bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ab initio calculations on urea
Authors: Elbert Stephen T. , Davidson Ernest R. 
Published: December 2004

AbstractMulticonfiguration wave functions constructed from contracted Gaussian‐lobe functions have been found for the ground and valence‐excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the 1A1(π → π*) state. The 1A1(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three‐configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.
Basis dependence of wave functions of the ground state and low‐lying excited states of planar formaldehyde
Authors: Tanaka Kiyoshi 
Published: December 2004

AbstractBy the use of three bases containing 3p STO's possessing different diffuseness, the electronic structure of several excited states (valence‐ and Rydberg‐type) of planar formaldehyde is investigated by limited CI calculations. The spatial extension of the valence‐type state is independent of the bases, whereas a diffuse orbital is important in describing the Rydberg‐type state. The natural orbital analysis shows that the Rydberg‐type states can be described as an outer electron plus an electronic cloud representing the ionized state to which the Rydberg states converge.
Ab initio computations of spin‐orbit interactions in polyatomic molecules. Splitting of sulfur LII,III states and singlet–triplet transitions in SO2
Authors: Bendazzoli G. L. , Palmieri P. 
Published: December 2004

AbstractSpin‐orbit interactions among the ground and the first few excited electronic states of SO2, are computed with ab initio molecular wave functions and Gaussian atomic orbitals. All spin‐other orbits contributions to the matrix elements are included.The computed intensity of the first singlet–triplet transition is found to be in broad agreement with experiment and sensitive to an extension of the configuration interaction expansion of molecular wave functions. Also, the splitting of sulfur LII,III states in SO2 is derived as an example of large spin‐orbit interactions among electronic states.
On a new partitioning of the Hamiltonian in many‐body calculation of pair‐correlation energies in closed‐shell systems
Authors: Mukhopadhyay Atri , Moitra Raj Kumar , Mukherjee Debashis 
Published: December 2004

AbstractWe introduce here a new partitioning of the Hamiltonian in calculating pair‐correlation energies using many‐body perturbation theory, by which we are able to eliminate the off‐diagonal particle–hole (p–h) ladders exactly to all orders in the perturbation expansion. In this formulation, the particle states turn out to be different for each distinct pair of hole states in the correlation energy calculation. We have also included the contributions of the diagonal particle–particle (p–p) and hole–hole ladders exactly to all orders. The effect of the off‐diagonal p–p ladders has been estimated for each pair by computing the third‐, foruth‐ and fifth‐order energies. For highly symmetric systems the present partitioning yields in general symmetry‐broken orbitals. Here one may use an average kind of partitioning for all the partners of the degenerate sets, which restores the symmetry and at the same time ensures cancellation of the p–h ladders exactly at the lowest order and approximately at the higher orders. Results are presented for a selection of 6π‐electron conjugated systems. The correlation energy for each pair is in excellent agreement with that obtained from a partial CI calculation involving all double excitations from this pair. The advantages of implementing the present scheme in larger systems has been discussed.
Application of many‐body green's functions to the calculation of molecular ionization potentials
Authors: Nerbrant Per‐olof 
Published: December 2004

AbstractWe present a method for calculating the one‐particle Green's function for molecules. The scheme is essentially that proposed by Schneider and Taylor [1]. The Green's function is obtained through the Dyson equation. Closed expressions result by using the functional derivation technique to truncate an infinite set of coupled equations. A physical interpretation of the approximation is given and a connection with the equations‐of‐motion method is pointed out. In a numerical application the ionization potentials are obtained for the molecules N2, H2O, NH3 and CH4.
On the scattering of slow protons by diatomic targets
Authors: Gianturco F. A. 
Published: December 2004

AbstractInelastic collisions leading to rotationally excited molecular targets and involving protons as projectiles are theoretically examined from the point of view of both the scattering equations in the close coupling formalism and the interaction potentials between the partners.A phenomenological approach is suggested for constructing such interactions and computational results are reported for simple diatomic targets. The inadequacy of more traditional ‘static’ approaches, when called for explaining dynamic couplings with open channels and centrifugal effects, is also discussed.
Asymptotic properties of inverses of cyclic overlap matrices in the large separation limit
Authors: Ra Øystein 
Published: December 2004

