bluuebunny/crossref_metadata_2025_split huggingface dataset:
Extended group function calculations for H2O, NH3 and CH4
Authors: Franchini P. F. , Moccia R. , Zandomeneghi M. 
Published: December 2004

AbstractWave functions expressed as antisymmetrized products of strongly orthogonal geminals have been evaluated for H2O, NH3 and CH4. The geminals have been expressed as linear combinations of 2 × 2 detors constructed with localized SCF‐MO's. Several ground state observables have been computed together with the electric polarizabilities and magnetic susceptibilities. In addition, a configuration interaction calculation limited to all possible double group excitations has been carried out.
Comparison of two perturbation methods
Authors: Singh S. R. , Smith V. H. 
Published: December 2004

AbstractPolikanov's perturbation method for the solution of the radial Schrödinger equation is shown to be equivalent to the usual Rayleigh‐Schrödinger method but with full normalization instead of intermediate noralization. The energy corrections are shown to be the same to all orders. The computational advantages of Polikanov's method are discussed. The method is illustrated by considering the two‐electron ions in the Hartree–Fock approximation.
Successive partitioning technique and Feenberg perturbation theory
Authors: Goscinski O. , Stepanov N. 
Published: December 2004

AbstractSuccessive partitioning technique, when based on a modified bordering method for solving a system of linear equations and a relation for the inverse of a partitioned matrix leads to Feenberg's perturbation theory. This sheds light on the properties of the expansion, its bracketing properties and the nature of the “counting operators” used sometimes in this connection.
A study of some potential energy functions for diatomic molecules
Authors: Jain D. C. 
Published: December 2004

AbstractA detailed study of the performance of the Morse, the Hulburt‐‐Hirschfelder and the Lippincott potential energy functions has been carried out by comparing them with the Rydberg–Klein–Rees potential energy curves for a number of electronic states of various diatomic molecules. Further, by employing the above potential energy functions, the radial Schrödinger equation has been solved for the vibrational energy levels, G(v), and the vibrational wave functions, ψv. The latter have been employed to determine the rotational constants, Bv, for the individual vibrational energy levels. Then the molecular constants, we, wexe, weye, Be, αe and γe for the various electronic states, have been computed by using the values of G(v) and Bv. A comparison of the molecular constants derived in this manner with the experimental ones reveals better agreement, in most cases, than that reported by previous workers.
Time‐dependent quantum theory II. Absorption of light by dimers: Quantum theory and classical analogy
Authors: Schurr J. M. 
Published: December 2004

AbstractThe quantum theory of light absorption by a pair of neighboring absorbers is developed in the point dipole approximation for the circumstance where excited states decay only by radiative damping. Comparison with classical local field theories, in which the monomers are represented by constant, frequency‐dependent complex polarizabilities, shows that these local field theories are valid for non‐harmonic absorbers only in the weak interaction limit, and only when there exist no states with both monomers simultaneously excited (e.g. one excited vibrationally, the other electronically) that are nearly degenerate with the single excitation states and also connected to them by appreciable transition moments. Failure of the local field theories is, thus, shown to be a consequence of the non‐harmonic nature of real absorbers.Using a general relation between the level‐shift function and complex polarizability, a recipe is formulated for calculating the complex polarizability and spectrum of a dimer.
Padé approximants to physical properties via inner projections
Authors: Goscinski O. , Brändas E. 
Published: December 2004

AbstractThe [N, M] Padé approximants to functions formally associated to power series expanssions are expressed in terms of expectation values of inverse matrices. These formulae, which can be derived by the inner‐projection technique, lead to a simple analysis of the properties of serveral approximation methods and their inter‐relationships, in particular Gaussian integration, continued factorization and Padé approximations, which are of current interest in the calculation of physical properties. A relation with Fredholm integral equations and expansions of the resolvent is also discussed.The use of operator inequalities in a systematic fashion is particularly convenient when both the function being approximated and the coefficients of the power series have physically meaningful expressions as moments of operators.
Analysis of reduced density matrices in the coordinate representation. II. The structure of closed‐shell atoms in the restricted Hartree–Fock approximation
Authors: Sperber Gunnar 
Published: December 2004

