bluuebunny/crossref_metadata_2025 huggingface dataset:
Reaction field effects on the electronic structure of carbon radical and ionic centers
Authors: Karelson Mati , Tamm Toomas , Katritzky Alan R. , Szafran Miroslaw , Zerner Michael C.
Published: January 2005

We examine solvent effects on carbon radical and ionic centers of HCXY by including a self‐consistent reaction‐field into the AM1 and MNDO electronic structure models to mimic dielectric effects. We find that such concepts as merostability are principally solvent effects, and that, as expected, molecules with large dipoles or with charge assymmetry are stabilized more by solvent than those with atoms that are more electrically neutral. Of some importance in this study is the finding that conformation is also dependent on solvation and that change in geometry must be considered if an accurate estimate is to be made of energy differences such as those examined in the calculations of merostabilization.
Coherence in disordered condensed matter. III: H+ and OH− ionic conductance and proton transfer reactions in aqueous solutions
Authors: Chatzidimitriou‐Dreismann C. A. , Brändas E. J.
Published: January 2005

The physical context of the high ionic conductivity of the H+ and OH− ions in aqueous solutions is considered. Quantum dynamical aspects of the mobility of these ions are investigated, especially within the framework of the complex scaling method (CSM). It is argued that the high ionic mobilities under consideration are due to quantum delocalization of the ions as well as mass interference of the H+ ions with water protons. This effect is formally described with the aid of the coherent‐dissipative structures, which become spontaneously created in the solution owing to the thermal motion. In particular, (1) a novel connection between the proton transfer rate (as measured by NMR) and the mobilities of the considered ions (as determined by conductivity measurements) is derived. Additionally, (2) a relation connecting the activation energies of the above reaction rates with the ionic mobilities is found. The above relations contain no fitting parameters. Comparisons with experimental results is made, which also reveals the predictive power of the present CSM treatment in the field of microdynamical processes in condensed phases.
Theoretical study of transitions involving three rydberg series in two‐valence electron systems: MgI and its electronic sequence
Authors: Martin Inmaculada , Barrientos Carmen , Gutiez Inmaculada
Published: January 2005

Oscillator strengths for transitions involving levels of three spectral series, 3s2 1S → 3snp1P0, 3s3p 1P0 → 3snd 1D, and 3s3p 1P0 → 3sns 1S, have been computed and analyzed for several ions of the MgI isoelectronic sequence (MgI‐TiXI). The quantum defect orbital (QDO) formalism has been used throughout, and the results are comparable to those of more complex theoretical procedures.
Quasiparticle band structure and exciton spectrum of hexagonal boron nitride using second‐order Møller‐Plesset many‐body perturbation theory
Authors: Vračko M. , Liegener C.‐M, , Ladik J.
Published: January 2005

Many‐body perturbation theory up to second‐order in the Møller‐Plesset partitioning has been used to calculate the quasiparticle band structure of hexagonal boron nitride, treated as a periodic two‐dimensional system. It was found that correlation leads to an essential narrowing of the fundamental gap. The exciton spectrum has been calculated using the first order irreducible vertex part with Hartree‐Fock and quasiparticle band structure data. It was found that the exciton binding energy amounts to about 10% of the fundamental gap (for the first singlet excitation).
From PPP‐MO theory to all‐valence electron calculations of ionic and excited states in organic molecules
Authors: Montero Luis A. , Alfonso Liz , Alvarez J. Raul , Perez Eduardo
Published: January 2005

PPP MO theory initiated in 1953 a reliable SCF approach to excited states in multielectronic systems. Further developments, such as Del Bene‐Jaffé's CNDO/S method, modified Pople's CNDO all‐valence electron (AVE) approach to improve the results but maintained some basic troubles of this theory which were not in the PPP method. A new approach to SCF‐MO‐NDO calculations with a priori chosen parameters and configuration interaction of singly excited states is developed in this paper from the ground considerations of the PPP method and it is called CNDOL. Thus, AVE NDO approximations have been modified in order to take into account independent monocentric terms in the Fock matrix elements for each kind of azimuthal quantum number atomic orbitals in the basis set. The only input data are Slater exponents, valence state ionization potentials, and electron affinities of each kind of atomic orbital. No parameters are empirically adjusted to fit results to experimental or previous accurate theoretical data. CNDOL is suitable for obtaining theoretically consistent results in polyatomic systems which are too large to be treated by more sophisticated procedures. Applications to hydrocarbons are shown which compare PPP results with the new AVE approach derived from it here.
The correlation potential for two‐electron atomic ions
Authors: Davidson Ernest R.
Published: January 2005

The correlation potential is computed for two electron atomic ions with atomic numbers from 1 to 10 using the charge density reconstructed from a natural orbital expansion of a Kinoshita‐like atomic wave function. Over the wide range of densities involved, the correlation potentials are not even approximately a local function of the density.
Extrapolation of finite cluster and crystal‐orbital calculations on trans‐polyacetylene
Authors: Weniger E. J. , Liegener C.‐M.
Published: January 2005