AbstractThis paper contains a discussion of asymptotic properties of inverses of cyclic overlap matrices relevant to LCAO theory based on localized orbitals. For inverse metric elements, Δ, and for large distances, R, between centres of localized orbital α in cell l and localized orbital β in cell l′, it is shown that:
one‐dimensional case:

two‐dimentional case:

three‐dimentional case:

These results are quite general and do not presuppose any restrictions as to symmetry (apart from the cyclic one), number of orbitals per unit cell, or vanishing of overlap integrals associated with neighbours beyond a fixed order. By way of illustration, explicit applications to one‐, two‐, and three‐dimensional, simple, mono‐orbitalic lattices with first‐neighbour overlap only are included.
Integral approximations on the basis of semiorthogonalized orbitals
Authors: Kashiwagi Hiroshi 
Published: December 2004

AbstractSemiorthogonalized orbitals are introduced as a basis of approximate ab initio molecular calculations. The semiorthogonalized orbitals are obtained from the usual overlapping atomic orbitals by means of a generalized Löwdin transformation. The new orbitals are characterized by two features: many overlaps between them are zero, and they are close to their corresponding initial atomic orbitals. On the basis of the semiorthogonalized orbitals, a scheme of integral approximations is proposed which allows one to carry out approximate ab initio calculations with less computational efforts and thus offers a possibility to perform calculations on much bigger molecules with limited computational facilities. Results are presented for model calculations on the anthracene π‐electron system. Orbital energies and electron distribution are quite close to the rigorously calculated values.
Calculation of j and jm symbols for arbitrary compact groups. III. Application to SO3 ⊃ T ⊃ C3 ⊃ C1
Authors: Butler P. H. , Wybourne B. G. 
Published: December 2004

AbstractThe methodology developed in earlier papers is used to compute the 6j symbols and 3jm factors that arise in the group chain SO3 ⊃ T ⊃ C1. The relevant character theory is given and the 2j and 3j symbols calculated. Selection rules are used to predict which j symbols or jm factors are necessarily zero, and then a set of 6j fundamentals computed for T. The complete set of primitive 6j symbols are then computed by application of the orthogonality and Racah backcoupling relations. Primitive 3jm factors are calculated for SO3 ⊃ T and T ⊃ C3 and, from these, all the 3jm factors for T ⊃ C3 and some of those for SO3 ⊃ T computed. A complete table of non‐equivalent 6j symbols for T and 3jm factors for T ⊃ C3 is given, together with a table for SO3 ⊃ T of all 3jm factors with j ≤ 2.
Spin‐Orbit coupling in aromatic hydrocarbons: A semiempirical evaluation of the radiative lifetimes of naphtalene and anthracene
Authors: Bendazzoli G. L. , Orlandi G. , Palmieri P. 
Published: December 2004

AbstractSinglet‐triplet spin‐orbit matrix elements, which govern the lowest 3B1u ← 1Ag transition in typical aromatic molecules like naphtalene and anthracene, are calculated with INDO molecular orbitals and the conventional spin‐orbit one‐electron hamiltonian. The correct order of magnitude of the triplet radiative lifetimes is obtained for the two molecules, when INDO MO coefficients are referred to a symmetrically orthogonalized basis. The possibility of using the semiempirical Hamiltonian is explored using ab initio wave‐functions for a few test cases. Reasonably accurate doublet‐doublet and singlet‐triplet matrix elements have been computed.
An interative scheme for electronic systems using one‐electron green's functions
Authors: Hyslop J. , Rees D. 
Published: December 2004