AbstractThe electron density and the Fermi hole of the ground states of the atoms He, Be, Ne, Mg, Ar, Ca, Zn, and Kr are studied in the restricted Hartree–Fock approximation. The use of single Slater‐type orbitals for the free atoms is also discussed.
On a formula of Braun and Rebane for variational bounds to overlap
Authors: Weinhold F. , Wang P. S. C. 
Published: December 2004

AbstractA recent formula of Braun and Rebane for variational bounds to overlap is shown to be closely related to the methods of Rayner and of Weinhold for the same problem, and is extended to give upper and lower bounds to overlap in both ground and excited states.
Spin‐free quantum chemistry. XIX. Particle‐hole and pairing symmetries
Authors: Matsen F. A. , Welsher T. L. , Yurke B. 
Published: December 2004

AbstractParticle–hole and pairing relationships are obtained within the framework of the unitary group formulation of the many‐electron problem using the concept of particle–hole conjugation. Besides the familiar relationships for alternant hydrocarbons, relationships among various pericyclic reaction paths are obtained.
Maximum and minimum overlap, localized, and hybrid orbitals for atoms and molecules by means of orthonormality‐constrained variation
Authors: Morikawa Tetsuo , I'haya Y. J. 
Published: December 2004

AbstractIt is shown that application of the orthonormality‐constrained variation method to the absolute squares of three kinds of overlap integrals leads to eigenvalue equations and of which the eigenvectors belonging to maximum (minimum) eigenvalues are the maximum (minimum) overlap, localized, and hybrid orbitals. In the eigenvalue equations, coupling operators similar to those used in SCF theory are found to occur. Connection of the maximum orbitals to a many‐shell model, a simple MO theory, and Löwdin's orthonormalization is also discussed.
On the windows in the absorption spectrum of an ensemble of oscillators embedding an impurity oscillator
Authors: Breton J. , Hardisson A. , Lefebvre R. , Mauricio F. 
Published: December 2004

AbstractThe windows in the absorption spectrum of a harmonic oscillator coupled to a bath can be given a two‐parameter form identical to the Fano windows in the absorption cross section of an atom with an autoionizing state.
On the structure of the diagonal conditions
Authors: Erdahl R. M. 
Published: December 2004

AbstractSome structural features of the diagonal problem of reduced density matrix theory are investigated by considering the cone Br introduced elsewhere. In some special cases, we discover a very tight connection between the group structure of Gr, the invariance group of Br, and the convex structure of Br. We also state a general theorem that relates the convex structure of Br to the group structure of Gr.
Comparisons of the RPA, SCRPA, Tamm–Dancoff, and full CI methods by analysis of their transition density matrices, oscillator strengths, and energy moments of oscillator strengths for the electric dipole transitions from the ground state of the B+ ion (frozen K‐shell model)
Authors: Larson Everett G. 
Published: December 2004

AbstractThe RPA, SCRPA, Tamm–Dancoff, and full CI methods are compared by analyzing their transition density matrices, oscillator strengths, and energy moments of oscillator strengths for the 1Sground – 1Podd transitions of the 4‐electron B+ ion in the frozen K‐shell approximation. It is found that the RPA gives transition density matrices that are aligned nearly as well as possible along those of the full CI, but have vector lengths that are significantly too long. The corresponding transition energies are significantly too small. These errors compensate to give oscillator strengths for the dominant transition that, for all forms of the oscillator strength, are within 1.6% of the corresponding full CI values. The SCRPA gives better transition density matrices than the RPA, but poorer oscillator strengths. The Tamm‐Dancoff approximation gives very good values for the mixed length‐velocity form of the oscillator strength. The RPA gives a static electric dipole polarizability that is nearly 20% larger than that of the full CI. The SCRPA gives a value 15% smaller than—and the Tamm‐Dancoff approximation gives a mixed length‐velocity value that is 11% larger than—that of the full CI. Other energy moments of oscillator strengths are also reported. Certain other approximations related to the RPA and the SCRPA are reported as well.
Reduced hamiltonian orbitals. II. Optimal orbital basis sets for the many‐electron problem
Authors: Absar Ilyas 
Published: December 2004

