bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic Thomas‐Fermi theory and electron subshell filling
Authors: Ten Hoor Marten J.
Published: January 2005

In previous work on electron subshell filling, the existence condition of the integrals involved has not been taken into proper account. As a result, part of the calculated subshell occupation numbers is meaningless.

In Theis' theory [9] the average number of electrons in a subshell is calculated as the difference between two integrals. With each of these integrals an existence condition is associated. Because of this, the number of electrons with angular momentum quantum number l can only be calculated for atoms of which the Z value is (much) larger than the corresponding empirical first‐appearance Z value. Thus, the range of Z for which such numbers can be calculated, is restricted considerably, especially for larger values of l.

Results obtained from a normalized version of Mason's approximation [13] to the exact Thomas‐Fermi function, have been compared with a least‐squares fit of the empirical subshell occupation numbers, and these are found to be as good as one may expect from a statistical theory.

A lower bound to each of the empirical first‐appearance Z values has been calculated. The results agree well with those reported in other work.
Geometrical energy derivative evaluation with MRCI wave functions
Authors: Shepard Ron
Published: January 2005

The theory of MCSCF and CI energy derivatives with respect to geometrical variations is briefly reviewed with special attention given to the MCSCF and MRCI energy gradients. A computational procedure is proposed for MRCI energy gradients that does not require the solution to any “coupled‐perturbed MCSCF” equations, it does not require any expensive direct‐CI matrix‐vector products involving derivative integrals, and it does not require any derivative integrals to be transformed from the AO basis to the MO basis. An additional feature is that it does not require any changes to existing MCSCF gradient evaluation programs in order to compute MRCI gradients. The only difference in the two cases is the exact nature of the data passed to the gradient evaluation program from the previous steps in the computational procedure. The additional effort required to compute the entire MRCI energy gradient vector is approximately that required for one additional iteration of the MRCI diagonalization procedure and for one additional MCSCF iteration. For large scale MRCI wave functions, the MRCI energy gradient evaluation should only require about 10% of the effort of computing the wave function itself. This computational procedure removes a major computational botleneck of potential energy surface evaluation.
Correlation balance in basis sets for electronic structure calculations
Authors: Gordon Mark S. , Truhlar Donald G.
Published: January 2005

The balanced addition of polarization functions to the 6–31G and 6–311G basis sets for correlated wave functions is evaluated using bond energy predictions at the MP2 and full MP4 levels as a measure of correlation‐balanced basis sets. The homolytic dissociations of the XH bonds in H2, CH4, NH3, H2O, and HF and the XY bonds in C2H6, NH2NH2, HOOH, and CH3OH are used as the basis for the evaluation. It is found that correlation balance is achieved for HH, XH, and XY bonds, particularly at the MP2 level, only if at least as many polarization sets, and sometimes more, are added to the hydrogens as are added to the heavy atoms.
The structure and properties of flavins: Molecular orbital study based on totally optimized geometries. II. Molecular orbital structure and electron distribution
Authors: Hall Lowell H. , Orchard Bradley J. , Tripathy Sukant K.
Published: January 2005

Based on MINDO/3 optimized geometries, molecular properties have been computed for lumiflavin and related methylated isoalloxazines. Excellent agreement with UV PES is obtained. Salient differences from previous work are identified in the interpretation of the spectra.

Computed partial atomic charges are presented and discussed for both oxidized and reduced forms, including cationic and anionic species. The most polar portion of the isoalloxazine system is pyrimidine‐like ring C because of the high polarity of the carbonyl groups. Otherwise, N(1) and N(3) are the most negative ring atoms in the oxidized and reduced forms forms; C(4a) is also very negative in reduced forms. N(5) and N(10) are more negative in reduced than in oxidized forms. It is shown in two‐electron reduction that bond length changes are quite localized to the diazadiene portion of the molecule, but that changes in partial charges extend to rings A and B. Only the OCNHCO moiety does not experience much change in charge.

Good correlation is obtained between MINDO/3 partial charges and proton NMR for both the aromatic and methyl protons on ring A, supporting the assignments by Grande and Müller. Also changes in 13C NMR spectra upon reduction are paralleled by changes in computed partial charges. The nature of two‐electron reduction is analyzed in terms of changes in HOMO/LUMO as well as geometry and charge distribution. A table of computed properties is included for all compounds studied: total energy, ΔHƒ, ionization potential, electron affinity, and dipole moment.
Basis sets generation: Relation between Adamowicz's and the maximum overlap method
Authors: Francisco E. , Seijo L. , Pueyo L.
Published: January 2005

It is shown that the method of Adamowicz for basis sets reduction [Int. J. Quantum Chem. 19, 545 (1981)] is, in practice, a particular case of the method of maximum overlap. The relationships between these two methods are discussed analytically and by means of a simple numerical example.
Valence‐bond calculations on N2 and isoelectronic species
Authors: Maclagan Robert G. A. R. , Simpson Richard W.
Published: January 2005