AbstractIn this paper an iterative generalization of the minimum principle proposed for electronic systems by Hall, Hyslop, and Rees is investigated. It is shown that this generalization still retains the advantage of using members of a larger class of trial wave‐functions, for example those with discontinuities, as initial approximations to the wave‐functions. This scheme has the advantage that, at each stage of iteration, an upper bound is obtained which is at least as good as that obtained previously.The theory is first applied to the hydrogen atom. It is then adapted to estimate the Hartree–Fock energy of the helium atom, the Hartree–Fock limit being obtained after a relatively small number of iterations.
Magneto‐optical properties of a spinless particle moving in a harmonic potential
Authors: Datta Sambhu , Richardson F. S. 
Published: December 2004

AbstractExact solutions to the quantum mechanical problem of an anisotropic oscillator in a one‐dimensional magnetic field are obtained. These solutions (eigenenergies and wave functions) are then applied to the problem of calculating the magneto‐optical properties of a charged spinless particle constrained to move in a harmonic potential field. General expressions for the dipole strengths and rotational strengths associated with radiation induced transitions between the eigenstates of this model system are developed, and these quantities are further related to observables of magneto‐optical absorption spectroscopy and Faraday effect studies.
Calculation of molecular electronic spectra by projected‐unrestricted Hartree–Fock theory
Authors: Schug John C. , Lengsfield Byron H. , Brewer Dana A. 
Published: December 2004

AbstractThis paper is concerned with a new application of projected‐unrestricted Hartree–Fock theory, namely, the calculation of electronic spectra for symmetric molecules. The excited electronic state is represented by a single determinant whose unrestricted nature allows for orbital rearrangement relative to the self‐consistent ground state. The self‐consistent calculation must be followed by spin projection to obtain appropriate spin eigenstates. It was necessary to develop modified procedures for portions of the spin projection calculation because our method of constructing the wave functions produces degeneracies among the natural orbitals. Illustrative calculations using the all‐valence‐electron INDO approximations produced results which compared favorably with configuration‐interaction treatments. The method described here should be most useful, however, in conjunction with ab initio calculations using flexible basis sets.
Theory of lone pairs. II. A moment analysis of localized molecular orbitals in ten‐electron hydrides
Authors: Daudel R. , Stephens M. E. , Csizmadia I. G. , Kozmutza C. , Kapuy E. , Goddard J. D. 
Published: December 2004

AbstractAb initio double‐zeta quality molecular orbital calculations have been carried out on an extensive series of ten‐electron hydrides. The Edmiston–Ruedenberg energy‐localized molecular orbitals were calculated and the total molecular and localized orbital densities analyzed in terms of dipole moments, contour plots, and a simplified model for the orbital density involving analysis of the first and second moments. The simplified moment analysis model may be easily visualized in terms of threedimensional geometric objects, spheres, and ellipsoids. The model summarizes the information on the effective functional distribution inherent in the more detailed orbital contour plots in a clear and concise manner.
Relativistic molecular orbitals for the double group D3h
Authors: Toivonen H. T. , Pyykkö P. 
Published: December 2004

AbstractRelativistic symmetry orbitals are given for the double group D3h. For atomic orbitals at the symmetry center a general expression is presented. The atomic orbitals of the s, p½, and p3/2 variety outside the center are also considered. The representation matrices are given in explicit form.
Ab‐initio and approximately rigorous calculations on small, medium, and large systems
Authors: Kaufman Joyce J. , Popkie Herbert E. , Preston Harry J. T. 
Published: December 2004

AbstractWe have explored two areas of approximately rigorous calculations for computing nonempirical wave functions for heavy and/or large molecules orders of magnitude faster than with conventional ab‐initio methods but with the same chemical accuracy.First, we have developed and used a series of programs (starting from our new fast sets of ab‐initio Gaussian SCF and SCF‐CI programs) incorporating ab‐initio effective core model potentials (MOD‐POT) which allow one to treat only the valence electrons explicitly, plus a charge conserving integral prescreening, which cuts down significantly on the number of integrals that have to be calculated, stored, or processed for a large molecule. We have named this latter procedure VRDDO (variable retention of diatomic differential overlap). With these MODPOT and MODPOT/VRDDO methods we have explored a variety of small, medium, and large systems ranging from electron affinities of atoms through to molecules of biological interest and large boron hydrides. The results compared to ab‐initio SCF or SCF/CI calcuations are very good, usually within 0.001 to 0.002 a.u. for orbital energies and gross atomic populations (GAPS) and even better along potential energy curves.Secondly, we have explored the use of the MS‐Xα method for less conventional molecules and properties than those for which it is customarily employed.
Variational localized‐site cluster expansions. VI. General theory revisited
Authors: Klein D. J. 
Published: December 2004

