bluuebunny/crossref_metadata_2025_split huggingface dataset:
Average virtual orbitals in configuration interaction studies with application to the low‐lying singlet states of the carbon monoxide and acetone molecules
Authors: Mogensen Benny J. , Rettrup Sten 
Published: January 2005

AbstractA method is described for obtaining a modified virtual orbital space that improves the rate of convergence in configuration interaction studies of electron correlation for both ground‐state and excited‐state properties. The validity of the method has been compared with other procedures by performing a series of test calculations of the electric dipole properties of the CO molecule in the X1Σ+ ground state and the first excited singlet states A1∏, I1Σ−, and D1Δ using different optimization schemes. The low‐lying Rydberg and valence excited states of the acetone molecule have been investigated using different N – 1‐electron potentials in the construction of virtual orbitals. The examples indicate that the presented average virtual orbitals (AVOs) constitute a compromise set of orbitals suited for simultaneously improving the description of several electronic states. © 1992 John Wiley & Sons, Inc.
Half‐projected Hartree–Fock natural orbitals for defining CAS–SCF active spaces
Authors: Bone Richard G. A. , Pulay Péter 
Published: January 2005

AbstractWe have extended the range of systems to which the half‐projected Hartree–Fock (HPHF) method has been applied, including the triplet state of the wave function. In our implementation, DIIS overcomes the convergence difficulties reported in earlier studies. HPHF allows generation of a symmetry‐broken wave function in regions of the potential energy surface where the RHF wave function is triplet‐stable. The fractionally occupied natural orbitals (FONOS) of the HPHF wave function are good starting vectors for CAS–SCF calculations. A CAS–SCF in the space defined by the HPHF FONOS should be used instead of the unrestricted natural orbital CAS–SCF method in regions of triplet stability and for small active space problems. We draw extensive comparisons between the results of both the UNO–CAS and HPNO–CAS methods and those of full CAS–SCF calculations. © 1993 John Wiley & Sons, Inc.
Accurate second‐order correlation energies for Mg and Ar
Authors: Flores Jesús R. , Redondo Pilar 
Published: January 2005

AbstractA p‐version finite element method has been used to calculate second‐order pair and total correlation energies for closed‐shell Mg and Ar. Comparison with the best results found in the literature suggests that the present values are the most accurate and that the method should perform comparatively better for heavier elements. © 1993 John Wiley & Sons, Inc.
Variation–iteration method in momentum space: Determination of Hartree–Fock atomic orbitals
Authors: De Windt Laurent , Defranceschi Mireille , Dehalle Joseph 
Published: January 2005

AbstractA method using the Svartholm iterative procedure to solve atomic Hartree–Fock equations in momentum space is defined and applied to the ground states of Be and B+. The calculated atomic orbital properties follow a monotonic and stable convergence, but with rates of convergence depending on each property. The evolution of the orbitals during the iterations is explained by the combined actions of the variational principle, the Svartholm iterative procedure, and the momentum space representation. © 1993 John Wiley & Sons, Inc.
Third‐order methods for molecular geometry optimizations
Authors: Vogel S. , Fischer T. H. , Hutter J. , Lüthi H. P. 
Published: January 2005

AbstractThird‐order optimization methods that require the evaluation of the gradient and initial estimates for the second and third derivatives are described. Update algorithms for the Hessian and the third‐derivative tensor are outlined. The direct inversion in the iterative subspace scheme is extended to third order and is combined with the third‐order update procedures. For geometry optimization, an approximate third‐derivative tensor is constructed from simple empirical formulas. Examples of application to Hartree–Fock geometry optimization problems are given. © 1993 John Wiley & Sons, Inc.
Ab initio study of the influence of aggregation on the infrared spectrum of acetonitrile
Authors: Mathieu Didier , Defranceschi Mireille , Delhalle Joseph 
Published: January 2005

