Library

feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
Filter
  • 1985-1989  (5)
  • 1
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electric field gradients (efg's) at the oxygen and hydrogen nuclei in CO,NO+, OH−, H2O, and CH2O, and at the chlorine, lithium, and hydrogen nuclei in HCl, LiCl, and FCl, calculated using ab initio quantum chemical methods, are reported. Using extended Gaussian basis sets, the efg's at the oxygen and chlorine nuclei were computed at the self-consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. The efg's at the hydrogen and lithium nuclei were computed as expectation values. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg's is discussed and, where possible, corrected for. The calculated efg's, together with the experimental nuclear quadrupole coupling constants, are used to estimate the 17O, 35Cl, and 2H nuclear quadrupole moments, and to test the quality of the correlated wave functions generated by the CI(SD) and CPF methods. The recommended values on the basis of the present calculations are −2.64±0.03, −8.2±0.2, and 0.278 fm2, respectively, for the 17O, 35Cl, and 2H nuclear quadrupole moments.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 2002-2013 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using ab initio SCF molecular orbital techniques, the electric field gradients (efg's) at the oxygen and hydrogen nuclei were calculated for water clusters ranging from dimer to pentamer in an attempt to reproduce the shift in 17O and 2H nuclear quadrupole coupling constants (qcc's) that is observed on going from ice to vapor. For 2H, where the qcc shift is due mostly to the change in O–H bond length, excellent agreement with the experimental vapor → ice shift was obtained. For 17O, the change in the qcc is found to be mainly electronic in origin, effectively due to the polarization of the charge associated with the oxygen atom, and approximately 75% of the observed qcc shift was reproduced. On the basis of the calculations, estimates of the 17O and 2H qcc's in liquid water were made that are consistent with the values obtained from an analysis of the available NMR relaxation data, provided that librational motions are properly taken into account. We also present results of SCF calculations on water interacting with a Li+, Na+, or Cl− ion, indicating that the effect of a nearby ion on the 2H and 17O qcc's is similar to that produced by H bonding.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electric field gradients (efg's) at the nitrogen nuclei in N2, NO+, NO, CN, and CN− and at the nitrogen and hydrogen nuclei in HCN, HNC, and NH3, calculated using ab initio quantum chemical methods, are reported. Employing extensive Gaussian basis sets, the efg's were computed at the self-consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg's, together with the experimental nuclear quadrupole coupling constants, are used to estimate the 14N and 2H nuclear quadrupole moments, and to test the quality of the correlated wave functions generated by the CI(SD) and CPF methods. The recommended values, on the basis of the present calculations, are 2.05±0.02 and 0.29±0.01 fm2, respectively, for the 14N and 2H quadrupole moments.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 4
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 89 (1985), S. 2151-2155 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 5
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 10 (1989), S. 939-950 
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Several improvements have been made to the gradient algorithms commonly used to optimize equilibrium and transition-state geometries at the semiempirical level. A gradient algorithm derived from a combination of a variable metric method (Davidon-Fletcher-Powell/Broyden-Fletcher-Goldfarb-Shanno) and Pulay's direct inversion in the iterative subspace method for geometry optimization (GDIIS) is compared with the variable metric method combined with an accurate linear search algorithm. The latter method is used routinely in the standard semiempirical program packages, MNDO, MOPAC, and AMPAC. The combined variable metric and GDIIS algorithm is also compared with GDIIS which uses a static metric. The performance of these algorithms is examined for a wide range of systems with respect to both choice of coordinate system (for cyclic molecules) and guess for the initial Hessian. The results show that the GDIIS method is up to ca. 40% more efficient than the variable metric combined with accurate line search algorithm: however, the exact savings vary depending on the coordinate system and initial Hessian. For noncyclic systems, variable-metric GDIIS is usually equal or superior to static-metric GDIIS, and consistently performs ca. 30% more efficiently than the variable metric combined with accurate line search algorithm. For the optimization of cyclic molecules, an improved estimate of the initial Hessian has increased the efficiency by at least a factor of two. Greater efficiencies (usually 〉40%) are also obtained when static-metric GDIIS is used to refine the geometry after the initial application of a transition-state search based on the variable metric combined with line search algorithm. On the basis of these results, we recommend several changes to the algorithms as currently implemented in the standard semiempirical program packages.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...