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  • 1
    Electronic Resource
    Electronic Resource
    An orbital-based density difference index for the comparison of electron density distributions (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 6693-6698 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new semiquantitative measure of the difference between two electron density distributions is proposed. Specifically, the natural orbitals of an accurate electron density, such as those given by a configuration interaction method, are used to expand the Kohn–Sham orbitals of an approximate density functional method. The difference between the two density distributions is then reduced to a single number, the orbital-based density difference index (DDI). With the reference densities calculated from quadratic configuration interaction calculations including single and double substitutions, DDIs were obtained for four diatomic molecules using three basis sets and seven approximate functionals. Results are also included for an additional six small molecules with a flexible extended basis set and the same set of functionals. These results show that the DDI leads to conclusions that are consistent with those obtained previously by visual comparison of density difference plots. The orbital-based density difference indices are very dependent on the choice of the functional, but are fairly stable with respect to the choice of the basis set. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474912
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  • 2
    Electronic Resource
    Electronic Resource
    An ab initio study of the series of fluorinated ethanes C2HnF6-n (n = 0-6): geometries, total energies, and carbon-carbon bond dissociation energies (1992)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 96 (1992), S. 6287-6290 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100194a035
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  • 3
    Electronic Resource
    Electronic Resource
    Structure of PLi4 and Its Comparison with SiLi4, CLi4, and Li4: The Importance of Li-Li Interactions (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 4941-4946 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100014a012
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  • 4
    Electronic Resource
    Electronic Resource
    Ab initio studies of reactions of hydroxyl radicals with fluorinated ethanes (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 13402-13411 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100036a014
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  • 5
    Electronic Resource
    Electronic Resource
    Radial moments of the electron density: Gas phase results and the effects of solvation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1113-1121 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The radial moments of the electron density, 〈rn〉, have been calculated for N2 and H2O at the quadratic configuration interaction (with singles and doubles) levels of theory by use of a variety of basis sets. An optimal basis set was chosen from the first set of calculations and then used with several conventional ab initio and density functional methods to compute the moments for a representative set of molecules: N2, CO, CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, CH3OH, and CH3SH. The effects of solvation on the radial moments were also studied using the Onsager model and an extension of the Tomasi polarized continuum model. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480666
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  • 6
    Electronic Resource
    Electronic Resource
    A combined quantum mechanics and molecular dynamics study of small Jahn–Teller distorted hydrocarbons: Another difficult test for density-functional theory (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 12059-12069 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Temperature, vibrational, and matrix effects on the geometry and hyperfine coupling constants of the methane and ethane radical cations are investigated with a combined quantum mechanics and molecular dynamics technique. Density-functional theory (the B3LYP functional) is implemented as the quantum mechanical method. Results obtained for the methane cation are discouraging. The hyperfine coupling constants (HFCCs) obtained from the simulations are in poor agreement with experimental results. These deficiencies are ascribed to the inadequacy of density-functional theory to describe the potential energy surface in this radical. Results obtained for the ethane radical cation with the identical method are more promising. The HFCCs obtained from the simulations are in better agreement with experimental results obtained at 4 K than those obtained from static, gas-phase calculations, indicating vibrational effects are important for this radical even at low temperatures. Temperature effects on the HFCCs in the ethane radical cation observed experimentally are also well reproduced by the simulations.© 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479141
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  • 7
    Electronic Resource
    Electronic Resource