AbstractOptimal orbital exponents are approximated by minimization of the reduced Hamiltonian orbital ground state energy. They appear to be as good as and are obtained at much less expense than the values derived by the usual SCF exponent optimization scheme. Partitioning of energy into 0‐energy, 1‐energy, and 2‐energy (Absar and Coleman, Int. J. Quant. Chem. 10, 319 (1976); Chem. Phys. Lett. 39, 60 (1976)) is used to study the variation in the electronic energy surface upon variation of orbital exponents. The 1‐energy operator, the natural orbitals of which are the reduced Hamiltonian orbitals, is compared with the SCF operator.
Representable reference density matrices applied to the graphical description of the Coulomb hole in the 1s2(2p)21S state of the B+ ion
Authors: Larson Everett G. 
Published: December 2004

AbstractRepresentable reference density matrices are applied to the graphical description of the Coulomb hole in a CI wave function for the first excited 1S state of the four‐electron boron ion B+. It is found that a satisfactory picture of the Coulomb hole emerges only when the symmetry of the correlated wave function is recognized in the construction of the reference full‐density matrix.
Study of the quality of Gaussian basis sets for carbon and silicon: Calculations on methane and silane
Authors: Daudel R. , Poirier R. A. , Goddard J. D. , Csizmadia I. G. 
Published: December 2004

AbstractA number of Gaussian basis sets for carbon and silicon have been examined in terms of the one‐electron properties of methane and silane. The convergence of the properties to their limiting values is not monotonic but, in general, a representation that involves five Gaussian functions per occupied atomic orbital on the heavy atom is sufficient to closely approach the limits. A relationship between the sizes and partitioned electronic energies is shown to hold to a good approximation for the Boys spatially localized molecular orbitals employed in this study.
Surface structure and energy spectra of localized states
Authors: Stȩślicka Maria , Kempa Krzysztof 
Published: December 2004

AbstractA simple model for surface reconstruction is developed utilizing the next‐nearest‐neighbor approximation within the framework of molecular‐orbital theory. Exact energy expressions for this model and various special cases are derived, and they are illustrated by numerical results. The influence of a very weak surface deformation on the energy spectrum of surface states is also discussed.
Cyanide–Isocyanide isomerization in the structural isomers of cyanogen isocyanate
Authors: Moffat J. B. 
Published: December 2004

AbstractThe cyanide‐isocyanide isomerization has been studied with ab initio calculations in an STO‐3G basis as applied to NCNCO, NCCNO, NCOCN, and NCONC, and the corresponding isocyanides. Geometry optimization has been performed on these cyanides, their isocyanides, and their hypothetical transition states. The energies of isomerization were calculated to be 42.2, 29.8, 44.6, and 41.4 kcal/mol, respectively, while the energy barriers were found as 84.3, 67.5, 107.9, and 106.8 kcal/mol. Overlap populations and atomic charges were employed to provide simple correlations of the results.
On the use of a MO polarized basis for the analysis of the interaction energy in molecular interactions: Application to amine complexes
Authors: Bonaccorsi R. , Palla P. , Cimiraglia R. , Tomasi J. 
Published: December 2004

AbstractA variant of the interaction energy decomposition scheme proposed by Morokuma which gives more emphasis to polarized MOS is presented and tested on complexes of amines with Li+, BH3, and SO2. A more systematic utilization of polarized MOS (i.e., of orbitals of the interacting molecules computed with the SCF formalism in the Coulombic field of the other molecular components of the system) is adopted, and the connection of this decomposition of the supermolecule interaction energy with perturbation approaches utilizing such polarized MOS is discussed.
Field theory of chemical reactions. I. Ideal space representation of A + BC → AC + B processes
Authors: Varracchio E. Ficocelli 
Published: December 2004

