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  • 1
    Digitale Medien
    Digitale Medien
    Electron correlation and relativistic contributions to dipole moments of heavy oxides and sulfides: SnO, PbO, SnS, and PbS (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2056-2066 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In order to resolve some puzzling discrepancies between theoretical and experimental dipole moment data for oxides of the Group IVa metals a series of nonrelativistic and relativistic high-level-correlated calculations have been performed for SnO and PbO. The same methods have been used to calculate the dipole moments of SnS and PbS for which the earlier results were found to be in reasonable agreement with experiment. The main attention of the present study has been focused on the treatment of the electron correlation contribution to dipole moment of highly ionic oxides. This contribution has been found to be strongly method-dependent. A comparison of different single and multiconfiguration methods shows that the coupled cluster approach with iterative treatment of single and double excitations and perturbative correction for the contribution of triple excitations, CCSD(T), offers presumably the most reliable scheme for the evaluation of the electron correlation contribution to dipole moments of the studied molecules. We have also verified the reliability of the earlier approximate (mass-velocity and Darwin terms) treatment of relativistic contributions by using the scalar Douglas–Kroll approximation. The PbO molecule has also been studied at the level of the Dirac–Hartree–Fock approximation with the results revealing a large spin-orbit contribution to its dipole moment. The present most accurate results, calculated by using the scalar Douglas–Kroll scheme and the CCSD(T) approximation for the electron correlation contribution, are found to give the oxide dipole moments systematically too low by about 0.1 a.u. In the case of the PbO molecule they are also compared with recent results obtained in the framework of the pseudopotential formalism. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.476367
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  • 2
    Digitale Medien
    Digitale Medien
    The point charge model of nuclear quadrupoles: How and why does it work (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 522-526 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The point charge model of nuclear quadrupoles is analyzed beyond the multipole approximation. The effect of the short-range expansion terms is shown to be responsible for the contamination of the calculated electric field gradients. Several regularities observed in earlier numerical calculations are elucidated and their analysis helps to devise proper computational strategies. The use of the point charge nuclear quadrupole model is strongly advocated in approximate relativistic calculations of electric field gradients at heavy nuclei. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.480543
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  • 3
    Digitale Medien
    Digitale Medien
    Inclusion of mean-field spin–orbit effects based on all-electron two-component spinors: Pilot calculations on atomic and molecular properties (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9667-9674 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An implementation of a two-component all-electron (jj/ωω) treatment of both scalar and spin–orbit relativistic effects in the MOLFDIR program suite is presented. Relativity is accounted for by Douglas–Kroll transformed one-electron operators: scalar (spin-free) and so called mean-field spin–orbit terms. The interelectronic interaction is represented by the nonrelativistic Coulomb operator. High-level correlated calculations of properties of several systems (FO, ClO, Cl, O2+, O2−, Tl, and TlH) where spin–orbit effects play a dominant role are presented and compared with other data. Agreement with Dirac–Coulomb(–Gaunt) reference values is in general very good. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1413510
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  • 4
    Digitale Medien
    Digitale Medien
    A theoretical study of the A 1Π←X 1Σ+ system of SiH+ (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 423-432 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Theoretical studies have been performed on the X 1Σ+ and A 1Π states of SiH+. The complete active space, self-consistent field (CASSCF) and many-body perturbation theory (MBPT) methods were employed for the X 1Σ+ and state while the A 1Π excited state potential energy curve was studied by using CASSCF, multireference contracted configuration interaction (MR CCI, (MR CEPA), and coupled pair functional (CPF) approaches. Excellent agreement was obtained with experiment for the X state spectroscopic parameters (CASSCF): re=1.505 (1.50 A(ring); D0=3.30 (3.22±0.03) eV; ωe=2161 (2155) cm−1; ΔG1/2=2093 (2089) cm−1, with experimental values within parentheses. The shape of the potential curve for the A state was very sensitive to the basis set and higher order electron correlation. The best results, obtained with the CPF method, were re=1.888 (1.871) A(ring); D0=0.15 (0.15±0.03) eV; ωe=438 (448) cm−1; ΔG1/2=318 (389) cm−1. The computed f0,0 oscillator strength for the A←X transition was 1.2×10−3 which falls between the laboratory value, (2.4±1)×10−3, and a value obtained from the solar spectrum, 0.5×10−3. The computed life time for the v'=0 state is 956 ns to be compared to the experimental value 1025±80 ns.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.455483
