state switch occurs for T > 1000 K for the O + HD(n = 1) → OD + H case and for T > 1500 K for the O + DH(n = 1) → OD + H reaction, and this may be a general phenomenon for excited‐state reactions at higher temperature. In the present case the switch occurs from an early variational transition state where the vibrationally adiabatic approximation is expected to be valid to a tighter variational transition state where nonadiabatic effects are probably important and should be included.
Analysis of the IGLO bond contributions to the 13C shielding tensors in the local bond frame
Authors: Facelli Julio C. , Grant David M. , Michl Josef 
Published: January 2005

AbstractA new method is presented to analyze the IGLO (individual gauge for localized orbitals) bond contributions in 13C chemical shielding. The IGLO bond contributions calculated in the molecular frame are rotated to a local bond frame, in which one component is selected along the bond. This procedure removes the explicit angular dependence of the IGLO bond contributions and allows a comparison of the bond contributions in different molecules.The results provide a new method to study the electronic basis of shielding interactions. The problems associated with the multiple gauge origins used in the IGLO method are discussed in their relationship to the bond contribution analysis.
Orthogonal Waller–Hartree spin eigenfunctions
Authors: Lim Tiong‐Koon 
Published: January 2005

AbstractIt was shown recently by the present author that the double symmetrization and the double antisymmetrization are essential in the spin‐dependent and the spin‐free formalisms, respectively, to perform the exclusion of all the unnecessary spin eigenfunctions and the selection of a unique set of linearly independent spin eigenfunctions. The double antisymmetrized Wigner matric basis and the Wigner double symmetrized matric basis are presented in this article for N up to 6. The double symmetrization or the double antisymmetrization also results in a direct expansion method for the calculation of the orthogonal spin coefficients; this direct method does not require the knowledge of the wave functions of the N − 1 electron system. The modified method of Graebenstetter can also be used to calculate these orthogonal spin coefficients.
The structure and properties of flavins: Molecular orbital study based on totally optimized geometries. I. Molecular geometry investigations
Authors: Hall Lowell H. , Orchard Bradley J. , Tripathy Sukant K. 
Published: January 2005

AbstractMolecular orbital calculations (MINDO/3) using energy minimized molecular geometries were performed on oxidized and reduced lumiflavin and related methylated isoalloxazines, including cationic and anionic species. Close agreement with experimental geometry, photoelectron spectra, and NMR data supports the importance of optimized geometries for these molecular systems and provides the basis for interpretation of chemical and biological properties. Oxidized forms are shown to be most stable in the planar configuration but also highly flexible about the N(5)—N(10) axis; only 1 kcal/mol is required for a 10° bend. N(10) is generally out of the plane slightly; also, C(9)‐methyl substitution introduces nonplanarity. The unsubstituted isoalloxazine is computed to be 0.76 kcal/mol (ΔH) less stable than its isomer, alloxazine. Calculations were also performed on enol as well as quinone‐methide tautomeric forms.Reduced flavin geometry depends on methyl substitution pattern: N(10) substituted forms are bent with typical fold angles around 155°, whereas the unsubstituted reduced form is planar. Both oxidized and reduced forms are also flexible.Proton affinities were calculated for protonation and deprotonation of oxidized and reduced forms. Protonation of oxidized forms is favored at N(1) by 10–12 kcal/mol and produces somewhat nonplanar isoalloxazinium ions. In addition, ΔH for the two‐electron reduction of lumiflavin is estimated to be −19.7 kcal/mol.In this paper investigations of geometric aspects are presented along with introductory and background material. Orbital structure and electron distribution studies are presented in paper II.
Explicit Galois resolvents for sextic equations
Authors: Hurley A. C. , Head A. K. 
Published: January 2005