Valence‐bond calculations are reported for the isoelectronic series of molecules and ions: N2, CO, BF, NO+ and CN−. The most important structures are NN, CO, BπF, N+O and CN. Hybridization of the 2s and 2p orbitals is important. Only two or three structures are required to obtain an energy lower than that obtained with the molecular orbital approximation. Structures in which the electronegative element loses a σ‐orbital or gains a π‐orbital are favored. π‐bonds tend to be favored over σ‐bonds. The bond in NO+ resembles that in CO, whereas that in CN− resembles the bonding in N2.
Algebraic structure count of rotagraphs
Authors: Cioslowski J.
Published: January 2005

An asymptotic formula describing a dependence of the algebraic structure count (ASC) of the even‐membered rotagraphs on its size (N) is derived from the backfolding theorem for the electronic band structure. This new result imposes very strict constrains on ASC as a function of N.
Optimal population analysis
Authors: Smith C. M. , Hall G. G.
Published: January 2005

An easy method of approximating the electron density of a molecule by expressing it as a sum of squares of atomic orbitals is described. The coefficients are determined by minimizing the error in the electric field. The optimal population analysis is an integration of this optimized density over the orbitals belonging to one atom. The Mulliken population analysis can be interpreted as the integral of a Mulliken density that is a similar expression with fixed coefficients. The two densities are compared in détail using calculations on the water molecule and, more briefly, HF, LiH, BH, and C2H4. The error in the densities is identified and examined. The optimal population analysis is strongly recommended as a practical improvement to the Mulliken populations.
On the localization of resonances
Authors: Siedentop Heinz K. H.
Published: January 2005

The notion of resonances in different settings is discussed. Two methods for localizing the resonances in the complex plane are given and applied to model potentials.
Maximum overlap method and the bond strength
Authors: (Liu Fan) Frank Liu , Zhan Chang‐Guo
Published: January 2005

According to the maximum overlap method and the concept that the overlap integral may be used as an indicator of bond strength, a further discussion of bond strength F, an universal formula of F, and the correlation between the bond strength discussed here and that defined by Pauling have been given in this paper. For the type of molecule MLk, under the approximations of k = 2 and using the projection method of the angular part of bond orbitals, the same results as those derived by Pauling can be obtained from the universal formula. From the same formula, two more equations containing g and h atomic orbitals have been derived.
Molecules in an electric field: Some structural and spectroscopic consequences
Authors: Banerjee Manas , Bhattacharyya Sankar Prasad
Published: January 2005

It is theoretically shown that under the influence of an appropriately directed external electric field (ε) in H2CO or a similar molecule (of the XYC = O type), the barrier to planar inversion in the 3n π* state decreases as the field strength increases. The molecule assumes a forced planar geometry in the 3n π* state rather abruptly when ε(applied) > ε(critical). This phenomenon of field‐induced change of structure of an isolated molecule has the attributes of a first order phase transition. The effects should be spectroscopically detectable in the inversion spectrum. The existence of another critical field strength at which the molecule makes a more or less continuous transition from the normal convalent to a purely ionic structure is demonstrated and rationalized through a simple model.
Hydrated carbonium ions as possible nitrosamine metabolites: An ab initio study
Authors: Reynolds Christopher A. , Thomson Colin
Published: January 2005

Various isomers of CH3(H2O) (where n = 1–3) have been studied using self‐consistent field gradient techniques. The calculations have largely employed a split valence (4‐21G) basis set, although the effects of polarization functions and electron correlation have been considered for a model system. A mechanism for the formation of CH3OH from CH3(H2O), involving linear hydration chains, is proposed, and the relevance of the results to the various proposed pathways for the decomposition of nitrosamine metabolites is discussed.
Basis set extension effects on the He2 interaction energy components
Authors: Sokalski W. A. , Roszak S.
Published: January 2005

Electrostatic, exchange, classical induction, and charge‐transfer components of the He2 interaction energy have been analyzed within our previously proposed SCF interaction energy decomposition scheme [36] employing full counterpoise correction of the basis set superposition error (BSSE). The results have been supplemented by dispersion energies evaluated within the variation‐perturbation approach. The basis sets used here have been systematically increased in order to investigate basis set extension effects and perform a thorough test of our decomposition scheme. The results obtained for He2 at its van der Waals minimum (R = 5.6 a.u.) indicate that interaction energy components corrected by the full counterpoise method display reduced basis set dependency, do not seem to be overcorrected by the Jansen–Ros–Boys–Bernardi approach, and converge toward the most accurate values available in the literature. The correct representation of occupied orbitals in He requires the use of at least 1s primitive is Gaussian orbitals. This explains the inconclusive character of previous attempts to resolve the BSSE overcorrection problem within smaller expansions.
A model calculation on the transport of excitation energy in a molecular crystal
Authors: Datta S. N. , Priyadarshy S.
Published: January 2005

We report a model calculation of the transport of a local (site) excitation in a doped molecular crystal containing one impurity. We do not consider the impurity as a direct trap for electronic excitations (zero trap depth) but assume that exciton‐phonon interaction is exclusively given by the coupling of excitons with the vibrational displacement of the impurity. The dynamical problem is solved by using a time‐dependent effective potential consisting of equilibrium average exciton‐phonon interaction and fluctuations around this average. Two correlation functions are computed using the slow phonon limit and assuming that the temperature of the system is 300 K.