AbstractA general and systematic localized‐site cluster expansion is described. It provides simple physically appealing and computationally feasible wave function ansätze, as well as, via extension of the cluster expansion, a unique expression for general, possibly exact, wave functions. We show that a variety of matrix element evaluation techniques are available and that they are sometimes exact though relatively simple. These techniques often proceed in terms of computational time proportional to the number of different types of sites, rather than their total number. Thus it seems we have a new and viable many‐body theory, which is expected to be most accurate and most readily implementable when a physical picture with slightly perturbed localized sites is relevant.
An efficient procedure to evaluate long‐range coulombic interactions within the framework of the LCAO–CO method for infinite polymers
Authors: Piela Lucjan , Delhalle Joseph 
Published: December 2004

AbstractA very efficient and exact method for calculating the long‐range effects in polymers is reported. The method is based on the multipole expansion within the Fock operator, and exact summation up to infinity is carried out. Only a small number of one‐electron integrals have to be considered, while in the traditional approach one has to compute a large number of two‐electron integrals. Results on LiH model polymer have been obtained with a remarkable accuracy within a negligible computing time.
Applicaition of the statistical method in the theory of intermolecular interactions
Authors: Kołos Włodzimierz , Radzio Elżbieta 
Published: December 2004

AbstractThe approximate way of calculating the supermolecular density applied in the Gordon and Kim method is examined. If the supermolecular density is approximated by a sum of the subsystem densities, the results for the individual contributions to the interaction energy (kinetic, potential) appear to be incorrect as compared with those obtained in the SCF approach and in first‐order perturbation theory. However, a considerable improvement of the statistical results is obtained if a more suitably calculated supermolecular density is used. All calculations have been carried out for the He2 system.
Orientation of the Eckart frame in a polyatomic molecule by symmetric orthonormalization
Authors: Jørgensen Flemming 
Published: December 2004

AbstractThe Eckart frame is a frame of orthonormal vectors ei, i = 1, 2, 3, which follow a vibrating molecule while it rotates. The frame is orientated by the so‐called Eckart condition, which Eckart found by requiring the Coriolis interaction to be minimal. In the present paper we shall see that Eckart's frame also can be characterized by a simple least squares property. Eckart showed how to construct the ei's by a certain orthonormalization of three vectors Fi, i = 1, 2, 3. We observe that this orthonormalization is in fact Löwdin's symmetric. Then we consider the least squares property in the light of the Carlson‐Keller theorem—which is given a slightly different form. It is shown that the complete solution to Eckart's condition can be generated from the one of interest by C2 rotations around axes in an orthonormal set which originates from the Fi's by Löwdin's canonical Orthonormalization.
Asymptotic behavior of the primitive function of different “symmetry‐adapted” perturbation schemes for the H ground state
Authors: Kutzelnigg Werner 
Published: December 2004

AbstractThe leading terms in the asymptotic 1/R expansion of the wave functions and energies of various “symmetry‐adapted” perturbation schemes for intermolecular forces, as well as for the “polarization approximation” (PA) are derived for the H ground state, both exactly (i.e., to infinite order in λ) and in the first two orders of the λ expansion. It is pointed out that only in the PA and the Hirschfelder–Silbey scheme is the formal primitive function Φ genuinely primitive, whereas Φ in the other schemes is asymmetric in a rather strange way. In this lack of genuine primitivity lies the reason why in these schemes the leading term of the 1/R expansion is only recovered to infinite order in λ and requires knowledge of the R−4 term of the wave function, provided that one uses the “internal” energy expression. Four different energy expressions are compared that behave differently in the different schemes. The results obtained here are basis independent, but the implications of the basis completeness problem are discussed as well.
On the defects of molecular symmetry‐lobe orbitals
Authors: Le Rouzo H. 
Published: December 2004