AbstractSeveral techniques for calculating the Galois resolvents of polynomial equations are discussed and implemented. In particular, the method of power sums, in conjunction with the symbolic algebra program muMATH, is used to derive a complete set of explicit algebraic resolvents for the general sextic equation. A simple example, drawn from the theory of crystal elasticity, illustrates the utility of these results in answering the question “When is a polynomial equation (with multinomial coefficients) solvable?”.
Chiral‐achiral molecular coupling: The effect on the chiral partner
Authors: Craig D. P. , Thirunamachandran T. 
Published: January 2005

AbstractPair coupling between a chiral molecule and an achiral molecule can induce weak circular dichroism in the achiral partner, as is well known in induced circular dichroism. Here the effect of the same coupling on the chiral partner is analyzed. The effect is an increase or decrease in the rotatory strength that may be detectable under conditions where the effect is enhanced by a near‐resonance.
The extended Koopmans' theorem Fock operator
Authors: Matos J. Mauricio O. , Day Orville W. 
Published: January 2005

AbstractBy expanding the wave function of a system of N particles in terms of products of functions of one and (N‐1) particles, the one‐particle, nonlocal operator F̂EKT (extended Koopmans' theorem) is determined. It is shown that although this operator is nonhermitian, its eigenvalues and eigenfunctions represent the ionization energies and occupied orbitals, respectively. The eigenfunctions of F̂EKT are the one‐particle functions that enter into the expansion of the wave function of the system as partners of the (N‐1)‐particle wave functions. The eingenvalues are also one‐particle energies that, multipled by the orbital occupancy probalities, enter the expression for the total N‐particle energy of the system.
Approximate solutions of Heisenberg Hamiltonians
Authors: Sanchez‐Marin J. , Malrieu J. P. , Maynau D. 
Published: January 2005

AbstractEven when each atom of a 2n‐center cluster or molecule only brings one active electron in one atomic orbital, the size of the Heisenberg Hamiltonian matrix increases as C. Simple truncations of this matrix would result in size consistence defects, as evident from the isomorphism between Heisenberg and configuration interaction (CI) matrices. Geometry‐dependent Heisenberg Hamiltonians derived from accurate ab initio calculations on the two‐center systems have proved to be very efficient for conjugated hydrocarbons and for alkali metals; in order to apply this approach to intermediate size systems (10–20 centers), a rational procedure is proposed consisting of the selection of a truncated set of determinants (of low energy) and a dressing of the truncated Hamiltonian matrix under the perturbation of the other determinants. The second order dressing is analogous to a so‐called “shift Bk procedure” or Generalized Degenerate Perturbation theory and is weakly dependent on an E0 parameter. Tests performed on various 8‐ and 10‐atom systems show the accuracy of the procedure. An iterative selection of the truncated basis set and proper choices of the E0 values allow one to obtain the whole lower part of the spectrum. The calculated geometries are satisfactory. Some preliminary applications are reported concerning the C12H14 dodecahexene linear chain, perfectly fitting with previous extrapolations.
Molecular electrostatic fields from bond fragments
Authors: Nagy Péter , Ángyán János G. , Náray‐Szabó Gábor , Peinel Gustav 
Published: January 2005

AbstractWe apply completely transferable, strictly localized molecular orbitals for the calculation of molecular electrostatic fields. This approach, derived from our previous bond fragment method for the calculation of molecular electrostatic potentials, reduces computational efforts drastically. The fields around small molecules containing first‐ and second‐row atoms are systematically overestimated as compared with ab initio calculations with a minimal STO‐3G basis set. However, deviations can be corrected by a simple multiplicative factor, which means that the overall shape of the potential and field around the molecule is correctly reproduced. Our approximate field can be used to determine possible hydration sites around molecules as proposed earlier by Peinel and coworkers. Application of the method is illustrated on the formamide molecule.
Electron correlation described by extended geminal models: The EXGEM4 and EXGEM5 models
Authors: Røeggen I. 
Published: January 2005