AbstractWith the aim of investigating the influence of direct intermolecular interactions between nitrile groups on the infrared features of nitrile polymers, vibrational frequencies and infrared intensities of acetonitrile and of its antiparallel dimer are compared. It is found that whereas frequencies are unchanged upon association infrared intensities are dramatically affected. In particular, the striking difference between the intensities of the CN stretching band measured in gas phase, on the one hand, and in the liquid, on the other, is already accounted for at the dimer level. Despite the poor reliability of absolute values of calculated intensities, it is interesting to note that consistent trends are obtained at various levels of theory. These results suggest that ab initio calculations of infrared intensities can be useful to interpret infrared measurements, thus contributing to the elucidation of molecular structure of materials. © 1993 John Wiley & Sons, Inc.
Redshifts and blueshifts of OH vibrations
Authors: Hermansson Kersti 
Published: January 2005

AbstractAb initio calculations of potential energy, dipole moment, equilibrium OH distance, force constants, and anharmonic frequencies, and correlation between these quantities, are presented for a water molecule and an OH− ion in a uniform electric field of varying field strength. It is explained why a bound H2O molecule in nature always experiences a frequency downshift with respect to the free molecule, and a bound OH−1 ion, either a downshift or an upshift. The frequency‐field variation is well accounted for by the expression ΔνOH α −E‖ · (d μ/drOH + 1/2 · ∂μ/∂rOH). A frequency maximum occurs at the field strength where ∂μ‖tot/∂rOH ∼ 0. Two cases can be discerned: (1) the frequency maximum falls at a positive field strength when dμ/drOH is positive (this is the situation for OH−), and (2) the maximum frequency falls at a negative field when dμ/drOH is negative (this occurs for water). In general, for an OH bond in a bonding situation where the intermolecular interactions are dominated by electrostatic forces, the nonlinearity of the frequency shift with respect to an applied field is governed by how close to the frequency maximum one is, i.e., by both dμ/drOH and ∂μ/∂rOH. Correlation curves between the external linear force constant, kext, and rOH,e are closely linear over the whole field range studied here, whereas the frequency vs. rOH,e and force constants vs. rOH,e correlation curves form two approximately linear, parallel branches, corresponding to “before” and “after” the maximum in the frequency vs. field curves. Each branch of the ν vs. rOH,e curves has a slope of ∼ − 16,000 cm−1/Å. © 1993 John Wiley & Sons, Inc.
Nonlinear optical properties of organic molecules. XII. Calculations of the hyperpolarizabilities of donor–acceptor polyynes
Authors: Morley John O. 
Published: January 2005

AbstractThe hyperpolarizabilities of polyynes containing a donor and acceptor group situated at either end of the molecular have been calculated at the sum‐over‐states CNDOVSB level using geometries derived from crystallographic data and ab initio studies. The calculated values increase with increasing chain length, though the maximum value obtained are considerably less than those for the related polyynes. Although the hyperpolarizability of the polyynes is dominated by one excited π‐electron state, there are a significant number of additional low‐lying excited states in the polyynes that contribute to the overall value by opposing the usual direction of charge transfer along the chain from donor to acceptor. © 1993 John Wiley & Sons, Inc.
Data parallel large‐scale molecular dynamics for liquids
Authors: Hedman Fredrik , Laaksonen Aatto 
Published: January 2005

AbstractAn efficient data parallel computational scheme is presented for large‐scale molecular dynamics (MD) simulations of liquids with short‐range interactions. The method is based on decomposition of the simulation cell into equally sized subcells, with the shortest side length equal to the cutoff radius. Inter‐ and intracell interactions are calculated in a coarse‐grained manner. A geometric sorting procedure, based on particle distances to subcell boundaries, is used to minimize the overall computations and the nonproductive communications. Using only nearest‐neighbor communications, an efficient scheme is developed for periodic updates of the contents of subcells due to the migration of particles. Special “null‐particles” are introduced, which act as buffers during the periodic updates and allow for a globally uniform algorithm during the calculations. Communication cost is about 7% of the total CPU time. The method is found to be linearly scalable with the number of particles, performing better as the ratio of virtual to physical processors increases. The MDcode is written in Fortran 90 and implemented on a CM‐200. The overall speed is approximately 5.9 μs. per MD step and per particle for 1 million particles and 5.5 μs for 5 million particles. The method should be easily transferred to other massively parallel computers of SIMD and MIMD type. © 1993 John Wiley & Sons, Inc.
SCF Wave functions for (CuO2)n lamella in crystal field of T′–Nd2CuO4
Authors: Acrivos J. V. , Stradella O. 
Published: January 2005