Sequences of total energies of trans‐polyacetylene were obtained by either finite cluster or crystal‐orbital calculations. Several different extrapolation methods were employed to improve the convergence of these two sequences. The properties of the extrapolation algorithms are discussed with emphasis on efficiency and reliability for short strings of physical input data. Four decimal digits were gained by extrapolation in the case of the finite cluster energies, but, at most, 2 decimal digits in the case of the crystal‐orbital energies. The extrapolation of the finite cluster energies gave results that are almost as good as the results produced by the extrapolation of the crystal‐orbital energies, and that are better than the unextrapolated crystal‐orbital energies.
The TST–CEQ calculations on the heavy‐light‐heavy Cl + HCl reaction
Authors: Ju Guan‐Zhi , Chen De‐Zhan
Published: January 2005

In this paper, the TST–CEQ calculated results including the reactive cross sections and rate constants for the Cl + HCl reaction at the specified vibrational state of the HCl are presented. It is shown that the oscillatory behavior of the system in the three‐dimensional world does not appear, and the larger the vibrational quantum number of HCl, the smaller the cross sections ¯QTST–CEQ (E, ν) are at a given total energy used in the calculations, but the larger the reactive rate, constants KTST–CEQ (T, ν) are at a given temperature. The KTST–CEQ (T, v) is the same as when first published, and the average reactive rate constants KTST–CEQ (T) are close to the ones by Garrett and Schatz et al.
Theoretical description of nuclear quadrupole coupling in light diatomic molecules
Authors: Vojtík J. , Paidarová I. , Špirko V. , Śavrda J. , Petráš M.
Published: January 2005

A practical procedure for calculation of nuclear quadrupole coupling constants for light diatomic molecules is discussed. The procedure is based on a molecular wave function that explicitly describes nuclear motion. The approach is capable of yielding quadrupole coupling constants for excited rovibrational levels of diatomic molecules in their ground and excited electronic states. An application of the procedure to the X1Σ+g and B1Σ+u states of HD and D2 is presented.
Variational calculations of the Wigner distribution function for selected anharmonic oscillators
Authors: Włodarz Joachim J. , Nowakowski Janusz
Published: January 2005

The quantum (ħ =1) and semiclassical (ℏ → 0) Wigner distribution functions were calculated for some anharmonic oscillators using a direct phase‐space variational method.
Squeezing transformation caused by frequency jumps for solving the problem of two coupled oscillators
Authors: Fan Hong‐Yi
Published: January 2005

For the Hamiltonian
equation image
a unitary operator that is closely related to the squeezing transformation caused by frequency jumps is found and its coordinate representation provides an alternative approach for calculating the wave function of the eigenstate of ℋ︁, whereas its normally ordered form is convenient for deriving the resolvent kernel in a coherent‐state basis.
Sur une tentative d'unification des théaories quantiques de la cancérisation par les polyacènes. II: Le rôle de la rèagion K dans le processus d'activation metabolique conduisant au cancérogène ultime. Théorie des régions M, L, et BK
Authors: Gayoso Jose , Kimri Said
Published: January 2005

This study is a unified approach to quantum theories of polyacen carcinogenesis. Part II is on the role of the K‐region in metabolic activation process leading to ultimate carcinogen and discusses the M, L, and BK theory. The saturation of the polyacen K‐region, or the transformation into a quinoid one, deactivates the carcinogenic process going through a diol epoxyde, which results in Bay‐region carbocation. K and B regions must be considered as two interdependent parts of the same structural entity, which may be termed the BK‐region. The M, L, and BK theory achieves a complete unification of the Pullman and Jerina and other theories.
Transition structures and energy barriers of pericyclic reactions in the CASSCF approach
Authors: Janoschek Rudolf , Kalcher Josef
Published: January 2005

Five energy hypersurfaces of the most examined pericyclic reactions have been investigated by using ab initio SCF, CASSCF, and the semiempirical AM1 methods. The systems are H4, H6, C3H6, C4H4, and C3OH4. Stationary points and their sets of harmonic vibrational frequencies have been calculated by means of analytical gradient techniques in the frameworks of the respective approaches. ZPE corrected energy barriers are based on single‐point calculations including dynamical correlation corrections by CAS‐CI(SD)+DC, CASCEPA, or MP2.
Covalent and ionic contributions to the bonding of atomic and molecular adsorbates on metal surfaces: A cluster model approach
Authors: Pacchioni G. , Bagus P. S. , Philpott M. R. , Nelin C. J.
Published: January 2005

The molecular cluster model approach for the description of processes at surfaces is presented. It involves using finite clusters that contain surface atoms interacting with atomic or molecular adsorbates. Accurate ab initio wave functions can be obtained for the clusters, and, thus, this makes it possible to describe the bonding in terms that are familiar for molecular systems. The adsorbate‐substrate interaction can be analyzed by new techniques developed to characterize the bonding as covalent or ionic. This is shown for F on Ag(111) and O and CO on Cu(100) systems. The measures of adsorbate ionicity are (a) the expectation value of a projection operator giving an indication of the number of electrons associated with the adsorbate, (b) the analysis of the dipole moment curve as function of the distance, and (c) the effect of an external electric field on the adsorbate geometry. The consequences of ionic adsorbates on metal surfaces is discussed for two selected topics: adsorption of thiocyanate on Ag electrodes and coadsorption of CO with ionic species.
Point group symmetry and cartesian force constant redundancy
Authors: Stanton John F.
Published: January 2005