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  • 5
    Digitale Medien
    Digitale Medien
    Estimates of relativistic contributions to molecular properties (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8122-8132 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A simple method for the estimation of the relativistic contribution to atomic and molecular properties is proposed. The method assumes that the dominant portion of relativistic contribution to different properties is accounted for by the Cowan–Griffin operator. In contrast to recently investigated variational relativistic and quasirelativistic approaches, the present method is based virtually on the triple-perturbation theory approach and can be easily executed in the framework of the finite-field perturbation schemes. The method proposed in this paper is applied to the evaluation of the relativistic contribution to electric properties of hydrogen halides. This contribution turns out to be completely negligible in the case of HCl. For the HI molecule, the relativistic correction to its dipole moment becomes almost as important as the electron correlation effects. A similar result is also obtained in the case of the dipole moment of AgH. Different possible applications of the present method are discussed. The major advantage of the proposed perturbation approach is a very simple computational structure which permits the calculation of relativistic corrections from any available nonrelativistic wave function.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459342
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  • 6
    Digitale Medien
    Digitale Medien
    Quadrupole moments of CuH, AgH, and AuH. A study of the electron correlation and relativistic effects (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 8248-8253 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: High-level correlated calculations of the quadrupole moment of the group-Ib hydrides have been carried out with large Gaussian-type-orbital/contracted-Gaussian-type-orbital basis sets. The nonrelativistic results obtained for CuH, AgH, and AuH at the self-consistent-field Hartree–Fock level of approximation and for the origin at the heavy atom are −2.16, −3.17, and −1.94 a.u., respectively. The electron correlation contribution computed by using the coupled-cluster approaches reduces those values to −0.25, −1.14, and −0.18 a.u., respectively. The relativistic corrections have been obtained from the recently developed quasirelativistic method. They have been found to be of increasing importance for the investigated series of hydrides. The final relativistically corrected values of molecular quadrupole moments are found to be small for CuH (+0.05 a.u.) and AgH (−0.36 a.u.) while the corresponding value for AuH is large and positive (+2.00 a.u.).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461303
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  • 7
    Digitale Medien
    Digitale Medien
    Quasirelativistic studies of molecular electric properties: Dipole moments of the group IVa oxides and sulfides (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 1345-1351 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Dipole moments of SiO, GeO, SnO, PbO, and of the corresponding sulfides are computed by using a variety of high-level correlated methods and polarized basis sets of Gaussian functions. The contribution of relativistic effects has been accounted for by the first-order quasirelativistic approach based on mass-velocity and Darwin terms. The results for sulfides are in good agreement with experiment while for oxides seem to show a progressive deterioration with the increase of the heavy atom nuclear charge. The discrepancies between the calculated and experimental dipole moments for oxides have been carefully analyzed for the PbO molecule in terms of the basis set choice and reliability of the calculated electron correlation and relativistic corrections. The pattern of relativistic corrections computed in the present study is compared with that which follows from recent quasirelativistic and fully relativistic calculations of Dyall. Some complementary calculations of the dipole moment of PbSe and PbTe seem to indicate that both the polarized basis sets and computational methods used in this study should not fail to predict correct values of the dipole moment of PbO and other oxides of the group IVa atoms. The rather unexpected discrepancies have no plausible explanation within the range of methods employed in our study and offer a challenge for more advanced, fully relativistic molecular calculations at the correlated level of approximation.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.464301