Transmission of the excitation energy over a distance of eight spacings takes place, electronically, within a few picoseconds. With the exciton‐phonon interaction switched on, calculated correlation functions diminish very rapidly with increasing time, indicating that an irreversible transfer of excitonic energy to the thermal bath takes place. Thus transmission of the excitation energy over such a distance (and without a high rate of trapping) is not an efficient process.
The functional derivatives of the Rayleigh's quotient—concept and applications
Authors: Cioslowski J.
Published: January 2005

The entity of generalized Rayleigh's quotient is introduced. A formalism involving calculation of the functional gradient and Hessian of the quotient reveals several properties that lean on the variational approach to the direct optimization of the ground state wave function. In particular, the link between a minimization of the generalized quotient and the solution to the “nonlinear Schrödinger equation” is elucidated. The concept of functional derivatives enables us to perform a direct optimization of the trial ket through the Newton–Raphson or the augmented Hessian method. Eventually, a “universal” perturbation theory is derived.
Method of local‐scaling transformations and density functional theory in quantum chemistry. III. The energy density functional: Spin‐restricted approach
Authors: Kryachko Eugene S. , Petkov Ivan Zh. , Stoitsov Mario V.
Published: January 2005

The rigorous derivation of the energy density functional is proposed within the framework of the spinfree, or spin‐restricted formulation of the energy density functional theory. It is shown particularly that the kinetic energy density functional is given by a sum of the Weizsacker term and the so‐called “modified” Thomas–Fermi one. The variational principle is formulated for the energy density functional theory in terms of the Euler–Lagrange equation, and the virial theorem is proposed.
Structure and properties of nonclassical polymers. V. On a class of nonalternant polymers with localized nonbonding bands
Authors: Polansky O. E. , Schuster P. , Ivanov C. I. , Tyutyulkov N.
Published: January 2005

The investigations on nonclassical polymers [4, 6, 12] have been extended to a new class of quasi‐one‐dimensional hydrocarbons. The latter are characterized by the occurrence of an infinitely narrow nonbonding band, originating from a new type of structural peculiarity. In contrast to the systems considered so far, the new polymers exhibit a strictly localized nonbonding band.
Hypervirial SCF treatment for vibrational energy levels of triatomic molecules
Authors: Zúñiga J. , Hidalgo A. , Bernabé F. , Fuster A. , Requena A.
Published: January 2005

The SCF method for the calculation of energy levels of triatomic molecules is applied using the hypervirial perturbative procedure for solving the coupled equations. This treatment allows to obtain in a recursive way the energy corrections and the expectation values required in the SCF treatment, avoiding the explicit calculation of the wave functions. A numerical application is made to the SO2 and O3 molecules, comparing our results with those obtained by other methods.
LCAO–MO similarity measures and taxonomy
Authors: Carbó Ramon , Domingo Llorenç
Published: January 2005

Similarity measures between pairs of molecular wave functions are described. They are based on the geometrical structure of the LCAO–MO framework and upon multivariate analysis ideas. The theoretical framework is presented, and formulae for some integrals needed are given. Two main measures, distance and correlation coefficients, are used. Distance and correlation matrices induce relationships in the whole MO set, which can be depicted through minimal spanning tree techniques. Furthermore, principal component analysis allows a two‐dimensional visualization of the Mo manifold geometrical relationships. Various examples are given in order to obtain information on how basis set, environment, excitation, bending, stretching, and electronegativity affect the induced order. For this purpose “ab initio” SCF–LCAO–MO calculations with double‐ and single‐zeta quality basis sets have been used for various simple molecular structures: H2O, NH3, CH4, N2, O2, C2, NO, CN, and CO. The results obtained can open the way to LCAO–MO taxonomy. Using this information, other areas of interest are connected with similarity measures (SCF and CI, localization procedures, etc.), proving in this manner their potential utility.
Pair‐excitation MCSCF treatment of small molecules in an optimized Slater–Transform–Preuss basis set
Authors: Guner O. F. , Shillady D. D. , Ottenbrite R. M. , Rao B. K. , Yurtsever E.
Published: January 2005

Pair‐excitation multiconfigurational self‐consistent field (PEMCSCF) treatment of 11 small molecules (LiH, BeH2, BH3, BF, CH4, C2H4, C2H2, CH2O, NH3, H2O, and HF) has been carried out in a minimum basis set of Slater Transform Preuss functions as fitted by six cartesian gaussians (STP‐6G). The advantages of accuracy without using a split basis are shown by comparison to familiar 4‐31G and 6‐31G calculations using molecular geometries optimized with STO‐6G basis sets. A benefit is shown for the use of minimum basis fitted to STP functions: they overemphasize long‐tail radial dependence to achieve long range basis sensitivity without increasing the basis size at the AO‐to‐MO transformation step in the configuration interaction portion of the MCSCF algorithm. Fully optimized STP‐6G parameters are given and appear to be transferable as shown for acrolein. A FORTRAN listing of the full least squares fitting algorithm is available* for in situ generation of STP‐6G orbitals energetically superior to 4‐31G, or a less accurate STP‐6G 1S, 2S, and 2P basis may be scaled directly as if they were STO‐6G functions, but with considerably lower energy that with an STO‐6G basis.
Conformation of p‐dimethylamino aza styryl dyes
Authors: Patnaik Lalit N. , Das Sarojini
Published: January 2005