A procedure for identifying redundancy in the Cartesian force constant matrix is described, and a prescription is given for generating the entire matrix of second derivatives from the minimal set of information. A proof is supplied which demonstrates that the number of nonredundant rows corresponding to a symmetry unique atom is necessarily less than three if the atom is invariant with respect to a symmetry operation of the point group other than the identity or inversion. Furthermore, only one row is required if the atom lies on a threefold or higher‐order rotation axis. An application of the procedure to the evaluation of harmonic force constants by numerical differentiation of gradient vectors is briefly described.
The rapid estimate of a gauss integral
Authors: Jiancheng Xuan , Shouping Jiang
Published: January 2005

We propose a fomula that directly gives an estimate of the value of a Gaussian integral through only one multiplication. Using this formula, we may neglect about 60%–80% of integrals in quantum chemical ab initio calculations, without any loss in precision theoretically. Such a large molecule as ATP with 183 STOS using the STO‐2G method has been calculated utilizing this technique, and only 31 CPU hours of VAX 11/730 were spent on integrals.
Band structure and electronic properties of lithium azide (LiN3)
Authors: Seel Max , Barry Kunz A.
Published: January 2005

Ab initio Hartree–Fock band structure and molecular calculations have been performed to study the electronic structure of LiN3 in a monoclinic C 2/m crystal structure. The total energy, band structure, density of states, and charge densities are computed. The calculated lattice energy (energy to separate the ions infinitely apart) of 8.6 eV agrees very well with 8.45 eV deduced from Madelung and London polarizability energies. The calculated split of the N 1s core bands of 5.0 eV compares favorably with the experimental X‐ray photoelectron value of 4.4 eV. This good agreement is not contributed to crystalline environment effects as proposed in earlier MO studies of N where the best values obtained were 5.1, 5.8, and 6.3 eV, but to the quality of the nitrogen core basis set. The calculated valence density of states supports one of two competing interpretations that peak III observed in the X‐ray photoelectron spectrum arises from contaminations or other extrinsic states.
Collision‐induced emission of O2(b1Σ → a1Δg) in the gas phase
Authors: Fink E. H. , Setzer K. D. , Wildt J. , Ramsay D. A. , Vervloet M.
Published: January 2005

In flow tube studies of the quenching of O2(b1Σ), broad band emission of O2(b):M collision complexes was found to appear under the discrete rotational lines of the 0–0 band of the b1Σ → a1Δg electric quadrupole transition at higher oxygen pressures and on addition of foreign gases. Bimolecular rate constants for the collision‐induced emission processes have been derived from the ratio of the intensities of the discrete lines and the continuum as well as from low‐resolution measurements of the relative intensities of the b → a and b → X bands as a function of O2 and added gas pressure. They range from ≈10−21 cm3 s−1 for He to ≈4 × 10−19 cm3 s−1 for PCl3 vapor.
Polarization propagator calculation of spectroscopic properties of molecules
Authors: Oddershede Jens , Sabin John R.
Published: January 2005

We propose 10 desirable features or criteria for general purpose methods to be applied to the calculation of a range of spectroscopic properties. We discuss the second‐order polarization propagator approximation (SOPPA) in light of these criteria by giving the actual computational expression used as well as a few numerical examples taken from the theory of NMR spectra (magnetic shieldings and spin‐spin coupling constants). We demonstrate that SOPPA comes close to fulfilling these criteria.
A realistic model for curve crossing in diatomic molecules
Authors: Broeckhove J. , Lathouwers L.
Published: January 2005

We present a model system for the study of curve‐crossing situations in diatomic molecules. All model parameters are determined by the potential energy topology. No coupling functions are fitted. The standard approximation schemes (Born–Oppenheimer approximation, adiabatic approximation, and the generator coordinate approximation) are then applied. The result of these numerical experiments suggest that the generator coordinate approximation is able to remove 80%–90% of the nonadiabatic effect on all levels throughout the crossing region.
The exact form of the Pauli potential for the ground state of two‐ and three‐level atoms and ions
Authors: Nagy A. , March N. H.
Published: January 2005

The square root of the electron density satisfies a Schrödinger equation having a correction term, called the Pauli potential, in addition to the Kohn–Sham potential. The Pauli potential is given in exact form for the ground state of two‐ and three‐level atoms and ions (e.g., Be and C). Expressions for the kinetic energy and the one‐body virial are also derived.
Improved quadrature methods for three‐center nuclear attraction integrals with exponential‐type basis functions
Authors: Homeier Herbert H. H. , Otto Steinborn E.
Published: January 2005