AbstractWithin the framework of the general extended geminal model, two new approximate models EXGEM4 and EXGEM5 are introduced. The models are tested against full CI calculations on the water molecule for three different nuclear configurations and a full CI potential energy curve for the LiH molecule in the ground state. On the basis of these calculations, it is suggested that the models will yield electronic correlation energies with an accuracy of 1–2% of the corresponding full CI result.
Accurate electron density and one‐electron properties for the beryllium atom
Authors: Esquivel Rodolfo O. , Bunge Annik Vivier 
Published: January 2005

AbstractAn accurate analytical electron density for the beryllium atom is obtained by using a fast and systematic method recently developed and tested for the neon atom. Asymptotic conditions both at the nucleus and at large distances are obeyed. A point‐by‐point comparison between our density and the one obtained from an almost “exact” configuration interaction wave function shows that differences are less than 0.5% for r between 0 and 5 bohrs and less than 1 % up to 9 bohrs. The accuracy of the density is also assessed by comparing results of density moments and x‐ray scattering factors.
Polyatomic SCF calculations with numerical orbitals. I. Introduction and computational experiments on the CYBER 205
Authors: Alexander S. A. , Monkhorst H. J. 
Published: January 2005

AbstractWe discuss a procedure for calculating numerical Hartree–Fock orbitals that can be applied to polyatomic systems. This approach is formulated in momentum space to avoid Coulomb singularities and uses fast Fourier transforms to solve integral convolutions. Results for a number of simple systems are presented.
The double many‐body expansion of potential energy surfaces from interacting 2S atoms
Authors: Varandas A. J. C. 
Published: January 2005

AbstractThe double many‐body expansion (DMBE) method has been applied to the many‐valued ground‐state potential energy surfaces from three and four interacting 2S atoms. New forms are suggested where the degeneracy manifold of the two Riemann sheets of the potential surface can be made identical to that predicted from the London equation that is correct to first‐order. Considerable flexibility can be introduced through adjustable coefficients to fit ab initio data, whereas the potential energy terms can be chosen to maintain the correct analytic properties at the conical intersection. Hence, the new forms may be used on the extrapolation of the potential energy from the lowest‐ to the upper‐Riemann sheets. Long‐range effects are semiempirically described from the general formalism of the DMBE method.
On the linearly driven parametric oscillator
Authors: Fernández Francisco M. , Castro Eduardo A. 
Published: January 2005

AbstractThe time‐dependent Schrouml;dinger equation for the oscillator H = α(t)p2 + β(t)x2 + γ(t)x is exactly solved. The time evolution operator is easily obtained by means of an operator algebra and the quantum‐mechanical equations of motion. The problem is reduced to solving the classical equations of motion. The method is shown to apply to multidimensional oscillators.
Renormalized inner projection—a case study: The octic oscillator
Authors: Vinette Francine , Cizek Jiri , Vrscay Edward R. 
Published: January 2005

AbstractThe upper and lower bounds of a harmonic oscillator with an octic perturbation are studied with the use of renormalized inner projection. It is shown that this relatively simple technique works even in the infinite coupling constant limit. Symbolic computation is very convenient and useful in these types of problems, where only a finite number of operations are required.
An analytic method for three‐center nuclear attraction integrals: A generalization of the Gegenbauer addition theorem
Authors: Weatherford Charles A. 
Published: January 2005

AbstractA completely analytic method for evaluating three‐center nuclear attraction integrals for STOS is presented. The method exploits a separation of the STO into an ‘evenly loaded’ solid harmonic and a OS STO. The harmonics are translated to the molecular center of mass in closed finite terms. The OS STO is translated using the Gegenbauer addition theorem; 1s STOS are translated using a single parametric differentiation of the OS formula. Explicit formulas for the integrals are presented for arbitrarily located atoms. A numerical example is given to illustrate the method.
A survey of optimization procedures for stable structures and transition states
Authors: Head John D. , Weiner Brain , Zerner Michael C. 
Published: January 2005