Conformational studies on some p‐dimethylamino β‐aza and β‐aza (with respect to the dimethylamino phenyl ring) styryl dyes derived from quinoline‐4, quinoline‐2, pyridine‐4, pyridine‐2, and benzothiazole‐2 have been carried out using the quantum mechanical PCILO (perturbative configuration interaction using localized orbitals) method. These molecules may be considered as heterocyclic analogues of benzylidene anilines whose conformations have been studied extensively by both theoretical and experimental methods to explain the difference of their spectra from the isoelectronic benzylidene compounds. The results of the present studies show that the β‐aza styryl dyes are nearly planar. In case of β‐aza styryl dyes, although the phenyl ring is coplanar with the central atoms, there is a substantial twist of the heterocyclic ring. These results are explained in terms of CT‐1 and CT‐2 effects and are used as a possible explanation for the observed spectral and sensitization properties.
Intersubband transition energies in quantum wells in n‐Type GaAs‐Alx Ga1 ‐ xAs heterostructures
Authors: Gomes V. M. S. , Oliveira G. M. G. , Leite J. R. , Chaves A. S.
Published: January 2005

Calculations of the single‐particle intersubband transition energies are carried out for a two‐dimensional electron gas confined in quantum well in GaAs‐Alx Ga1 ‐ x As heterostructures. A variety of samples is considered with different quantum well widths, electron densities, and aluminum concentrations. The calculations are based on a simultaneous self‐consistent solution of Poisson and Schrödinger equations, including exchange‐correlation effects. The results are compared with experimental data. In most cases the calculated subband energy spacings as well as the Fermi energies are in good agreement with the available experimental data. In addition, for a particular single asymmetric quantum well, the shift of the electron‐hole recombination energy is calculated as a function of the two‐dimensional electron density. The results emphasize the importance of the exchange‐correlation effects as have been observed experimentally.
A method for constructing diabatic states by interpolation on the reduced density matrix
Authors: Kučar Jasna , Pavlović Milica , Hendeković Josip
Published: January 2005

Avoided crossing of adiabatic potential energy curves is considered. A new scheme for the adiabatic‐diabatic transformation is developed that is based on an interpolation performed on reduced one‐electron density matrices. The procedure is tested on the N2 molecule.
Vibrational frequencies and assignments for some isotopomers of urcail using a scaled ab initio force field
Authors: Császár Pál , Harsányi László , Boggs James E.
Published: January 2005

Vibrational frequencies and IR band intensities for 18 isotopomers of uracil, including deuterated 15N and 18O species, have been calculated using the scaled ab initio force field of Ref. 1. The results obtained are compared with available experimental data, and a number of refinements in former assignments are proposed. The good agreement between the calculated and experimental frequencies confirms the reliability of the scaled quantum mechanical‐force field.
Three‐dimensional numerical integration for electronic structure calculations
Authors: Boerrigter P. M. , Te Velde G. , Baerends J. E.
Published: January 2005

Two three‐dimensional numerical schemes are presented for molecular integrands such as matrix alements of one‐electron operators occuring in the Fock operator and expectation values of one‐electron operators describing molecular properties. The schemes are based on a judicious partitioning of space so that product‐Gauss integration rules can be used in each region. Convergence with the number of integration points is such that very high accuracy (8–10 digits) may be obtained with obtained with a modest number of points. The use of point group symmetry to reduce the required number of points is discussed. Examples are given for overlap, nuclear potential, and electric field gradient integrals.
Excited state electron density differentials: A new technique for studying HOMO/LUMO excitations
Authors: El‐Issa B. D. , Makhyoun M. A. , Salsa' B. A.
Published: January 2005

A new technique for the study of charge transfer processes is presented. The method is based on electron density differentials and employs multiple scattering Xα molecular wave functions. The method is discussed with reference to an excitation of an electron from the HOMO to the LUMO of bis(glycinohydroxomato) nickel (II).
Variational energy derivatives and perturbation theory
Authors: Sellers Harrell
Published: January 2005

In this note we discuss the variational forms of the energy derivatives and the method of obtaining them. We show that perturbation theory can be formulated in terms of stationary energy derivatives and that this formulation extends the idea of the Hylleraas functional to infinite order and excited states.
On the resolution of a double determinant
Authors: Lim Tiong‐Koon
Published: January 2005

The resolution of a double determinant into a sum of spin‐free orthogonal spin eigenfunctions is presented; its equivalent resolution of a spin function product θM0 is also given. Furthermore, a more general resolution of a Slater determinant into all spin eigenfunctions with S ≥ M is also obtained. Present work has also provided a method to calculate the linear coefficients in the expression of the primitive θMi in terms of the linear combination of spin eigenfunctions X̃ (S, M).
Considerations on Gaussian expanded generator coordinate wave functions
Authors: Mohallem José R. , Trsic Milan
Published: January 2005