The numerical properties of a two‐dimensional integral representation [J. Grotendorst and E.O. Steinborn, Phys. Rev. A 38, 3857 (1988)] of the three‐center nuclear attraction integral with a special class of exponential‐type orbitals (ETO'S), the B functions [E. Filter and E.O. Steinborn, Phys. Rev. A 18, 1 (1978)] are examined. B functions span the space of ETO'S. The commonly occurring ETO'S can be expressed in terms of simple finite sums of B functions. Hence, molecular integrals for other ETO'S, like the more common Slater‐type orbitals, may be found as finite linear combinations of integrals with B functions. The main advantage of B functions is the simplicity of their Fourier transform that makes the derivation of relatively simple general formulas for molecular integrals with the Fourier transform method possible. The integrand of the integral representation mentioned above shows sharp peaks causing, in the case of highly asymmetric charge distributions, slow convergence of the quadrature method used by Grotendorst and Steinborn. New quadrature schemes are presented that use quadrature rules based on Möbius transformations. These rules are well suited for the numerical quadrature of functions that possess a sharp peak at or near a single boundary of integration [H.H.H. Homeier and E.O. Steinborn, J. Comput. Phys., 87, 61 (1990)]. Numerical results are presented that illustrate the fact that convergence of the new quadrature schemes is about a factor two faster in case of highly asymmetric charge distributions.
Maximum overlap symmetry orbitals
Authors: Zhan Chang‐Guo , Zheng Fang
Published: January 2005

Based on the principle of maximum overlap, a new simple method is suggested for constructing the symmetry orbitals of arbitrary molecules and the delocalized molecular orbitals of molecules that do not involve the rings with an odd number of atoms. All these orbitals, called “maximum overlap symmetry orbitals,” are determined by an extended maximum overlap criterion and form the bases for the irreducible representations of the molecular point symmetry group. The theoretical analysis and the numerical results show that the obtained molecular orbitals are close to those obtained from the customary LCAO method, and calculation by the proposed method requires less computing time than does the LCAO method, thus illustrating a fact that the method is not only a reasonable approximation of the LCAO method, but simpler and feasible in large molecular systems.
Construction of orthogonal subspaces
Authors: Banerjee Ajit
Published: January 2005

An orthogonalization procedure is presented that allows construction of at least (n−m) vectors orthogonal to {Xj}, j equals; 1, m, by linear combinations solely among {ηi}, i equals; 1, n, n>m, and 〈Xj/ηi〉≠0. An important application of the procedure is in effective core potential methods for which valence orbitals can be constructed that are orthogonal to the core orbitals and yet involve no component of the core. Thus, a separate calculation for only the valence electrons can be performed without any explicit reference to the core electrons (orbitals).
A density functional for the correlation energy, deduced in the framework of the correlation factor approach
Authors: Moscardó Federico , San‐Fabián Emilio
Published: January 2005

An approximate density functional is deduced from a wave function within the correlation factor method. The new functional does not include terms depending on the gradient of the density, but shows the simplicity of local density functionals without spin polarization. However, it includes correctly the inhomogeneity effects and, also, the nonlocal nature of an electronic system. The approach adopted here stresses the goodness of the expression taken by Colle and Salvetti for building a correlation factor and, at the same time, allows us to gain light on the nature of the deficiencies of those functionals obtained, up to now, from the perspective of the Hohenberg and Kohn theorem.
Quantum rigid dipole in a permanent electric field. I. Rigorous treatment
Authors: Kryachko Eugene S. , Yanovitskii Oleg E.
Published: January 2005

A behavior of a quantum rigid and pointlike dipole in a permanent electric field is under study. A dependence of a number of librational energy levels on the strength of the electric field is investigated on the basis of some rigorous results for the appropriate normal Jacobian matrices.
Atoms and electron‐pair bonds in a molecule: An approach within the context of extended geminal models
Authors: Røeggen I.
Published: January 2005

Within the framework of extended geminal models, the meaning of the electron‐pair bond concept is substantiated. If there exist two‐centered electron‐pair bonds in a molecule, it is demonstrated that the molecule can be partitioned into neutral fragments. The total electronic energy can then be written as a sum of intra‐ and interfragment energies. If the electron‐pair bonds are only on the two‐center type, the advocated procedure leads to the definition of an atom in a molecule. As test cases, the bonding in H2 and HF, the difference in equilibrium geometries of H2O and H2S, and the origin of the ethane rotational barrier are analyzed.
Quantum localization over a potential barrier
Authors: Atabek O. , Lefebvre R. , Sucre M. Garcia , Gomez‐Llorente J. , Taylor H.
Published: January 2005

The ability of a harmonic barrier to support resonances is examined. Stabilization graphs calculated from box quantization have the features expected for resonances at an energy equal to the barrier maximum. The search for a wave packet of maximum lifetime leads also to this energy. These findings agree with the energies associated with the poles of the S matrix.
First‐Order geometrical response equations for state‐averaged multiconfigurational self‐consistent field (SA‐MCSCF) wave functions
Authors: Bak Keld Lars , Boatz Jerry , Simons Jack
Published: January 2005

The first‐order geometrical response equations for a state‐averaged multiconfigurational self‐consistent field (SA‐MCSCF) calculation are derived. This derivation is carried through from first principles to final working equations suited for computer implementation. The final equations are expressed such that the energies and wave functions must be known only for the internal SA‐MCSCF states. In the derivation, the special but important case where two or more internal states have equal weighting factors is treated in a manner fully consistent with all other cases. Except for introducing extra equations that are straightforward to solve, the case where two or more internal states have equal weighting factors introduces no new complications.
Ab initio MRD–CI calculations on cubane (neutral, carbocation, carboanion) and dissociation of nitrocubanes based on localized orbitals
Authors: Chapman Douglas A. , Kaufman Joyce J. , Buenker Robert J.
Published: January 2005