AbstractAn SCF analysis has been carried for ensembles that simulate the CuO2 conduction layer in the tetragonal layer crystal T′—Nd2CuO4 (Fig. 1). In this work, the CuO2 layer is described by a planar macromolecule, (CuO2)n, subject to the crystal field produced by the point charges in the ionic layers of D4h symmetry The computations were carried out using the KGNMOL, HONDO, and KGNGRAF codes in MOTECC ‐91. The computations were carried out with different oxygen and copper basis sets and energy convergence to less than 10−8 Hartrees. The purpose of the SCF computation was to estimate the cohesive energy of the ensembles, the electron density for the individual molecular orbitals, and the excess correlation energy, to ascertain the nature of the CuO bond in the conduction layer. The results indicate the following: (1) The cohesive energy of the ensembles (measured by the SCF energy plus the correlation energy, above the atomic values): ΔUc ≡ ΔESCF + ΔEC = −4.35 to −4.17 Hartrees per CuO bond as n increases from 4 to 9. Further insight was obtained by considering the electrostatic energy contributions to ΔUc; Eelectrostatic (ensemble) → EModeling (infinite lattice) were evaluated by replacing the oxygen and copper atoms by point charges determined by a Mullion population analysis. The larger oxygen basis set (13, 8/5,3) gave consistent results for the different ensembles of ΔEcovalent ≡ ΔUc − Eelectrostatic = −1.1 Hartrees per CuO bond. (ii) The electron density indicates that covalent bonds are formed and that the oxygen atoms play an important role in the structure stability. The covalent bonds formed indicate that nominal ionic valences do not apply. Mulliken population analyses gave valences of the order of one at the copper and oxygen atoms. The CuO bond orders are 0.47 between neighboring atoms and 0.048 for those separated by two atoms. (iv) The covalent nature of the CuO bonds in (CuO2)n was compared to that for the H2 molecule using as a measure the electron density and the excess correlation energy. The excess correlation energy per CuO bond above the atomic values is one order of magnitude lower that that for the H2 molecule and that for the CC bond in alternant hydrocarbons. © 1993 John Wiley & Sons, Inc.
Ewald summation of multipolar interactions at an arbitrary order on a two‐dimensional lattice
Authors: Lambin Ph. , Senet P. 
Published: January 2005

AbstractThere exist problems in condensed‐matter theory that require evaluating infinite Bloch sums of multipolar potential r−l−1Yl,m(θ,φ) on a periodic lattice. For an arbitrary multipolar order l, tractable formulas are given for summing such interactions on a two‐dimensional Bravais lattice and evaluating their Bloch sums at a point outside as well as inside the plane of the lattice. The approach used is the Ewald method, which consists of separating the original series in rapidly converging sums in reciprocal and real spaces. Computational aspects of the present formulation are briefly reviewed. © 1993 John Wiley & Sons, Inc.
Complex scaling and self‐adjoint dilations
Authors: Kurasov P. B. , Elander N. 
Published: January 2005

AbstractComplex scaling of the Schrödinger equation on the halfaxis with a nontrivial boundary condition at the origin is investigated. A self‐adjoint dilation of the corresponding dissipative operator is constructed. The relations between the scattering problems for the operator and it's dilation are clarified. © 1993 John Wiley & Sons, Inc.
Accurate algebraic densities and intracules for heliumlike ions
Authors: Koga Toshikatus , Kasai Yoshiaki , Thakkar Ajit J. 
Published: January 2005

AbstractAccurate electron number densities and interelectronic distance (intracule) densities are given in simple algebraic form for the ground states of H−, He, Li+, Be2+, B3+, and Ne8+. The densities are obtained from systematic sequences of increasingly accurate Hylleraas‐type wave functions. The most accurate densities are obtained from 204‐term wave functions that lead to variational energies no more than 16 nano‐Hartrees above the exact the exact ones. The algebraic permit the evaluation of systematic sequences of many expectation values. Some moments of the densities are evaluated in this manner. © 1993 John Wiley & Sons, Inc.
Redshifts and blueshifts of OH vibrations
Authors: Hermansson Kersti 
Published: January 2005

