bluuebunny/crossref_metadata_2025_split huggingface dataset:
Use of effective core potentials in perturbation corrections to the Koopmans theorem: Vertical ionization potentials of Cl2, ClN3, and ClNCO
Authors: Langhoff Stephen R. , Jaffe Richard L. , Chong Delano P. 
Published: December 2004

AbstractThe valence‐shell vertical ionization potentials of Cl2 were calculated by perturbation corrections to the Koopmans theorem using a traditional effective core potential based on a Phillips–Kleinman derivation and an improved effective core potential obtained by Christiansen, Lee, and Pitzer. Comparison of the results with an all‐electron calculation demonstrated the reliability of the Christiansen–Lee–Pitzer effective core potential, which was then used to compute the vertical ionization potentials of ClN3 and ClNCO. The results shed new light in the interpretation of the photoelectron spectra of these molecules.
Quantum study of the ground state of a series of unsaturated boron–nitrogen compounds by the MNDO method. I. Geometries and stabilities
Authors: Maouche Boubekeur , Gayoso José 
Published: December 2004

AbstractA MNDO study of molecular geometries, enthalpies, formation, atomization, bond separation, and hydrogenation of a series of unsaturated boron–nitrogen compounds, linear with two and three members and cyclic with three, four, five, and six members are presented. For all these molecules, MNDO calculations are in excellent agreement with available ab initio calculations or experimental data. The high rotational barrier in aminoborane H2BNH2, 30.6 kcal/mol, and the length of bond in iminoborane HBNH, 1.183 Å, imply strong double and triple BN bond character in these two molecules. In the odd membered heterocycles, examination of the molecular geometries and energies of equilibrium states shows that in all cases, the stability of the compounds grows with the number of boron atoms and decreases with that of the nitrogen atoms. Moreover, compared study of the two BN‐fulvenes with their homologous hydrocarbon shows that only BN (BB)‐fulvene has a polyenic structure similar to that of fulvene.
Multiconfigurational Hartree–Fock response functions
Authors: Jørgensen Poul , Swanstrøm Peter , Yeager Danny L. , Olsen Jeppe 
Published: December 2004

AbstractThe linear, quadratic, and cubic response of a multiconfigurational Hartree–Fock state to a time independent one‐electron perturbation has been derived. A comparison between the exact response functions as obtained from Rayleigh–Schrödinger perturbation theory and the multiconfigurational Hartree–Fock response functions allows a identification of matrix elements of the perturbation operator between the ground and excited states and between excited states. We discuss some ambiguities which result from such an identification.
Excited states of linear polyenes in the SCF–RPA method
Authors: Baldo Marcello , Pucci Renato , Tomasello Pasquale 
Published: December 2004

AbstractIn the framework of the PPP Hamiltonian, we have studied the low‐lying spectra of all trans linear polyenes with the dielectirc function method. It is shown that higher order processes, not included in the RPA scheme as local field correction (LFC) and self‐energy (SE) effects can be introduced in a effective way by a suitable parametrization of the residual coulomb interaction. This parametrization is fixed along the polyene series both for singlet and triplet states, at variance with previous RPA calculations. Comparison with the most refined CI calculations, as well as with the experimental findings shows very good agreement. Furthermore, chain length and geometry dependence of the electron–electron correlation is briefly discussed.
Calculation of excited‐state properties of some small molecules: Comparative study of the CNDO/S and CNDO/2‐vn−1 potential methods
Authors: Sannigrahi A. B. , Mukherjee D. C. , De B. R. 
Published: December 2004

AbstractThe transition energy and geometry of the lowest excited (nπ*) singlet and triplet states of CO, CS, HNO, H2CO, HFCO, and F2CO molecules are calculated by CNDO/S and CNDO/2‐VN−1 potential methods, and the results are compared with those of experimental and ab initio theoretical studies, wherever available. In the calculation of the vertical transition energy, the performance of the CNDO/S method is seen to be generally more satisfactory than that of the CNDO/2‐VN−1 potential method, while the reverse is true for the excited‐state geometry. The CNDO/S method as such fails to describe the geometry of the excited state, but a combined version (CNDO/S‐2) of CNDO/S and CNDO/2, as well as the CNDO/2‐VN−1 potential method is fairly successful in this regard.
Ab initio MO Calculations of benzene + TCNE and naphthalene + TCNE complexes with STO‐3G π‐split basis set
Authors: Watanabe Yoshitaka , Kashiwagi Hiroshi 
Published: December 2004