Ab initio MRD–CI calculations based on localized orbitals were carried out for cubane (neutral, carbocation, carboanion) both in our customary MODPOT basis set and in an all‐electron 4–31G basis set. The calculated MRD–CI charge distributions on C1 (the skeletal atom from which the H− or H+ was removed) (ab initio MODPOT neutral 4.221, carbocation 3.796, carboanion 4.282; all‐electron 4–31G neutral 6.171, carbocation 5.717, carboanion 6.078) indicate that the + or ‐ charge does not remain localized on C1 but redistributes itself. This has significant implications for preparative reactions of energetically substituted cubanes. The MRD–CI population analyses differ somewhat from the SCF population analyses, especially in the calculated total overlap populations. To investigate this effect on electrostatic molecular potential contour (EMPC) maps generated from SCF or MRD–CI wave functions, we wrote additional routines to calculate EMPC maps from MRD–CI wave functions. The EMPC maps generated from SCF or MRD–CI wave functions are different if the molecule needs an MRD–CI multideterminant wave function to describe it adequately. The EMPC map is a one‐electron property. One‐electron properties are derived from the 1‐matrix. The 1‐matrix is different for SCF or MRD–CI wave functions. Thus, all the one‐electron properties (EMPC maps, population analyses, bond deviation indices, etc.) are different when calculated from SCF or MRD–CI wave functions if MRD–CI wave functions are necessary to describe a system properly. We calculate these one‐electron properties from the 1‐matrix from the final natural orbitals. Our preliminary calculations for the dissociation pathway indicate it takes more energy to dissociate a bond in 1‐nitrocubane than in octanitrocubane. Even in their ground electronic states at equilibrium geometry, both 1‐nitrocubane and octanitrocubane require MRD–CI wave functions to describe them properly. The c2 of the single determinant SCF wave function is only 0.8401 for 1‐nitrocubane and 0.8300 for octanitrocubane. There are contributions from skeletal excitations as there are for cubane itself as well as excitations involving the nitrogroup. As the bond in nitrocubane is dissociated to 8.00 bohrs, the c2 of the SCF contribution drops to only 0.4606 (1‐nitrocubane) and 0.4445 (octanitrocubane). At this C1N1 intermolecular distance, the largest excitations are in the C1N1 bond: (C1N1)2 → (C1N1*)2, (C1N1) → (C1N1*). We also calculated the first electronically excited state for the dissociation pathway for selected points for both 1‐nitrocubane and octanitrocubane.
Calculating energy spectrum of one‐dimensional quantum systems with rational potential P(X)/Q(X)
Authors: Rivas‐Silva J. F. , Campoy G. , Palma A.
Published: January 2005

The solution of the Schrödinger equation for rational potentials is given using an extension of a previous numerical method. The comparison with other methods shows that ours is a good alternative to those that use the Taylor series approximation of wave function. We illustrate the algorithm considering the Mitra and the Newton–Thomas potentials as examples.
The electronic structure of parallel β‐pleated sheets in proteins: An ab initio computation including electron correlation
Authors: Suhai S.
Published: January 2005

The electronic structure of a 2D polyglycine network with a pleated sheet structure has been computed at the Hartree–Fock level and by including electron correlation effects within the second order of many‐body perturbation theory (electron polaron model). The influence of the size of the atomic basis set and of the extension of the virtual space has been investigated both for single‐ and many‐particle properties. Comparison with the energy of the corresponding single chains showed that interchain interactions (mainly hydrogen bonding) provide an extra stabilization for the 2D network by 7.4 and 10 kcal/mole per glycine residue at the Hartree–Fock and correlated levels, respectively. The energy dispersions are rather anisotropic for all bands whose widths are about 0.5–1 eV along the polypeptide backbones and 0.1–0.2 eV in the perpendicular direction (hydrogen bonds). The HF value of the fundamental energy gap is reduced by 4 eV to 9.2 eV for electron polarons. The wave functions and interaction integrals obtained can be used to calculate further optical and lattice vibrational properties.
Ab initio SCF calculations on [V10O28]6−: A benchmark for the classical calculation and processing of molecular integrals on large Gaussian basis sets
Authors: Rohmer Marie‐Madeleine , Ernenwein René , Ulmschneider Michel , Wiest Roland , Benard Marc
Published: January 2005

A vector efficient implementation of the McMurchie and Davidson algorithm for the calculation of one‐ and two‐electron molecular integrals is presented, as available in the Cray version of the ASTERIX program system. The implementation and performance of a vector‐oriented strategy for the generation and processing of the P supermatrix is also discussed. This program system has been applied to the ab initio SCF computation of the ground‐state wave function for the [V10O28]6− ion, with a basis set of triple‐zeta quality for the valence shell of oxygen generating 1404 GTOS and 574 CGTOS for the complete system. The performance and the bottlenecks of the integral calculation are discussed as a function of the integral classes. Two‐dimensional maps of the electrostatic potential are presented for this molecule and compared to experimental information about proton fixation.
A contracted bromine basis set for use in calculation of molecular energies
Authors: Curtiss L. A. , Binning R. C.
Published: January 2005

