bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractBased 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

AbstractIt 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

AbstractValence‐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.
Optimal population analysis
Authors: Smith C. M. , Hall G. G. 
Published: January 2005

AbstractAn 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.
Maximum overlap method and the bond strength
Authors: (Liu Fan) Frank Liu , Zhan Chang‐Guo 
Published: January 2005

AbstractAccording 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

AbstractIt 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

AbstractVarious 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

AbstractElectrostatic, 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

AbstractWe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractSimilarity 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

AbstractPair‐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

AbstractConformational 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

AbstractCalculations 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

AbstractAvoided 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

AbstractVibrational 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

AbstractTwo 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

AbstractA 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).
On the resolution of a double determinant
Authors: Lim Tiong‐Koon 
Published: January 2005

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractTwo 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

AbstractThe 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

AbstractAn 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.