AbstractAb initio calculations of potential energy, dipole moment, equilibrium OH distance, force constants, and anharmonic frequencies, and correlations between these quantities, are presented for a water molecule and an OH− ion in a uniform electric field of varying field strength. It is explained why a bound H2O molecule in nature always experiences a frequency downshift with respect to the free molecule, and a bound OH− ion either a downshift or an upshift. The frequency‐field variation is well accounted for by the expression ΔνOH ∝ −E∥·(dμ/drOH + 1/2 · ∂μ/∂rOH). A frequency maximum occurs at the field strength where ∂μ/∂rOH ∼ 0. Two cases can be discerned: (1) the frequency maximum falls at a positive field strength when dμ/drOH is negative (this is the situation for OH−), and (2) the maximum frequency falls at a negative field when dμ/drOH is positive (this occurs for water). In general, for an OH bond in a bonding situation where the intermolecular interactions are dominated by electrostatic forces, the nonlinearity of the frequency shift with respect to an applied field is governed by how close to the frequency maximum one is, i.e., by both dμ/drOH and ∂μ/∂rOH. Correlation curves between the external linear force constant, kext, and rOH,e are closely linear over the whole field range studied here, whereas the frequency vs. rOH,e and force constants vs. rOH,e correlation curves form two approximately linear, parallel branches, corresponding to “before” and “after” the maximum in the frequency vs. field curves. Each branch of the v vs. rOH,e curves has a slope of ∼ −16,000 cm−1/Å. © 1993 John Wiley & Sons, Inc.
Helical region of the potential energy surface of α‐aminoisobutyric acid: A theoretical study
Authors: Alemán Carlos , Perez Juan J. 
Published: January 2005

AbstractThe helical region of the potential energy surface of blocked α‐aminoisobutyric acid (Aib) dipeptide has been studied by using ab initio and semiempirical quantum mechanical methods, as well as force‐field‐derived methods. Depending on the method, an α‐helix or a 310‐helix is found to be the energy minimum. The conformations obtained from computations performed at the ab initio quantum mechanical level, as well as by using the AMBER force field, are in excellent agreement with X‐ray data. Semiempirical results display some important differences with regard to experimental data. On the other hand, the CVFF force field predicts no energy minimum in the helical region of the Aib potential energy surface. © 1993 John Wiley & Sons, Inc.
Geometrical, spectroscopic, and magnetic properties of an oxygen atom adsorbed on the Ni(100) surface
Authors: Goursot Annick , Mele Franca , Russo Nino , Salahub Dennis R. , Toscano Marirosa 
Published: January 2005

AbstractSpin‐polarized linear combinations of Gaussian‐type orbital–model core potential–local spin density (LCGTO–MCP–LSD) computations have been performed for oxygen chemisorption on a Ni(100) surface simulated by four different clusters. Results show that the oxygen atom chemisorbs preferentially on the fourfold hollow site with an equilibrium distance of 1.931 Å and a vertical vibrational frequency of 401 cm−1. The corresponding experimental values are 1.960 Å and 423 or 430 cm−1. A satisfactory agreement with experiment is also found for the adsorption energy (6.7 vs. 5.6 eV). The bridge position lies at only 0.4 eV above the fourfold hollow one. It is found that oxygen adsorption leaves the bare cluster total spin magnetic moment unchanged, but induces appreciable reductions of the local atomic moment on the surface nickel atoms. © 1993 John Wiley & Sons, Inc.
Second‐order Jahn–Teller instability and the activation energy for Al+(1S) + H2 → AlH+(2∑+) + H
Authors: Rusho Jon , Nichols Jeff , Simons Jack 
Published: January 2005