The recently developed integral discretization technique for the generator coordinate method is applied to discuss some features of Gaussian expanded wave functions. Using the hydrogen atom ground state, it is shown that the nuclear cusp problem has an interesting relation with the generator coordinator weight function, which allows the production of accurate wave functions by an integration criterion.
Reduced‐diabatic vibrational close coupled treatment of molecular dissociation dynamics
Authors: Pernot P. , Atabek O. , Beswick J. A. , Levy B.
Published: January 2005

A close coupled treatment in a vibrational adiabatic representation is applied to the study of molecular photodissociation dynamics. The procedure which is developed here involves three steps: transformation from a diabatic to an adiabatic basis set, truncation of the adiabatic basis set, back transformation to a reduced–diabatic basis set. In the two model cases which are studied, dissociation spectra show complicated peaks and dips, patterns interpreted in terms of shape and Feshbach resonances associated to vibrational predissociation with a relatively high potential barrier in the excited state. An important reduction in the number of channels required for a given final accuracy can be reached by using the reduced–diabatic basis set instead of the usual diabatic one. This is very promising for studying energy partitioning in molecular systems with several internal degrees of freedom taking part in the dynamics.
Further evaluation and development of charge similarity indices for choosing molecular analogues
Authors: Richard Ann M. , Rabinowitz James R.
Published: January 2005

Two molecular charge similarity index (CSI) methods are further evaluated for practical application: one method based on a simple CNDO‐type approximation to the electron density function and the other based on an ab initio pseudo total charge density function. The test system consists of isosteric analogues of dimethyl ether and methoxy acetic acid. The effects of differences in skeletal structure on the CSI measure of electron density similarity about corresponding atoms is estimated, and two new developments are presented for application of the ab initio‐based method: (1) an INDO‐type approximation which improves the efficiency of the CSI calculation; and (2) a FOCUS feature which enables comparisons of local molecule regions.
Effects of contraction and reduction of basis set size on the He2 interaction energy components
Authors: Roszak S. , Sokalski W. A.
Published: January 2005

The effect of basis set contraction and elimination of primitive Gaussian orbitals on the He2 interaction energy components have been studied within the SCF counterpoise corrected approach supplemented by a dispersion term calculated within the variation‐perturbation scheme. Despite elimination of almost half of the primitive Gaussian functions from the saturated sp basis set and complete contraction of the remaining ones, the components of interaction energy in He2 suffer a remarkably small loss of accuracy except for the short range charge transfer contribution.
An algebraic Hamiltonian for electronic and nuclear degrees of freedom based on the vector model
Authors: Holme T. A. , Levine R. D.
Published: January 2005

An approach which considers both nuclear and electronic motions for molecular systems is derived using algebraic techniques within the vector model. The dynamical symmetries of the Hamiltonian thus derived are examined and shown to account for the analytic diagonalizability of the LEPS level potential function for molecular systems. The initial application is to the H3 collision system, demonstrating the ability of the model to treat systems without bound states. Extensions to more complicated systems are also considered, and these efforts allow for the determination of the algebraic source of the coupling in the “diatomics in molecules” (DIM) potential surface. In general, these terms in the DIM approach preclude analytical diagonalization of the Hamiltonian, which may also be understood in terms of the dynamic symmetry of the corresponding algebraic description.
QSAR and Mechanistic studies on the genotoxic compounds including environmental effects
Authors: Miertuš S. , Frecer V. , Májeková M.
Published: January 2005

The effects of a proximate condensed environment as the solvent and cellular structured patterns (biopolymers, membranes, etc.) play an important role in determination of the courses of molecular processes in biology. We present here the background of methods developed for such an environmental effects estimation combining the continuum and discrete models. Their applications within theoretical studies into the mechanisms of carcinogenic action of alkylating N‐nitrosocompounds are shown. The results given cover four different areas, namely the quantitative structure‐activity relationship, mechanistic studies into their metabolic activation reactions, interactions of the ultimate carcinogens with DNA, and finally their genetic consequences.
Normally ordered Franck–Condon operator: A new approach for the calculation of Frank–Condon factors
Authors: Hong‐Yi Fan , Zaidi H. R.
Published: January 2005

The idea of a Franck–Condon (FC) operator is introduced, and its normally ordered form is obtained through the newly developed technique of “integration within an ordered product of operators (IWOP).” It is shown that the FC operator leads to a new approach for the calculation of FC factors. The results of existing theories are viewed, and the connection between the FC operator and the “squeeze‐operator” is pointed out.
Structure and electronic structure of polyacene
Authors: Kertesz Miklos , Lee Yong S. , Stewart James J. P.
Published: January 2005

It is shown that infinite long polyacene chains may have three energetically close but structurally distinct isomers (a symmetrical, sym, form and two lower symmetry forms: one with double bonds in a trans and another isomer with double bonds in a cis pattern). The energetics is based on solid state MNDO theory. We discuss that the symmetrical form has a substantial energy gap Eg in the Hartree–Fock approach owing to exact exchange terms, which are nonlocal. Broken symmetry Hartree–Fock (HF) solutions for polyacene are also described. An angularly distorted structure suggested earlier on Jahn–Teller grounds is found to be energetically not favorable.
Local energy as an indicator of the accuracy of wave functions and probability densities for helium
Authors: Deal Walter J.
Published: January 2005