AbstractAb initio SCF MO calculations have been carried out on benzene + TCNE (tetracyanoethylene) and naphthalene + TCNE complexes with the STO‐3G, STO‐3G π‐split (STO‐3G for π orbitals and a split basis for π orbitals), and 4–31G basis sets. The interaction energy, gross charges, dipole moment, and the electron density in the middle plane of the complexes have also been evaluated. The STO‐3G π‐split basis set is appropriate for the calculation of large π–π stacking complexes from two points of view, production of reliable results and ease of computations. The approximation scheme based on the semiorthogonalized orgitals is revealed to be very efficient to save CPU time and storage in such calculations. The stable conformation and the charge‐transfer interaction of the two complexes are discussed on the basis of the calculated quantities.
Reduction of the excited state into the ground state of a super‐Hamiltonian
Authors: Katriel Jacob 
Published: December 2004

AbstractA reformulation of the Hylleraas–Undheim–MacDonald variational procedure for excited states, involving the application of Löwdin's supersecular equation to a variational problem concerning the ground state of an appropriate super‐Hamiltonian, is proposed. Some formal applications are presented.
Electronic correlation contribution to the three‐body potentials for water trimers
Authors: Habitz P. , Bagus P. , Siegbahn P. , Clementi E. 
Published: December 2004

AbstractA number of trimers of water molecules have been computed with an extended basis set in the Hartree–Fock and in the direct CI approximations. It has been verified that the three‐body interaction energy can be calculated within the Hartree–Fock method. Therefore, the correlation corrections to the Hartree–Fock level are essentially additive and do not contribute significantly to three‐body effects.
Calculations of core‐electron binding energies by transition operator method followed by Rayleigh–Schröudinger perturbation theory
Authors: Chong Delano P. , Minato Tsutomu , Mukherjee Prasanta K. 
Published: December 2004

AbstractVertical ionization potentials for core orbitals of HF, H2O, and CO are calculated with the ab initio transition operator method. The results are improved by third‐order Rayleigh–Schrödinger perturbation theory. The effect of basis sets is also investigated. The computed core‐electron binding energies are compared with the experimental values and with those obtained by other theoretical methods. The comparison shows that the present approach leads to reliable core ionization energies, the average absolute deviation from experiment being only 0.4 eV for the cases studied.
Molecular basis set generation: Accurate slater basis sets for LiH− ground and excited state and Li2− ground state
Authors: Adamowicz Ludwik , McCullough E. A. 
Published: December 2004

AbstractA procedure for generating basis sets for diatomic molecule electronic structure calculations is described. In essence, this procedure maps the results of nearly exact numerical Hartree–Fock calculations into basis set form. Two applications of the procedure are proposed: (a) generation of very high accuracy basis sets, and (b) investigation of basis sets for unusual systems. The latter application is illustrated by some results for diatomic anions.
Theoretical study of equilibrium CoF bond distance of CoF6n− complexes (n = 4, 3, and 2) in crystals
Authors: Miyoshi Eisaku , Kashiwagi Hiroshi 
Published: December 2004

AbstractThe equilibrium CoF bond distance and the a1g vibrational frequency of CoF6n− complexes (n = 4, 3, and 2) in crystals were investigated from a theoretical point of view. Ab initio SCF MO calculations were performed for the isolated CoF6n− complexes in order to obtain the potential energy curves against the CoF distance. These potential energy curves were improved by considering the effect of the surrounding ions. It is shown that the effects of the next‐nearest neighbors (eight K+ ions) and the next‐next‐nearest neighbors (six Co2+ ions) are very important in determining these properties for “ionic” KCoF3 crystal. On the other hand, for Cs2CoF6 crystal, in which the CoF bond has considerable degree of covalency, the shape of the potential energy curve is almost completely determined by the nonelectrostatic component of the intramolecular interaction of the isolated CoF62− unit.
Parametrization and comparative analysis of the BFGS optimization algorithm for the determination of optimum linear coefficients
Authors: Kari Roy 
Published: December 2004