AbstractA new quantal approach to the problem of chemical reactions, of the type A + BC → AC + B, is proposed. The general formalism is based on a change of representation for the Hamiltonian system, such that in the new (ideal space) representation the transformed Hamiltonian may present, simultaneously, information on both the “reactants” and the “products” of the dynamical process. The change of representation is carried out by means of a unitary transformation that physically corresponds to creating bound BC and AC molecules, respectively, starting from free atoms. The transformation is explicitly realized by using models available in the literature, and the ideal space Hamiltonian is fully obtained, showing that it contains, in its structure, all the channels (inelastic, reactive, dissociative, etc.) available to the system. A discussion of the dynamics, at the light of the new Hamiltonian, seems to indicate that the theory will be particularly suited for numerical applications to the problem of chemical reactions and, more generally, that it may lead to a “unified” approach for both the subreactive and rearrangement regimes, in atom‐diatom encounters.
Symmetrization of operator matrix elements
Authors: Taylor Peter R. 
Published: December 2004

AbstractThe method of Dupuis and King for generating matrix elements of a totally symmetric one‐electron operator in terms of symmetry‐distinct integrals only is generalized to the case of nontotally symmetric operators. For operators constructed from two‐electron integrals, explicit reduction of integral processing to permutationally inequivalent symmetry‐distinct integrals only is described, while for one‐electron operators further reductions are derived using double coset decompositions. Finally, some computational consequences of this approach are briefly discussed.
Multiple solutions of the valence‐universal coupled‐cluster equations for Be, B+, and C2+
Authors: Jankowski K. , Malinowski P. 
Published: December 2004

AbstractSets of nonlinear equations for the cluster amplitudes of the valence‐universal coupled‐cluster (VU–CC) method have been solved to obtain physically meaningful multiple solutions for Be, B+, and C2+. The wave operator is taken in Lindgren's normal ordered exponential form and the completeness of the model space is postulated. The cluster operator is restricted to its one‐ and two‐electron components that are represented in terms of radial amplitudes defined by the configurational excitations (VU–CCSD/R method). Five solutions giving rise to 10 approximate energies of four 1S states are obtained and discussed. These are the first multiple solutions documented for a nonmodel system. Some attention is paid to the problem of the efficiency of various methods in obtaining alternative solutions and to some consequences of the availability of alternative solutions. © 1993 John Wiley & Sons, Inc.
Collision‐Induced intensity of the b1Σg+−a1Δg transition in molecular oxygen: Model calculations for the collision complex O2 + H2
Authors: Minaev Boris F. , Lunell Sten , Kobzev G. I. 
Published: December 2004

AbstractAb initio configuration interaction (CI) calculations have been performed for the O2 + H2 complex in a trapezoidlike collision arrangement with C2v symmetry. The potential energy surfaces of the four lowest states of this van der Waals complex (arising from the X3 Σg−, a1 Δg, and b1 Σg+ states of the oxygen moiety), as well as the collision‐induced b1 Σg+ − a1 Δg electric dipole transition moment (Mb–a), have been analyzed for different CI expansions, using as a reference determinant the restricted open‐shell Hartree–Fock (ROHF) function for the ground state of the complex H2(X1 Σg+) + O2(X3 Σg−). The geometry optimized at the ROHF/6–311G** level was refined by a partial optimization at the CI level scanning the intermolecular distance. The equilibrium distances for the X, a, and b states have been found to be a slightly different in the region 3.02–2.98 Å. The larger binding energy of the b1 Σg+ state (2.96 kJ/mol) in comparison with the a1 Δg (2.1 kJ/mol) and ground X3 Σg− states (1.35 kJ/mol) presumably could be explained as resulting from charge‐transfer interactions. A good convergence of the calculated transition moment Mb–a for the larger CI expansions (approximately 50,000 configuration‐state functions) has been obtained. The calculated collision‐induced intensity of the b1 Σg+‐a1 Δg and a1 Δg−X3 Σg− transitions in molecular oxygen are in reasonable agreement with recent experimental data for several foreign gases. © 1994 John Wiley & Sons, Inc.
The effects of static quartic anharmonicity on the quantum dynamics of a linear oscillator with time‐dependent harmonic frequency: Perturbative analysis and numerical calculations
Authors: Sarkar Pranab , Bhattacharyya S. P. 
Published: December 2004