AbstractIt is proved that two molecular symmetry‐lobe orbitals, belonging to different irreducible representations, can have a non‐negligible overlap. Using a previously reported multipolar analysis of Gaussian‐lobe orbitals (GLOs), it is demonstrated that such defects occur when individual symmetry orbitals (SOs) are both contaminated in a given Y1m subspace, even if such contaminations are very small. A numerical application illustrates this result in the case of the NH3 molecule, and it is shown that axial‐GLOs allow for the exact cancellation of the symmetry defects.
Heuristic intermolecular potential function for the methane–water interaction based on ab initio quantum‐mechanical calculations
Authors: Harrison S. W. , Swaminathan S. , Beveridge David L. , Ditchfield Robert 
Published: December 2004

AbstractAn analytical potential function for the pairwise interaction of methane and water is reported. The function is representative of 225 ab initio quantum‐mechanical calculations of the intermolecular interaction using 6–31G self‐consistent‐field molecular‐orbital theory. The statistical parameters of the curve fitting are given and isoenergy contour maps of the interaction energy are presented and discussed.
Half‐projected Hartree–Fock model for computing potential‐energy surfaces
Authors: Smeyers Yves G. , BruceñNta Angel M. 
Published: December 2004

AbstractThe possible uses of the half‐projected Hartree–Fock (HPHF) scheme for computing potential‐energy surfaces for both singlet and triplet states are investigated. For that purpose, a new procedure based on the simultaneous solution of two eigenvalue equations is presented for calculating the HPHF function. Applications are given in the cases of the singlet ground state and lowest triplet state of the lithium molecule. In both cases, the HPHF scheme is found to yield the correct general shape of the curves. The features of these curves are discussed. Finally, it is concluded that the HPHF model seems to be useful as a tool for determining potential‐energy surfaces, especially in the cases of large molecules where more sophisticated methods are unmanageable.
Equations of change in configuration space
Authors: Hirschfelder Joseph O. , Yang Kuo‐Ho 
Published: December 2004

AbstractThis paper is a summary of a detailed treatment, given elsewhere, in which (gauge invariant), equations of change (at points in configuration space) of arbitrary dynamical properties of an N‐particle quantum‐mechanical system in the presence (or absence) of an arbitrary (time‐ and space‐varying) classical electromagnetic field are derived for an arbitrary Hamiltonian by making use of both the Maxwell and the Schrödinger time‐dependent equations. The properties at a point in configuration space are defined so as to be closely related to observables and the forces are related to their classical counterparts. The rates of change of linear momentum and angular momentum are closely related to the classical Lorentz force and the Lorentz torque, respectively. The rate of change of energy is the work done on the system by E(t), the time‐dependent part of the external electromagnetic field. The reason for E(t) and its definition of the energy for a system in a time‐varying electromagnetic field is explained. The Breit‐Pauli Hamiltonian is required to treat “state‐to‐state” molecular problems and the detailed equations of change using this Hamiltonian are used as examples.
Ab initio molecular‐orbital study of the binding of ZnII with SH2 and SH−
Authors: Kothekar V. , Pullman A. , Demoulin D. 
Published: December 2004

AbstractThis paper reports an ab initio molecular‐orbital (MO) study of binding of SH2 and SH− with ZnII. The mechanism of binding of ZnII with these ligands is investigated using a detailed analysis of the energy decomposition and of the electronic distribution. The dependence of the results on the choice of the basis set for sulfur (in particular the effect of incorporation of diffuses p and d orbitals) on the geometry of ligand binding, the binding energy, and the proton affinity of SH− are investigated. Comparison made with the corresponding results concerning the binding of OH2, OH−, and NH3 shows that sulfur binding is less favorable although more covalent. Both sulfur ligands show a marked preference for angular conformations for binding with the metal ion. The effect of ZnII binding on the ease of deprotonation of H2S is quite similar to the corresponding effect found earlier for H2O.