AbstractThis paper examines the numerical performance of the BFGS variable‐metric algorithm and compares this performance with that of the DFP, SR1, and OC algorithms. Numerical results indicate that the BFGS algorithm is far superior to the DFP and SR1 algorithms and comparable to the OC algorithm. Although the BFGS algorithm offers a viable method for the direct determination of localized molecular orbital coefficients, the method is not yet competitive with the more traditional methods.
Calculation of localized molecular orbitals
Authors: Leonard Joseph M. , Luken William L. 
Published: December 2004

AbstractTwo techniques are presented for reducing the effort required to determine localized orbitals based on the energy or density criteria. The first, population weighted editing of the electronic repulsion integrals, reduces the effort required for each iteration of the localization procedure. The second, damping/extrapolation of the transformation matrix, reduces the number of iterations required to reach convergence. Numerical results are provided for methane and formaldehyde (for the editing method) and for carbon monoxide and boron fluoride (for the damping/extrapolation technique).
Ab initio calculations on sulfur‐containing compounds. II. One‐electron properties of H2S
Authors: Poirier Raymond A. , Daudel Raymond , Kari Roy E. , Csizmadia Imre G. 
Published: December 2004

AbstractClosed‐shell RHF one‐electron properties are calculated for H2S using a total of 41 different s, p basis sets and two polarized basis sets (6–31G* and 6–31G**). Total energies and geometries alone are not a comprehensive criteria for selecting the best basis sets. It is shown here that the comparison of a number of one‐electron properties can serve as an excellent criteria for testing basis sets. The quality or reliability of a basis set is taken as being its agreement with a large uncontracted s, p basis set (s, p limit).
Theoretical studies on the interaction of proteins with base pairs. I. Ab initio calculation for the effect of H‐bonding interaction of proteins on the stability of adenine–uracil pair
Authors: Sarai Akinori , Saito Minoru 
Published: December 2004

AbstractThe hydrogen‐bonding interaction of protein with the adenine–uracil base pair was investigated by the ab initio MO method (STO‐3G level). We found that the stability of the base pair was greatly affected by the hydrogen‐bonding interaction of several residues of protein in different ways, depending on whether the interacting species is charged or neutral, whether the interaction is made from the major groove or minor groove side, and which of the two, adenine or uracil, is hydrogen bonded. These results were interpreted as the cooperative interaction between the base pair hydrogen bonds and external ones. The implications of the present results to biological functions were also discussed.
Numerical solution of Hartree–Fock equations for a polyatomic molecule: Linear H3 in momentum space
Authors: Defranceschi M. , Suard M. , Berthier G. 
Published: December 2004

AbstractWe give an example of multidimensional numerical solutions in momentum space, for the Hartree–Fock equations of the simplest molecule with more than two electrons and two nuclei.
Fitting electron densities of molecules
Authors: Hall G. G. , Smith C. M. 
Published: December 2004

AbstractCompact, convenient expressions for the electron density are derived using a fitting functional. Results are given for H2O, CH4, HF, NH3, and PH3, and show the accuracy obtainable using differing numbers of functions on each center. The definition of an atomic charge using these expressions is discussed and it is shown that the Bader virial‐partitioning definition, in which the density is integrated over a volume around each nucleus, leads to convergent results.
Theory and computational methods for studies of nonlinear phenomena in laser spectroscopy. I. General formalism
Authors: Dmitriev Yuri Yu. , Roos Björn O. 
Published: December 2004

AbstractA general formalism will be outlined, which uses steady‐state wave functions for the study of nonlinear phenomena occurring in molecular systems interacting with intense electromagnetic fields. The steady‐state approach has the advantage of being free from the secular divergencies and normalization terms which appear in a perturbation expansion of the time‐dependent wave function. A physical interpretation of the steady states will be given by considering the interaction between the molecule and the quantized electromagnetic field. The steady states appear as states of the combined system molecule plus electromagnetic field, with eigenvalues corresponding to the energy levels of the combined system in a semiclassical approximation. Evolution operators will be introduced and used to derive formulas for n‐photon transition probabilities between molecular states both in the ordinary configuration Hilbert space and in the composite Hilbert space spanned by the steadystate wave functions.
Theory and computational methods for studies of nonlinear phenomena in laser spectroscopy. II. Calculations of steady‐state wave functions
Authors: Roos Bjöurn O. , Dmitriev Yuri Yu. , Hotokka Matti 
Published: December 2004