AbstractThe effects of quartic anharmonicity on the quantum dynamics of a linear oscillator with time‐dependent force constant (K) or harmonic frequency (ω) are studied both perturbatively and numerically by the time‐dependent Fourier grid Hamiltonian method. In the absence of anharmonicity, the ground‐state population decreases and the population of an accessible excited state (k = 2, 4, 6…) increases with time. However, when anharmonicity is introduced, both the ground‐ and excited‐state populations show typical oscillations. For weak coupling, the population of an accessible excited state at a certain instant of time (short) turns out to be a parabolic function of the anharmonic coupling constant (λ), when all other parameters of the system are kept fixed. This parabolic nature of the excited‐state population vs. the λ profile is independent of the specific form of the time dependence of the force constant, Kt. However, it depends upon the rate at which Kt relaxes. For small anharmonic coupling strength and short time scales, the numerical results corroborate expectations based on the first‐order time‐dependent perturbative analysis, using a suitably repartitioned Hamiltonian that makes H0 time‐independent. Some of the possible experimental implications of our observations are analyzed, especially in relation to intensity oscillations observed in some charge‐transfer spectra in systems in which the dephasing rates are comparable with the time scale of the electron transfer. © 1995 John Wiley & Sons, Inc.
Localized planar atomic hybrids
Authors: Rees D. , Hall G. G. 
Published: December 2004

AbstractThe derivation of localized atomic hybrids in the plane is discussed using their definition as simultaneous eigenfunctions of the x and y position operators, when these are represented by commuting matrices in a finite basis. The use of alias functions, as introduced in the preceding paper, to produce commuting matrices is described and illustrated. Full results are obtained for any regular polygon. The extension to three dimensions allows the sp2 trigonal, sp2d square, and sp2d2 pentagonal hybrids to be derived. The theory also allows the radial factors to differ so that several shells of hybrids can be found at once. The double trigonal configuration is described as an example. The implications for any calculation of a molecular dipole moment are discussed. © 1995 John Wiley & Sons, Inc.
A semiempirical study of heterocycle oligomers and polymers in different dielectric media
Authors: Jüuurimäaue Tiit , Strandberg Marek , Karelson Mati , Calais Jean‐Louis 
Published: December 2004

AbstractFour common five‐membered heterocycles—pyrrole, phosphole, thiophene, and furan—and their oligomers with the chain length of 2, 4, 6, and 10 units have been studied quantum chemically using the semiempirical PM3 parameterization. The oligomers of pyrrole and phosphole with the homolytically dissociated NH bond and PH bond, respectively, and oligomers of thiophene and furan with one electron removed per monomer unit (4n + 2 π‐electron bipolaron systems) have also been studied. The electronic properties of the respective polymers were extrapolated from the oligomer data. Bulk polymer effects on the electronic structure were modeled using the self‐consistent reaction field theory in the multicavity approximation (MCa SCRF). © 1995 John Wiley & Sons, Inc.
Electric‐field gradients and magnetic hyperfine parameters of square‐pyramidal [M(CN)5]3− (M = Co, Rh, and Ir) complexes
Authors: Nogueira S. R. , Guenzburger Diana 
Published: December 2004

AbstractDensity functional self‐consistent spin‐polarized calculations with the discrete variational method were performed to obtain the electronic structure of the paramagnetic complexes [Co(CN)5]3−, [Rh(CN)5]3−, and [Ir(CN)5]3− of square‐pyramidal geometry. All electrons were kept in the variational space. Electric‐field gradients and magnetic hyperfine parameters at the metal site were computed with the molecular charge and spin densities obtained and compared with experimental values derived by electron paramagnetic resonance. It was found that the Fermi interaction is critically dependent on the angle between the axial and equatorial CN ligands. © 1995 John Wiley & Sons, Inc.
SINDO1 calculations of vibrational frequencies of adsorbed molecules
Authors: Jug Karl , Ahlswede Bernd , Geudtner Gerald 
Published: December 2004

AbstractA scaling procedure is used for the force constants generated by the SINDO1 method in internal coordinates to achieve better agreement with experimental frequencies of molecules. The procedure is subsequently used to calculate frequency shifts for adsorbed molecules. The results for CO2 and CO adsorption at NaCl cluster surfaces are in good agreement with experimental data. © 1995 John Wiley & Sons, Inc.