AbstractWe examine a variety of methods for obtaining the stable geometry of molecules and the transition states of simple systems and summarize some of our findings. We find the most efficient methods for optimizing structure to be those based on calculated gradients and estimated second derivative (Hessian) matrices, the later obtained either from the Broyden–Fletcher–Goldfarb–Shanno (BFGS) quasi‐Newton update method or from approximations to the coupled perturbed Hartree–Fock method. For uncovering transition states we find particularly useful a variety of the augmented Hessian theory used to uncover regions of the potential energy hypersurface with one and only one negative eigenvalue of the Hessian matrix characterizing the catchment region of the transition state. Once this region is found we minimize the norm of the gradient vector to catch the nearest extreme point of the surface. Examples of these procedures are given.
A practical solution to the “unknown normalization” problem
Authors: Macías A. , Martín F. , Riera A. , Yáañez M. 
Published: January 2005

AbstractWe analyze the theoretical basis of a procedure to determine an unknown normalization factor in discretized wave functions, which we have successfully used in a series of calculations of resonance widths for atomic and molecular systems. By reducing this determination to that of a suitable interpolation function for the energy eigenvalues, the problem is easily solved when atomic basis sets are chosen according to simple rules. Illustrations of our procedure are presented for atomic, molecular, and model systems; renormalized wave functions are compared with the exact ones for these model systems. The resulting method of renormalized continuum wave functions has a wide range of application in the study of long‐lived quasibound states (predissociation, autoionization, photoionization, unimolecular reactions, etc.).
Construction of linearly independent relativistic symmetry orbitals for finite double‐point groups including time reversal symmetry
Authors: Meyer J. 
Published: January 2005

AbstractA formalism is developed for the construction of relativistic symmetry‐adapted molecular basis functions under consideration of time reversal invariance. The theory is applicable to the finite double point groups Cn, Cnh, Sn, Cnv, Dn, Dnd, Dnh, T, Th, Td, O, and Oh. It is based on the LCAO method. A projection operator technique is employed to construct molecular symmetry orbitals from atomic orbitals. The search for linearly independent basis function is simplified by means of group theoretical considerations.
Hall transformation of exp( – r): Correction of the results
Authors: Ten Hoor Marten J. 
Published: January 2005

AbstractOwing to an error in his determination of the repulsion integral and the kinetic energy. Hall's results [1] for the ground state of atomic two‐electron systems are wrong. Here these results have been corrected. For Z ≤ 5, the results have been obtained by determining the pertinent integrals numerically: but for Z ≥ 5, these can be found sufficiently accurately from Z expansions.
Linear integrability of wave functions
Authors: Koga Toshikatsu , Thakkar Ajit J. 
Published: January 2005

AbstractFiniteness is usually imposed as a condition for physical admissibility of a wave function. Examination of this condition in both position‐ and momentum‐space shows that finiteness of the wave function at the origin in one space implies “linear” integrability in the reciprocal space, except in some pathological cases. Some implications of this result are discussed.
Extrapolation of the total energy of polymers to the bulk limit using generalized padé approximants
Authors: Cioslowski J. , Bessis D. 
Published: January 2005

AbstractGeneralized Padé approximants are used to extrapolate the total energy of polymers described by alternant Hamiltonians to the bulk limit. The method provides an upper bound to the energy. The origin of (quasi) periodic oscillations of the energy per unit cell as the function of the number of unit cells is enlightened through analysis of the moments of the Hamiltonian.
Optical activity of small peptide chains with right‐handed α‐helix structure
Authors: Volosov Andrey 
Published: January 2005

AbstractThe optical activity of oligopeptides in the conformation of the right‐handed α‐helix was calculated by the direct semiempirical quantum chemical CNDO/OPTIC method. The oligomers of glycine and alanine from dimer up to pentamer were considered. The comparison with results obtained using the model of interacting groups (MIG) based on the perturbation approach was carried out. The calculational results show that the circular dichroism (CD) of oligopeptide α‐helices is essentially different from the CD of the peptide polymers in the same conformation. The comparison between results obtained by the CNDO/OPTIC method and by the MIG leads to doubts about the reliability of the use of the MIG to calculate rotatory strengths of nπ* transitions of oligopeptides in α‐helix conformation.