AbstractMethods will be developed for variational calculations of steady‐state wave functions and the corresponding quasienergies. These functions appear as solutions to the time‐dependent Schrödinger equation for a molecule interacting with an oscillating electromagnetic field. A variation principle is used to obtain coupled equations for each Fourier component (FC) of the steady‐state wave function. The FCs are expanded in configuration state functions using the structure of the complete active space (CAS) SCF approach. Two methods are outlined, one which is based on a common set of molecular orbitals for all FCs and one based on an individual optimization of both CI coefficients and molecular orbitals for each FC. The latter method leads to molecular orbital sets for the different FCs which are mutually nonorthogonal. Formulas are derived for the calculation of higher‐order dynamic susceptibilities for steady states. An application is made to the H2 molecule, and results are presented for the frequency‐dependent polarizability and hyperpolarizability along the molecular axis. The static values obtained are α = 6.413 (6.387) and γ = 676 (674) atomic units, with exact numbers within parentheses.
Direct MRCI method for the calculation of relativistic many‐electron wavefunctions. I. General formalism
Authors: Esser M. 
Published: December 2004

AbstractThe formalism of a quasi‐ or full‐relativistic multireference CI method has been developed and implemented. The scheme is appropriate for the calculation of molecular systems in which the relativistic effects are of the same order of magnitude as the correlation contributions. In this contribution some important symmetry aspects of a relativistic many‐electron wave function are discussed and the consequences for the CI matrix structure are shown. An efficient CI strategy in the form of a direct CI is presented, which avoids the construction of the whole CI matrix. Based on a determinantal expansion of molecular spinor products, the individual one‐ and two‐electron molecular integrals are processed, and the molecular symmetry is easily accounted for by a proper linear combination of Slater determinants in the CI starting vector. For an efficient CI organization some powerful techniques of the graphical unitary group approach have been transferred to the relativistic case.
The metric properties of the reduced nuclear configuration space
Authors: Mezey Paul G. 
Published: December 2004

AbstractA proof is given for the triangle inequality of distance function d in the reduced nuclear configuration space M, used in the differentiable manifold model of potential‐energy hypersurfaces [P. G. Mezey, Int. J. Quantum Chem. Quantum Chem. Symp. 17, 137 (1983)]. This result completes the proof that M is indeed a metric space with metric d.
Conformation of p‐dimethylaminobenzylidene‐p‐nitroaniline, p‐nitrobenzylidene‐p‐dimethylaminoaniline, their stilbene and azobenzene derivatives
Authors: Patnaik Lalit N. , Das Sarojini 
Published: December 2004

AbstractPCILO computations have been carried out on the conformation of p‐dimethylaminobenzylidene‐p‐nitroaniline [I(m)], p‐nitrobenzylidene‐p‐dimethylaminoaniline [I(n)] and the corresponding stilbene [II(a)] and azobenzene [II(b)] derivatives. The aniline rings in Im and In are found to be twisted out of the plane containing the central atoms by 60° and 30°, respectively. The two phenyl rings in case of II(a) are twisted out of plane in opposite directions by 30° each. II(b) was found to be planar. The results have been compared with the earlier experimental findings and used as a possible explanation for the visible absorption spectra of the four molecules.
An alternative computational strategy for direct space hartree–fock calculations on extended model chains
Authors: Delhalle Joseph , Harris Frank E. 
Published: December 2004

AbstractBased on previous analyses of slowly convergent exchange lattice sums entering the configuration space restricted Hartree–Fock–Roothaan scheme for chain systems, an alternative computational strategy is developed. Within the present formalism, the traditionally used finite Fourier transform of k‐dependent LCAO density matrices are by‐passed and an advantageous computational organization is obtained.
The valence bond description of conjugated molecules i.SCFlevel
Authors: Hiberty P. C. , Ohanessian G. 
Published: December 2004