A contracted [9s6p2d] basis set derived from Dunning's (14s11p5d) primitive Gaussian set for bromine has been used in ab initio molecular orbital calculations of the dissociation energies of HBr, CH3Br, and Br2, the ionization potentials of Br and HBr, and the electron affinity of Br. The calculated energies are within 0.1 eV of the experimental values. This is similar to the accuracy obtained in a previous study, also using a contracted [9s6p2d] basis set, of the dissociation and ionization energies of the GeHn, AsHn, and SeHn hydrides.
Ab initio relativistic effective potentials with spin‐orbit operators. VI. Fr through Pu
Authors: Ermler W. C. , Ross R. B. , Christiansen P. A.
Published: January 2005

Ab initio averaged relativistic effective core potentials (AREP), spin‐orbit (SO) operators, and valence basis sets are reported for the elements Fr through Pu in the form of expansions in Gaussian‐type functions. Gaussian basis sets with expansion coefficients for the low‐energy states of each atom are given. Atomic orbital energies calculated under the j–j coupling scheme within the self‐consistent field approximation and employing the AREP'S in their unaveraged form (REP'S) agree to within 10% of orbital energies due to numerical all‐electron Dirac–Fock calculations. The accuracy of the AREP'S and so operators is also shown to be good through comparisons of calculated so splitting energies with all‐electron Dirac–Fock results.
The COLUMBUS Standard Integral File Structure: A proposed interchange format
Authors: Shepard Ron
Published: January 2005

The integral file structure used in the COLUMBUS Program System is described. This file structure is proposed for use as an interchange format for the exchange of information between various electronic structure codes. Access to the integral and density matrix arrays stored in the file structure is simplified by a supporting subroutine library. This library is portable across various computers and is readily available to programmers from the COLUMBUS distribution files. This library provides for the efficient processing of individual records, including the use of asynchronous I/O and the vectorized processing of packed orbital labels. The individual arrays are identified in a self‐defining and extensible manner, allowing for the addition of new integral types as demanded by the application. The format of the individual records is also self‐defining, allowing for the use of various packing and data compression methods within each record without burdening the calling program with unnecessary complications.
Hyperspherical Strumian basis functions
Authors: Avery J. , Herschbach D. R.
Published: January 2005

A Sturmian basis set is a set of solutions to the Schrödinger equation, with the potential scaled in such a way that all the members of the set correspond to the same value of the energy. We discuss, in particular, the set of Sturmian basis functions corresponding to solutions of the d‐dimensional hydrogenlike wave equation. These hydrogenlike Sturmian functions are expressed in terms of Laguerre polynomials and hyperspherical harmonics. When they are used as a basis for solving the many‐particle Schrödinger equation, the secular equations take on a simple form [Eq. (59)]. The necessary integrals are evaluated explicitly, and the possibility of combining the hyperspherical technique with dimensional scaling is discussed.
The second‐order energy contribution from the spin–orbit interaction operator to the potential energy curve of Cr2
Authors: Vahtras Olav , Ågren Hans , Jørgensen Poul , Jørgen Hans , Jensen Aa. , Helgaker Trygve
Published: January 2005

The second‐order Rayleigh–Schrödinger energy correction to the Born–Oppenheimer potential energy due to the spin–orbit interaction can be expressed as a linear response function evaluated at zero frequency. We have calculated this energy contribution to the Cr2 singlet ground state X1 Σ+g potential energy function for a multiconfiguration self‐consistent field (MCSCF) wave function. The calculations show that the effect of spin–orbit interaction is small and of the same magnitude for the whole potential energy curve.
Ab initio studies of the electronic structure and density of states of metallic beryllium
Authors: Ross R. B. , Ermler W. C. , Kern C. W. , Pitzer R. M.
Published: January 2005

Hartree–Fock–Roothaan studies are reported for low‐lying electronic states of metallic beryllium as modeled by a moiety of 135 beryllium atoms. The system corresponds to 16 coordination shells of a central Be with internuclear separations derived from the lattice constants of the bulk metal. The calculations become tractable by use of the full D3h symmetry of the system at both the integrals and self‐consistent‐field stages and by employing ab initio effective potentials for the 1s electrons of each beryllium atom. Ionization potentials, binding energies, orbital energies, electric field gradients, nuclear‐electrostatic potentials, diamagnetic shielding constants, second moments, and Mulliken populations are calculated for selected electronic states. The calculated ionization potential for the lowest state agrees to within 10% of the experimental bulk work function. A density‐of‐states analysis for that state is reported and compared with band structure calculations.
Löwdin α‐function, overlap integral, and computer algebra
Authors: Jones Herbert W.
Published: January 2005

A commercial computer algebra program, Mathematica, is used to generate the C matrix that characterizes our implementation of the Löwdin α‐function method as applied to Slater‐type orbitals. An example of a two‐center overlap integral is done to show how the arbitrary precision capability of Mathematica can overcome severe cancellation errors encountered with programming in FORTRAN. This strategy is capable of being generalized to other multicenter molecular integrals. Mathematica programs are included.
CH Bond dissociation of acetylene: Local density functional calculations
Authors: Habibollahzadeh Dariush , Murray Jane S. , Grodzicki Michael , Seminario Jorge M. , Politzer Peter
Published: January 2005