The local energy is examined as an indicator of the accuracy of approximate wave functions for the ground state of helium. It is observed that at a given point (1) an inaccurate local energy may or may not correspond to an inaccurate value of the wave function or probability density, but (2) a value of the local energy within 0.1 a.u. of the ground‐state energy corresponds to a value of the approximate wave function or probability density within about 10% of that for the ground‐state wave function.
Orthogonalization in momentum space
Authors: Calais Jean‐Louis
Published: January 2005

Since the overlap integral between two functions in position space is the same as the overlap integral between their counterparts in momentum space, there is an intimate connection between orthonormalization procedures in the two spaces. It is pointed out that in certain cases this situation can be used to simplify the orthogonalization.
Scattering of fast electrons and X‐rays from molecules: CH4 and C2H2
Authors: Tripathi A. N. , Smith Vedene H. , Thakkar Ajit J.
Published: January 2005

Self‐consistent‐field (SCF) wave functions are used to calculate cross sections for the elastic and inelastic scattering of fast electrons and x‐rays from CH4 and C2H2 molecules. The effects of basis set choice and free rotation on these cross sections are investigated. The utility of an approximate scheme to correct SCF inelastic cross sections for the effects of electron correlation is examined. The probability density for the interelectronic distance, or radial intracule density, is obtained and discussed.
OH stretching force constant in associated methanol species and the cooperativity effect
Authors: Shivaglal M. C. , Singh Surjit
Published: January 2005

The effect of molecular interaction on the OH stretching force constant of methanol (MeOH) is reported for its associated species. The various electron donors (D) and acceptors (A) considered include organic molecules such as methanol, dimethylether, acetone, acetonitrile, dimethyl formamide, pyridine, and ions such as F−, Cl−, Li+, and H+. The variation in the OH stretching force constant of MeOH…︁D species on interaction with the electron acceptor such as in the species is explained on the basis of the cooperativity effect. (CE). The effect is discussed in terms of the relationship CE = (ΔF/F) × 100, where ΔF is the reduction is force constant of the hydrogen‐bonded OH stretching mode of the associated methanol species MeOH…︁D when the lone pair electrons on oxygen of the methanol molecule are involved in hydrogen bonding with A, and F is the hydrogen‐bonded OH stretching force constant of the species when the lone pair electrons are free. The cooperativity effect (CE) is found to increase with increasing electron acceptor and electron donor capacities of A and D. The calculated force constants are compared with the experimental results.
Finite chain approximation for linear and nonlinear polarizabilities of polydiacetylene and polybutatriene
Authors: Kirtman Bernard
Published: January 2005

Static longitudinal polarizabilities αzz and cubic hyperpolarizabilities γzzzz are obtained by the FPT‐INDO method for finite chain C4N+2H2N+4 (N = 1–15) models of polydiacetylene and polybutatriene. For the acetylenic structure the onset of saturation first becomes evident at about C14C18 for the linear polarizability and C22C26 for the cubic term. The values per C4H2 unit are accurately extrapolated to the infinite chain limit. In the case of the butatrienic structure, no saturation is observed through C54.
The hyperfine coupling constants of 19F: An ab initio MRD‐CI basis set study
Authors: Karna S. P. , Grein F. , Engels B. , Peyerimhoff S. D.
Published: January 2005

The isotropic (aiso) and dipolar (Adip) hyperfine coupling constants of 19F were obtained from MRD‐CI wave functions using a variety of basis sets. In series I, increasing numbers of d functions were added to a 5s4p contracted Huzinaga/Dunning basis. In series II, the 5s3p basis set was uncontracted in several steps until 9s5p was reached, to which were added from one to three d‐polarization functions. CI parameters (selection thresholds and the number of reference configurations) were also varied. A study of the R dependence of aiso and Adip was performed. The best values obtained at Re are 260 G for aiso and 308 G for Adip, compared with experimental values of about 280 G for aiso and 320 G for Adip.
Potentials of an alternative scheme for ab initio polymer band structure calculations. Illustration on the chain of hydrogen atoms
Authors: Fripiat J. G. , Delhalle J. , André J. M. , Calais J. L.
Published: January 2005

An alternative scheme for ab initio polymer band structure calculations based on a Filon‐type quadrature is proposed. This scheme avoids the explicit calculation and the storage of the “troublesome” Fourier transforms of the LCAO density matrix elements and is a first step towards a better control of the convergence of the different lattice sums appearing in the configuration space LCAO‐SCF‐CO method. The potential of the proposed technique is illustrated by a minimal basis set calculations on an infinite chain of H atoms.
Role of doubly excited states in calculations of optical activity
Authors: Volosov Andrey
Published: January 2005

It is shown that doubly excited states play an important role in calculations of the optical activity of molecules with well‐conjugated electron systems, such as the DNA bases. In some significant cases it is necessary to include a large number of excited states in the configuration interaction (CI) to obtain a reliable, converging result. A new version of the CNDO/OPTIC method, which includes doubly excited states in the CI, is proposed. As an application, the electric transition moments in different pyrimidines are considered. The calculated results agree with experimental data and results obtained from ab initio calculations and INDO calculations using doubly excited states in the CI.
Graphical techniques in the configuration interaction approach based on pure slater determinants
Authors: Wasilewski J.
Published: January 2005