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  • 8
    Digitale Medien
    Digitale Medien
    Electron correlation and relativistic effects in the coinage metal compounds. II. Heteronuclear dimers: CuAg, CuAu, and AgAu (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2991-2999 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Electric properties of heteronuclear dimers of the coinage metals are calculated at the level of the CCSD(T) approximation applied to 38 electrons of the valence and next-to-valence atomic shells. The relativistic effects are accounted for by using the scalar approximation to the Pauli hamiltonian. Both the pure relativistic and mixed relativistic-correlation contributions to energies and electric properties are computed. All calculations have been carried out by using the recently developed first-order polarized basis sets of the coinage metal atoms. In the non-relativistic approximation all studied dimers show only a moderate degree of polarity; the non-relativistic CuAg turns out to be the most polar dimer with the Cu(−)Ag(+) polarity. The relativistic effects considerably reduce the negative value of the CuAg dipole moment, change the sign of the CuAu dipole moment, and make the AgAu molecule the most polar species in the series. Simultaneously, the parallel component of the dipole polarizability shows only a small relativistic contraction. The calculated quasirelativistic interaction potentials have a correct behavior in the vicinity of their minima and give the Re and ωe values in complete agreement with experiment. Much less satisfactory are the dissociation energy data which seem to suffer from the single reference configuration approximation. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.470487
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  • 9
    Digitale Medien
    Digitale Medien
    Relativistic effects on electric properties of many-electron systems in spin-averaged Douglas–Kroll and Pauli approximations (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 1995-2003 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Relativistic effects and electron correlation effects on the dipole moments of the coinage metal hydrides are investigated and compared employing one-component (scalar) relativistic approximations based on the mass–velocity and Darwin operator and, alternatively, the Douglas–Kroll-transformed spin-averaged no-pair Hamiltonian. The former of the two operators is found to perform quite accurately for CuH and AgH. For AuH the limits of the Pauli approximation seem to be reached, as can be inferred from a comparison with the values obtained within the spin-averaged Douglas–Kroll no-pair formalism. The coupled cluster calculations in the Douglas–Kroll no-pair approximation for relativistic effects establish the dipole moment values of the coinage metal hydrides as equal to 1.05 a.u. for CuH, 1.14 a.u. for AgH and 0.52 for AuH. The corresponding non-relativistic results are 1.14 a.u., 1.36 a.u., and 1.22 a.u., respectively. Some formal problems arising in applications of the Douglas–Kroll no-pair approximation are discussed. It is shown that the Hellmann–Feynman theorem leads to a rather complicated form of the first-order energy change due to external perturbation. The usual expectation value formula is, however, valid through terms proportional to 1/c4 and can be used in most applications. The invariance property with respect to a shift in the external potential is addressed for the Douglas–Kroll no-pair approximation in a finite basis set. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.472067
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  • 10
    Digitale Medien
    Digitale Medien
    A CAS SCF study of reactive interactions between Be(3 P) and H2(1ε g + ) (1992)
    Springer
    Theoretical chemistry accounts 81 (1992), S. 417-424 
    Details
    ISSN: 1432-2234
    Schlagwort(e): Be(3 P) ; H2(1ε g + ) ; CAS SCF ; Adiabatic energy surface
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Summary TheC 2v symmetry section of the Be(3 P)+ H2(1ε g + ) adiabatic energy surface is investigated by using the CAS SCF method. The small active space CAS SCF calculations in the valence approximation are followed by a perturbation treatment of the dynamic, core, and core-valence contributions in the framework of the CASPT2 method. The possibility of the nonradiative chemical deactivation of the lowest triplet state of Be by the insertion mechanism is studied. The structure of the3 B 2 reaction intermediate BeH2 is established. The calculations show that the symmetric dissociation of this intermediate into Be(1 S) and 2H(2 S) does not involve any barrier beyond the endothermicity of the corresponding reaction. The hydrogen abstraction mechanism via a linear configuration is shown to possess the activation barrier of about 25 mH.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01134865
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