AbstractThe interaction of Al+ (1S) ions with H2 on the lowest electronic energy surface is studied using ab initio electronic structure methods. A Cs symmetry transition state is located and found to have the geometry of a product AlH+ ion loosely bound to a H atom, consistent with the Hammond postulate for this endothermic reaction. Locating this transition state, beginning at geometries that characterize vibrationally cold H2 and translationally hot Al+, posed special challenges to the commonly used “hill‐climbing” algorithm because of regions of geometrical instability along the path thus generated. This instability was found to be a result of second‐order Jahn–Teller coupling with a low‐lying 1B2 electronic state. In addition to these primary findings, a weakly bound T‐shaped Al+ ——— H2 C2v van der Waals complex is found that lies only 242 cm−1 below the Al+ and H2 asymptote, with HH internuclear separation only slightly distorted from the equilibrium bond distance of H2 and AlH distance (3.5 Å) much longer than the covalent bond length in AlH+ (1.6 Å). The locally stable but thermodynamically unstable linear HAlH+ (1∑) species and, of course, the H + AlH+(2∑+) reaction products have also been identified as critical points on the ground‐state surface. Where known, the geometries and energies that we calculte agree well with experimental data. © 1993 John Wiley & Sons, Inc.
On the applicability of the screened‐Coulomb exchange model in Kohn–Sham density functional studies
Authors: Proynov E. I. , Salahub D. R. 
Published: January 2005

AbstractThe screened‐Coulomb exchange model used first in the Xα method and later on in Kohn–Sham density functional theory is reexamined. Based on the well‐elaborated framework of the local spin‐density approximation, we show that this model is not well suited when systems with a finite number of electrons are concerned, because it does not respect the pair‐density sum rule. A proper modification of the model is proposed by reformulating it in terms of a screened‐exchange hole and ensuring the sum rule for this hole. As a result, it is shown how the static screened exchange in finite systems is accompanied by a conjugate antiscreened one. The possible consequences of this effect on the application of the screened‐Coulomb exchange model are discussed. © 1994 John Wiley & Sons, Inc.
Applications of the molecular orbital graph theory: A topological method to calculate aN–K based on aN
Authors: Zhao Honggang , Li Jiping , Hu Shixian , Zhang Jicai 
Published: January 2005

AbstractA formula of the coefficient aN–K of the eigenpolynomials connected with the zero‐order term's coefficient of the eigenpolynomials corresponding the k‐order‐induced subgraph's molecular fragments has been induced. From this formula, aN–K can be calculated and the contributions of the induced subgraph's molecular fragments to the stability and reactivity of the molecules are revealed. © 1994 John Wiley & Sons, Inc.
Building blocks for electron density in free molecules and in condensed matter phases
Authors: March N. H. 
Published: January 2005

AbstractWith the density functional theory as the underlying framework, the modeling of the ground‐state densityp(r), using fragments or bonds as appropriate, is considered in both free molecules and in some condensed phases. Emphasis is placed on bringing the models into contact with both first‐principles quantum mechanics and available diffraction experiments. Clusters of alkali metals are specifically referred to, since cluster science affords a bridge between the quantum chemistry of small molecules and condensed phases. Finally, amorphous silicon and both solid and liquid alkali metals are considered. © 1994 John Wiley & Sons, Inc.
Spin virial theorem in the density functional theory
Authors: Nagy Á. 
Published: January 2005

AbstractThe spin virial theorem is derived in the density functional theory. The theorem establishes a relation between the differences of spin‐up and ‐down kinetic and potential energies. The theorem is useful for checking the accuracy of spin orbitals. As an illustration, the example of the Xα method is studied. © 1994 John Wiley & Sons, Inc.
Potential energy surfaces of carbon dioxide
Authors: Xantheas Sotiris S. , Ruedenberg Klaus 
Published: January 2005

AbstractSeveral bent valence states of CO2 are characterized by means of full‐valence‐space MCSCF calculations. The ground state potential energy surface exhibits a double well corresponding to a ring minimum, with C2vsymmetry (1A1) and a 73.1° OCO angle, in addition to the linear (1σ) global minimum. The transition state for the ring opening process, which has a barrier of 12.1 kcal/mole with respect to the ring minimum, is however found to have Cs symmetry. Double minima are also shown to exist for the 1A2, 1B1 and 1B2 excited states. However, in these cases all minima are bent. Cross sections through the ground state potential energy surface corresponding to the two collinear exchange reactions O(1D) + CO(1σ+) → OC(1σ+) + O(1D) C(3P) + O2(3σ) → CO(1σ+) + O(1D) are also calculated and their energy contour maps are reported. The latter reveals the existence of a stable linear intermediate with the structure COO. © 1994 John Wiley & Sons, Inc.