The CH bond dissociation energy of acetylene was computed by both ab initio approaches and density functional theory in a local density approximation (DFT–LDA). Structures and energies for acetylene and its dissociation products (the ethynyl and hydrogen radicals) are presented and compared. Using directly computed HCCH and HCC· energies and the exact H· value, the DFT–LDA calculations are found to yield CH dissociation energies ranging from 129 to 131 kcal/mol, in good agreement with recent experimental and the highest level theoretical results. The DFT–LDA results show little dependence upon the computational procedure used to obtain geometries.
Electronic states and nature of the chemical bond in the molecule CrC by all‐electron ab initio calculations
Authors: Shim Irene , Gingerich Karl A.
Published: January 2005

All‐electron ab initio Hartree–Fock (HF), valence configuration interaction (CI), and multiconfiguration self‐consistent‐field (CASSCF) calculations have been applied to investigate the electronic states of the CrC molecule. The molecule is predicted as having four low‐lying electronic states, 3∑−, 5∑−, 7∑−, and 9∑−, separated by an energy gap of 0.55 eV from the next higher‐lying state, 1∑−, which is followed by the states 5Π and 7Π. The four lowest‐lying electronic states are due to the coupling of the angular momenta of the 6Sg Cr+ ion with those of the 4Su C− anion. The chemical bond in the 3∑− ground state can be viewed as a quadruple bond composed of two σ and two π bonds. One σ bond is due to the formation of a molecular orbital that is doubly occupied. The remaining bonds, i.e., one σ and two π bonds, arise from valence‐bond couplings. The π bonds originate from the valence‐bond couplings of the electrons in the C 2pπ orbitals with those in the Cr 3dπ orbitals. The σ bond originates from the valence‐bond coupling of the C 2pσ electron with an electron in the Cr 4s, 4p hybrid that is polarized away from the C atom.
Implications of unitary invariance for gradient theory
Authors: Jayatilaka Dylan , Handy Nicholas C.
Published: January 2005

This paper shows how the invariance of the energy to unitary rotations in the occupied orbitals and in the virtual orbitals can be used to determine some elements of the derivative molecular orbitals, which are important for gradient theory. Solution of the usual coupled‐perturbed Hartree–Fock equations for these elements is thus unnecessary, eliminating considerable effort. We also give proofs which show clearly that special conditions are required.
Shape similarity and shape stability along reaction paths: The case of the PPO → OPP isomerization
Authors: Luo Xincai , Arteca Gustavo A. , Mezey Paul G.
Published: January 2005

An analysis of molecular shape has been performed along an ab initio reaction path of the PPO → OPP isomerization. In a first approximation, we have used the fused‐sphere (van der Waals) model to represent shape. The results show that there is no one‐to‐one correspondence between the species defined in terms of shape and those based on an energy criterion. In contrast to the cases of most other isomerizations in triatomic molecules (e.g., the HCN isomerization), for the PPO isomerization there exists a stable intermediate. For the elementary reaction from the linear PPO (C∞v, reactant) to the cyclic P2O (C2v, intermediate), we found no distinct molecular shape type for the transition structure (between the reactant and the intermediate). In terms of shape, the nuclear configuration of the transition structure is more similar to that of the intermediate than that of the reactant. Using the PPO → OPP reaction as an example, in this work we discuss the implications of this result regarding possible measures of shape similarity and shape stability along reaction paths. We have performed a molecular‐shape analysis of the electron density contour surfaces to test the reliability of the fused‐sphere approximation. A comparison between the van der Waals and electron isodensity surface models at critical points shows that the former model can be qualitatively correct for molecular‐shape descriptions along some reaction paths.
Monte Carlo determination of the oscillator strength and excited state lifetime for the Li 22S → 22P transition
Authors: Barnett R. N. , Reynolds P. J. , Lester W. A.
Published: January 2005

A recently developed Monte Carlo method is used to compute the 22S → 22P transition dipole moment of Li. This approach employs a guided Metropolis random walk with quantum Monte Carlo “side” walks to sample the required probability distributions. The transition dipole moment is employed to obtain the oscillator strength and excited‐state lifetime. Our most accurately converged calculations yield an oscillator strength of 0.742(7) and excited‐state lifetime of 27.41(35) ns. These results are in excellent agreement with precise experimental measurements of 0.742(1) and 27.29(4) ns, respectively. In addition, single‐state expectation values are computed for both states. Monte Carlo parameters, such as the time step size and the convergence time, are varied in order to study their effect on computed results.
Spline methods for multiconfiguration Hartree–Fock calculations
Authors: Fischer Charlotte Froese , Guo W. , Shen Z.
Published: January 2005