    Density functional theory investigation of hyperfine coupling constants in peroxyl radicals (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 7738-7748 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The geometries and 17O hyperfine coupling constants in several peroxyl radicals have been determined through the use of density functional theory. Becke's three-parameter hybrid exchange functional (B3) together with the correlation functional of Lee, Yang, and Parr (LYP) in combination with a variety of basis sets was used to study basis set effects. Subsequently, the effects of different gradient-correlated functionals were also examined. Results comparable to experimental values are obtained for all of the alkyl peroxyl radicals at the B3LYP level with IGLO-III or s-shell decontracted IGLO-III, 6-311G(d,p), 6-311+G(2df,p), and the augmented correlation-consistent polarized-valence triple-zeta basis set of D. E. Woon and T. H. Dunning [J. Chem. Phys. 98, 1358 (1993)], R. E. Kendall, T. H. Dunning, and R. J. Harrison [J. Chem. Phys. 96, 6796 (1992)], and T. H. Dunning [J. Chem. Phys. 90, 1007 (1989)]. Calculations imply that the spin density ratio between the inner and outer oxygens is 0.3:0.7, supporting earlier theoretical work [S. L. Boyd, R. J. Boyd, and L. R. C. Barclay, J. Am. Chem. Soc. 112, 5724 (1990)]. Erratic and strongly fluctuating results are exhibited for the fluoroperoxyl radical. Geometries close to the experimental values can be obtained at the B3LYP level, but at the expense of considerable spin contamination. A high degree of spin contamination can also be observed in calculations of the hyperfine coupling constants for this molecule. Possible explanations for the apparent failure to obtain converged results for FOO, apart from the considerable spin contamination, include vibrational, multireference, and matrix effects. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473774
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  • 8
    Electronic Resource
    Electronic Resource
    Structure sensitivity and cluster size convergence for formate adsorption on copper surfaces: A DFT cluster model study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9562-9568 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure sensitivity and cluster size convergence for formate adsorption on the Cu(100), Cu(110) and Cu(111) surfaces have been investigated systematically using density functional theory and the cluster model containing up to 40 Cu atoms. The copper core–valence correlation effect on the adsorbate–surface interaction is examined by using three different basis sets and effective core potentials. The calculated geometries and vibrational frequencies are in good agreement with experimental data even on the small clusters and are not surface sensitive. However, the adsorption energies show strong dependence on the surface structure and the cluster size. The adsorption energies are shown to converge very well for the large clusters, and the activity of the Cu planes for formate adsorption is in the order of Cu(110)〉Cu(100)〉Cu(111), the same as that observed experimentally for methanol synthesis. Regardless of the basis set, cluster size and surface structure, all results show an anionic formate adsorption species. The chemisorption mechanism and the local structure of formate on the three copper surfaces are essentially very similar. Some discussion about cluster modeling is presented. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481573
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  • 9
    Electronic Resource
    Electronic Resource
    A spin-density polarization index (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2824-2830 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By analogy with the Carbo overlaplike similarity measure, a spin-density polarization index (SPI) is defined. This index gives the Euclidean distance between the α and β electron densities for an open-shell molecule. An extensive study of the O2 molecule reveals that the SPI is highly dependent on the theoretical method, but is less sensitive to the basis set provided flexible extended basis sets are used. The radial distribution of the O2 spherically averaged electron density also reveals that near the center of the bond there is an inversion of the α and β electron densities and that there is a minimum in the β electron density near the nuclei within the internuclear region. Finally, the SPI is calculated for a variety of small molecules and the results are compared with those obtained for O2. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475673
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  • 10
    Electronic Resource
    Electronic Resource
    The convergence of basis set contractions: A case study of the molecular hyperfine structure of NH〈sup LOCATION="PRE"〉142 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 6270-6274 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The convergence of basis set contractions is examined at the multireference configuration interaction level with respect to the 14N isotropic hyperfine coupling constant in NH2, a case that is very sensitive to the choice of the basis set and the degree of correlation recovery. The contraction schemes studied include the atomic natural orbital (ANO) approach and two Hartree–Fock-based contraction methods: Raffenetti's general contraction method and the segmented method. Contractions of a (13s8p2d) primitive set ranging from [4s3p2d] to [8s3p2d] in the s space and from [7s3p2d] to [7s6p2d] in the p space are employed. It is found that the ANO contraction yields the smoothest and fastest convergence, although all three contractions converge to the uncontracted results. Thus, the ANO contraction starts to converge at [6s3p2d] while the other two converge at [7s4p2d]. The correlation recovery of the core electrons versus that of the valence electrons is also studied. In contrast to previous speculation, it is found that the correlation due to core electrons is recovered well with the simple choice of the most populated ANOs. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475133
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