AbstractAiming at developing an approximate method to get good quality valence bond functions for any closed‐shell delocalized system, we analyze their SCF functions in terms of weighted bonding schemes, and investigate the general rules relating the weights of ionic structures to those of covalent ones. The butadiene case is treated in details, and the results are generalized to all closed shell singlet conjugated molecules. It is demonstrated that, contrary to what an intuitive generalization of the two‐electron‐two‐center model would predict, each bond is not described as a half‐covalent, half‐ionic entity, but that the ionic component is significantly preponderant. Some consequences on the behavior of conjugated systems under the influence of an electric field are discussed.
The hydrogen molecule for a two‐dimensional system
Authors: Silva A. Ferreira Da , Fabbri M. 
Published: December 2004

AbstractThe two‐dimensional (2D) energy of the hydrogen molecule is carried out by the Heitler‐London method. The 2D integrals (which are more localized compared to 3D ones) are performed in the light of the 3D Slater integrals. A discussion of such 2D systems is briefly outlined for doped semiconductors.
Local behavior of the kinetic energy in density functional theory
Authors: Zorita M. L. , Alonso J. A. , Balbas L. C. 
Published: December 2004

AbstractThe local behavior of several approximate kinetic energy functionals is analyzed, for the case of free atoms and ions, by comparison with the local kinetic energy of Hartree–Fock theory. The atomic electron densities used are, in all cases, Hartree–Fock electron densities. The kinetic energy functional obtained by the gradient expansion method (with a small number of terms) is, locally, not very accurate, but its integrated value is fortuitously accurate, due to a strong cancellation of errors. Functionals which have the Weizsäcker term tw = (Δ ρ)2/8ρ as a key ingredient are more accurate locally. The explicit incorporation of the shell structure and nonlocal density effects into the kinetic energy functional leads to the best results. The motivation for this work is that only a kinetic energy functional with an accurate local behavior will give good electron densities on solution of the Euler equation derived from it.
On the rotational invariance of the fock equations in indo methods using d functions
Authors: Schulz Joachim , Iffert Rüdiger , Jug Karl 
Published: December 2004

AbstractThe rotational invariance of the Fock equations is investigated. It is demonstrated that the inclusion of d functions requires additional hybrid integrals which do not occur in the original version of INDDO. Calculations are performed with SINDO1 on binding energies, ionization potentials, and dipole moments to show the differences between the various levels of approximation.
Two‐Dimensional, fully numerical molecular calculations. IV. hartree–fock–slater results on second‐row diatomic molecules
Authors: Laaksonen Leif , Sundholm Dage , Pyykkö Pekka 
Published: December 2004

AbstractFully numerical two‐dimensional Hartree–Fock–Slater calculations are reported for the diatomic molecules B2, C2, N2, CO, O2, F2, and BF. The model is identical to Becke's but the numerical method is different. Fully numerical ground‐state total and orbital energies are reported for the first time. The basis‐set truncation error in the LCAO calculations of Dunlap, Connolly, and Sabin influence the third and fourth decimals (in a.u.) of ET and ϵi respectively. Benchmarks of improved accuracy are provided for dissociation energies, bond lengths, and electronic multipole moments for future assessment of basis‐set errors.
Numerical method for the open‐shell restricted hartree–fock density‐matrix direct calculation
Authors: Mestechkin M. M. 
Published: December 2004

AbstractA new computation procedure for direct calculation of the density matrix in the LCAO version of the restricted Hartree–Fock–Roothaan open‐shell theory is analyzed. It is proved that the procedure is quadratically convergent and stable to the round‐off errors independently of the Fock operator spectrum. The dependence of the limit matrix of the initial matrix is examined.
Ab initio calculations on H transfer in the HF trimer
Authors: Heidrich D. , Köhler H.‐J. , Volkmann D. 
Published: December 2004

AbstractSystematic ab initio LCAO MO calculations with basis sets of increasing size have been performed to analyze the changes in relative stabilization energies of the two cyclic (D3h and C3h symmetry) and the noncyclic HF trimers. The results form the basis for a discussion of cyclic H‐transfer reactions.