The earlier numerical multiconfiguration Hartree–Fock atomic structure package was not designed with high‐performance computers in mind. In this paper, some new algorithms based on spline–Galerkin methods are described that are appropriate for concurrent/vector architectures. The goal is to improve the level of numerical accuracy by several orders of magnitude using fewer basis functions than points in a numerical grid. Of critical importance is the robustness of the code: The most serious problems in the numerical implementation were associated with orthogonality constraints. In a spline basis approach, the orthogonality requirements can be integrated into quadratically convergent update procedures. These procedures are evaluated for a number of cases.
A heuristic potential function for intramolecular and intrasupermolecular chemical reactions
Authors: Nagaoka Masataka , Okuno Yoshishige , Yamabe Tokio , Fukui Kenichi
Published: January 2005

A heuristic fitting procedure to obtain an analytical potential function for describing a reactive potential energy surface in the neighborhood of the intrinsic reaction coordinate (IRC) has been developed. For discussion, the pairwise potential function form, ∑anr−n, is assumed in order to fit ab initio quantum mechanical calculations of intramolecular (or intrasupermolecular) interaction energies and its use is found advantageous because all the calculation can be carried out by the linear least squares method. Normal modes perpendicular to IRC are utilized to prepare an initial data base for the potential fitting in the neighborhood of IRC. Some trial molecular dynamics (MD) simulations are performed in order to check the fitted potential function and, unless they lead to reasonable energies within the tolerance assumed, their results are utilized to construct an improved data base (the dynamic sampling). The present systematic optimization procedure has been applied to the proton transfer reaction of the formamidine–water (FW) system. The normal mode analysis in both the transition state (TS) and the stable state (SS) regions suggests that the present fitted potential function can reproduce satisfactorily the Born–Oppenheimer (BO) adiabatic surface obtained by ab initio molecular orbital (MO) calculations. We conclude that our procedure works well for the chemical reaction molecular dynamics (CRMD) simulation.
Ab initio calculations of the ground‐state potential energy surface for the CF bond decomposition in n‐fluoropropane within the method for approximation of the frozen molecular fragment
Authors: Roszak S. , Kaufman Joyce J.
Published: January 2005

The recently proposed ab initio method for calculation on many‐electron molecular systems with the approximation of the inactive part of a molecule by a frozen molecular fragment was tested further in a case of the dissociation reaction of the CF bond in n‐fluoropropane. Results from the Hartree–Fock, multiple reference double‐excitation configuration–interaction and second‐order Møller–Plesset methods are presented. The reproduction of potential energy surfaces as well as the reproduction of electron density distribution are in excellent agreement with extended basis‐set calculations. Different choices of fragments to be frozen have been examined.
Molecular quantum similarity measures and N‐dimensional representation of quantum objects. II. Practical applications
Authors: Carbó R. , Calabuig B.
Published: January 2005

Newly developed classification and graphical representation techniques are discussed in order to visualize quantum similarity computational results. Some examples of molecular quantum similarity applications involving the fields of (a) structure–property relationships, (b) structure–activity relationships, (c) MO taxonomy, and (d) conformer taxonomy are presented.
A simplified three‐level model describing the molecular third‐order nonlinear optical susceptibility
Authors: Dirk Carl W. , Cheng Lap‐Tak , Kuzyk Mark G.
Published: January 2005

A simplified three‐level model for γ, the molecular third‐order nonlinear optical susceptibility, is presented and discussed. The perturbation theory‐based approach suggests that there are three primary avenues to optimizing molecular four‐wave mixing susceptibilities and that with each of these is associated a particular class of molecular electronic structures. The three electronic structure classes consist of (1) conjugated donor–acceptor dipolar molecules with a large secondorder susceptibility, β; (2) even‐member conjugated chains such as ‐enes, ‐ynes, and ene–ynes with large two‐photon dominated susceptibilities; and (3) “charged” odd‐member conjugated chaines with large linear absorption dominated third‐order susceptibilities such as squaryliums (perhaps, more generally, the polymethine dyes). Classes (1) and (2) have been known and investigated in the past, while recent results of ours suggest the existence of the third and perhaps final class.
New algorithm for high‐order time‐dependent hartree–fock theory for nonlinear optical properties
Authors: Sekino Hideo , Bartlett Rodney J.
Published: January 2005

A new formalism and algorithm is developed for solving the general‐order time‐dependent Hartree–Fock (TDHF) problem. It is shown that for any order a generalization of the TDHF equations can be derived where all lower‐order solutions constitute a constant term. This makes it very easy to obtain high‐order solutions. As the space required for the mapping of density matrices to Fock matrices in a problem of a giver order is largely reduced, we can perform the most time‐consuming steps within the core memory of the machine and easily manipulate vector products via optimum routines. The second hyperpolarizability γ is obtained from the secondorder TDHF solution via a 2n rule. The formalism also allows for expressing all terms in the equation diagrammatically, which provides additional physical insight and a more systematic evaluation of terms. To illustrate the method, TDHF results are presented for trans‐butadiene and carbon monoxide for several optical processes, including correlation corrections to their static hyperpolarizabilities obtained via coupled cluster (CCSD) and many‐body perturbation theory. The hybrid TDHF/CCSD method provides excellent agreement with the DC–SHG experiments (χ||(2) = 11.4 x 10−32 esu/mol compared to 12.9 ± 11.4 × 10−32 esu/mol and χ||(3) = 149 compared to 144 ± 4 × 10−39 esu/mol).