A graphical approach to the configuration interaction in the basis of pure Slater determinants is presented. The formulation based on the spin‐separated two‐slope graph (SSTSG), enabling the selection of determinants with the fixed Ms value, has a direct relation to the well‐known concept of the group‐function product. The commonly used excitation criterion and the spatial (Abelian) symmetry properties are analyzed in terms of the graph's internal structure. The Slater formulae for the Hamiltonian matrix elements between determinants, in the particle‐hole formalism and in the spin‐separated form, are related to different classes of loops within graphs. Some aspects of implementation within both the matrix‐element‐driven (ME) and integral driven (ID, direct) CI algorithms are discussed. The presented formulation, of a general complete active space (CAS) CI type, is a basis of the Graphical Determinantal Configuration Interaction (GDCI) computer program.
Ultraviolet photoelectron and absorption spectra of substituted phenols
Authors: LaFemina John P.
Published: January 2005

The spectroscopically parametrized CNDO/S3 model is used to provide a quantitative description of the valence electron photoemission and ultraviolet absorption spectra of a series of substituted phenols and anisoles. The CNDO/S3 density of valence states was used to determine the molecular conformation of these molecules via analysis of gas‐phase photoemission spectra. These conformations, in turn, were used to quantitatively determine the ultraviolet absorption spectra.
A representation of the McClelland method of graph splitting. Stack graphs
Authors: Shen Mingzuo
Published: January 2005

McClelland's rules on graph splitting can be represented using the generalized graph notation. Generalized graphs are edge‐ and vertex‐weighted graphs, which are becoming important to chemical problems. By this the McClelland method of graph splitting has a wider range of applications. “Stack graphs” are constructed from identical “base graphs” by connecting corresponding vertices from one base to another. Their eigenvalues are related to the eigenvalues of the base graph. Two‐ and even three‐layered graphs may be used as a simple model for the inter‐ring interaction in a cyclophane.
The PPP‐CI scheme as a fundamental physical model
Authors: Del Re Giuseppe
Published: January 2005

Parametrized computational procedures and simplified physical models of molecular reality are often related to one another but are by no means the same thing. With special reference to work by Koutecky and by Cizek, Paldus et al., it is shown here that the PPP model rests on a simple but physically sound electrostatic picture of a molecular π system, and its intrinsic difference from an independent particle model is emphasized. As an illustration of the heuristic power which makes physical models powerful tools of theoretical research, ground state charge transfer GSCT, in situ molecular electronegativity, and the principle of electronegativity equalization are derived within the PPP‐CI scheme, no use being made of molecular orbitals, and the assumptions on which the CT theory rests are made explicit.
Localization in aromatic and conjugated hydrocarbons. Shape studies on canonical PPP one‐electron eigenfunctions
Authors: Pipek J. , Varga I. , Nagy T.
Published: January 2005

The localization properties of the π‐electron system of some aromatic and conjugated hydrocarbon molecules have been investigated. The π orbitals have been obtained using a standard PPP‐SCF procedure. The shape analysis was carried out using the novel concept of the relation between the localization measure and structural entropy. According to our investigations the π‐electron system of these aromatic molecules is more localized than expected. The eigenstates seem to show one‐dimensional characteristics with a decay rate generally faster than exponential. Linear chains, on the other hand, have rather delocalized orbitals. As a byproduct we have found that in certain systems some states exhibit step‐function–like behavior, similar to bond‐centered charge density waves.
Modification of DEWAR‐PI to include ring strain
Authors: Dewar Michael J. S. , Dennington, Roy D.
Published: January 2005

Previous work has shown that the heats of formation of unstrained conjugated molecules can be reproduced with surpring accuracy by a semiempirical SCF MO treatment (DEWAR‐PI) based on the Pariser‐Parr‐Pople (PPP) π SCF MO approximation. The original version failed to allow for ring strain. This deficiency has now been remedied in a new version (DEWAR‐PI2).
The half‐projected Hartree‐Fock model for computing thermally “forbidden” pericyclic reactions and biradical processes. I. Formulation and results for some singlet biradical species
Authors: Olivella Santiago , Salvador Jose
Published: January 2005

The possible use of a half‐projected Hartree‐Fock (HPHF) wave function correlating a single pair of electrons, being defined as a linear combination of two unrestricted Slater determinants which are identical except that all the α and β spin functions are interchanged, is investigated in relation to the computation of the potential energy surfaces involving singlet biradical or biradical‐like species. The results of calculations, within the framework of the MINDO/3 semiempirical Hamiltonian, for a number of prototypical biradical molecules indicate that, in general, the HPHF model provides a geometrical and energetical description that is equivalent to that obtained from a conventional two‐configuration self‐consistent‐field wave function.
How to find a saddle point
Authors: Smith Colin M.
Published: January 2005

This paper explains a method for finding saddle points on a multidimensional surface and shows how it may be used to define saddle‐point seeking curves that have properties similar to well‐known orthogonal trajectories. It is shown that a gradient extremal is a special case of one of these curves, and its chemical significance as the path, defined by local criteria, which starts from a stable structure and leads to a transition state, is discussed briefly in relation to the intrinsic reaction coordinate. It is emphasized that this theory gives a natural method for locating points that have Hessians of similar structure to those of transition states.
Analytical expressions and recursion relations of two‐center harmonic oscillator matrix elements of arbitrary functions
Authors: Drallos P. J. , Wadehra J. M.
Published: January 2005

The matrix elements of various analytical functions f(X), X being the internuclear separation, are required for the description of transition probabilities and other molecular properties. These matrix elements can be conveniently estimated by assuming vibrational wave functions of two relatively dispalced linear harmonic oscillators of arbitrary frequencies to represent the vibrational levels of two electronic states of a molecule. Using this assumption, analytical expressions for the matrix elements of an arbitrary analytical function f(X) are obtained. Useful recursion relations among these matrix elements are derived and an elegant graphical representation of the recursion relations is obtained. These graphical representations are utilized to obtain new more general recursion relations among matrix elements of the arbitrary function f(X).
Optimum structures and vibrational frequencies of (SIC)2 clusters
Authors: Fitzgerald G. B. , Bartlett Rodney J.
Published: January 2005

(SiC)2 clusters occupy an intermediate place between the well‐studied clusters Cn and Sin. Of the five closed‐shell isomers investigated in this paper, the rhombic form is found to be the lowest in energy at the MBPT(4) level. Analytic SCF and MBPT(2) second derivatives are used to predict vibrational frequencies for the isomers.
Triplet‐singlet splitting of the alkali metal clusters: Example of the Li6 clusters
Authors: Koutecký J. , Boustani I. , Bonačića‐Koutecký V.
Published: January 2005

The influence of the geometry on the specifics of the electronic structure of Li6 clusters is studied in detail, since planar and three‐dimensional lithium hexamers are found to be of comparable stability. The ab initio CI investigation of the Li6 isomers yields almost degenerate lowest singlet and triplet states for certain cluster geometries. Simple criteria for the energy gap between the lowest triplet and singlet state are derived, and their applicability is demonstrated.
Ab initio calculations of nuclear quadrupole coupling constants of low‐lying rovibrational levels in the X1Σ+ and a1Σ+ states of all isotopic species of LiH
Authors: Paidarová I. , Vojtík J. , Češpiva L. , Š;vrda J. , Špirko V.
Published: January 2005

The 2H, 6Li, and 7Li quadrupole coupling constants of the low‐lying rovibrational levels in the X1∑+ and A1∑+ electronic states of 7Li2H, 6Li2H, 7Li1H, and 6Li1H are calculated from molecular wave functions which explicitly describe nuclear motion. Except for the lithium coupling constants in the A1∑+ state, the vibrational dependence of the nuclear quadrupole coupling is found to be significant for all the studied isotopic species. On the other hand, the rotational dependence appears to be important only for the deuteron coupling constant of 7Li2H and 6Li2H in the A1∑+ state. Special attention is paid to relative magnitudes of the Li and D quadrupole coupling constants in the 7Li2H and 6Li2H isotopic species. The information about the relative magnitudes and their changes with vibrational excitation is used to outline a theoretical approach to the interpretation of the hyperfine structure of the spectra of these species.
Transfer of electronic excitation energy in partially disordered polymer chain
Authors: Pálszegi Tibor , Kyseľ Ondrej
Published: January 2005

A new form of the generalized master equation for electronic excitation transfer in a partially disordered polymer chain was derived. Within this equation, in order to differentiate the transfer in ordered parts of the partially disordered polymer chain, we decomposed the memory function of electronic excitation into several parts. These parts are memory functions associated with various types of subdynamics of electronic excitation. The partially disordered polymer chain where the disorder is caused by conformational defects is discussed.
Molecular integrals in the generalized hylleraas–CI method
Authors: Lu Yannan , Huang Zuqia
Published: January 2005

In the generalized Hylleraas–CI method, the original correlation factor rvij is multiplied by a Gaussian geminal. Using the approach of generating functions, the general formulas of molecular integrals in this method are derived over Cartesian Gaussian orbitals. From differentiations of the generating functions, the expanding length in the incomplete Gamma functions is reduced, and some cancellations presented in other approaches are avoided. Preliminary calculations for H2 and H2—H2 systems are carried out over STO‐3G basis. The results are encouraging.
Splitting of n‐fold rotationally symmetric graphs
Authors: Shen Mingzuo
Published: January 2005

R. A. Davidson's rules for splitting an n‐fold rotationally symmetric graph can be derived from a unitary transformation on the adjacency matrix. The McClelland and D'Amato rules are special cases with n = 2 and n = 3, respectively.
A population analysis based on hybrid orbitals
Authors: Ming Li
Published: January 2005

By a way of partitioning of Mulliken overlap populations, the hybrid orbitals of central atoms and ligands are obtained by the CNDO method. The dipole moments of some hybrids are calculated using the hybrid orbitals obtained, and they are mostly in accordance with experimental values. In addition, overlap integrals may be decomposed into components, and then σ, π, and δ hybrid orbitals can be obtained.