The Experts below are selected from a list of 219 Experts worldwide ranked by ideXlab platform
Martin Kaupp - One of the best experts on this subject based on the ideXlab platform.
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Hyperfine Coupling constants of the nitrogen and phosphorus atoms a challenge for exact exchange density functional and post hartree fock methods
Journal of Chemical Physics, 2010Co-Authors: Martin Kaupp, Alexei V Arbuznikov, Andreas Heselmann, Andreas GorlingAbstract:The isotropic Hyperfine Coupling constants of the free N(S4) and P(S4) atoms have been evaluated with high-level post-Hartree–Fock and density-functional methods. The phosphorus Hyperfine Coupling presents a significant challenge to both types of methods. With large basis sets, MP2 and coupled-cluster singles and doubles calculations give much too small values for the phosphorus atom. Triple excitations are needed in coupled-cluster calculations to achieve reasonable agreement with experiment. None of the standard density functionals reproduce even the correct sign of this Hyperfine Coupling. Similarly, the computed Hyperfine Couplings depend crucially on the self-consistent treatment in exact-exchange density-functional theory within the optimized effective potential (OEP) method. Well-balanced auxiliary and orbital basis sets are needed for basis-expansion exact-exchange-only OEP approaches to come close to Hartree–Fock or numerical OEP data. Results from the localized Hartree–Fock and Krieger–Li–Iafrat...
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scalar relativistic calculations of Hyperfine Coupling tensors using the douglas kroll hess method
Chemical Physics Letters, 2004Co-Authors: Irina Malkin, Olga L Malkina, Vladimir G Malkin, Martin KauppAbstract:Abstract Scalar relativistic calculations of Hyperfine Coupling tensors have been implemented for the first time at the Douglas–Kroll–Hess level, with proper transformation of Hyperfine operators up to second order. Implementation into the ReSpect program has allowed the assessment of the new approach for a number of atoms and diatomic molecules, in comparison with experimental data and other calculations. While neglect of the transformation of the operators leads to meaningless Hyperfine results, DFT calculations with operators transformed to first order agree already relatively well with previous ZORA results. Full transformation to second order leads to larger isotropic Hyperfine Couplings.
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Scalar relativistic calculations of Hyperfine Coupling tensors using the Douglas–Kroll–Hess method
Chemical Physics Letters, 2004Co-Authors: Irina Malkin, Olga L Malkina, Vladimir G Malkin, Martin KauppAbstract:Abstract Scalar relativistic calculations of Hyperfine Coupling tensors have been implemented for the first time at the Douglas–Kroll–Hess level, with proper transformation of Hyperfine operators up to second order. Implementation into the ReSpect program has allowed the assessment of the new approach for a number of atoms and diatomic molecules, in comparison with experimental data and other calculations. While neglect of the transformation of the operators leads to meaningless Hyperfine results, DFT calculations with operators transformed to first order agree already relatively well with previous ZORA results. Full transformation to second order leads to larger isotropic Hyperfine Couplings.
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relativistic spin orbit effects on Hyperfine Coupling tensors by density functional theory
Journal of Chemical Physics, 2004Co-Authors: Alexei V Arbuznikov, Juha Vaara, Martin KauppAbstract:A second-order perturbation theory treatment of spin-orbit corrections to Hyperfine Coupling tensors has been implemented within a density-functional framework. The method uses the all-electron atomic mean-field approximation and/or spin-orbit pseudopotentials in incorporating one- and two-electron spin-orbit interaction within a first-principles framework. Validation of the approach on a set of main-group radicals and transition metal complexes indicates good agreement between all-electron and pseudopotential results for Hyperfine Coupling constants of the lighter nuclei in the system, except for cases in which scalar relativistic effects become important. The nonrelativistic Fermi contact part of the isotropic Hyperfine Coupling constants is not always accurately reproduced by the exchange-correlation functionals employed, particularly for the triplet and π-type doublet radicals in the present work. For this reason, ab initio coupled-cluster singles and doubles with perturbative triples results for the ...
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mechanisms of epr Hyperfine Coupling in transition metal complexes
Journal of the American Chemical Society, 2000Co-Authors: Marketa Munzarova, Pavel Kubacek, Martin KauppAbstract:A detailed quantum chemical analysis of the underlying principles of Hyperfine Coupling in 3d transition metal complexes has been carried out. The explicit evaluation of one- and two-electron integrals for some atomic systems has been used to understand the spin polarization of the core shells. While spin polarization enhances the exchange interaction of the 2s and 2p shells with the singly occupied orbitals, the opposite spin polarization of the 3s and 3p shells arises from the required orthogonality to the 2s and 2p shells, respectively. Core-shell spin polarization in molecules is found to be proportional to the spin population in the valence 3d orbitals but to depend little on other details of bonding. In contrast, the spin polarization of the valence shell depends crucially on the overlap between the singly occupied and certain doubly occupied valence orbitals. Large overlap leads to pronounced spin polarization of these orbitals and, among other things, likely to spin contamination when using UHF wa...
Hans Agren - One of the best experts on this subject based on the ideXlab platform.
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role of zero point vibrational corrections to carbon Hyperfine Coupling constants in organic π radicals
Journal of Chemical Physics, 2013Co-Authors: Xiao Chen, Zilvinas Rinkevicius, Kenneth Ruud, Hans AgrenAbstract:By analyzing a set of organic π radicals, we demonstrate that zero-point vibrational corrections give significant contributions to carbon Hyperfine Coupling constants, in one case even inducing a sign reversal for the Coupling constant. We discuss the implications of these findings for the computational analysis of electron paramagnetic spectra based on Hyperfine Coupling constants evaluated at the equilibrium geometry of radicals. In particular, we note that a dynamical description that involves the nuclear motion is in many cases necessary in order to achieve a semi-quantitatively predictive theory for carbon Hyperfine Coupling constants. In addition, we discuss the implications of the strong dependence of the carbon Hyperfine Coupling constants on the zero-point vibrational corrections for the selection of exchange-correlation functionals in density functional theory studies of these constants.
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zero point vibrational corrections to isotropic Hyperfine Coupling constants in polyatomic molecules
Physical Chemistry Chemical Physics, 2011Co-Authors: Xing Chen, Zilvinas Rinkevicius, Kenneth Ruud, Hans AgrenAbstract:The present work addresses isotropic Hyperfine Coupling constants in polyatomic systems with a particular emphasis on a largely neglected, but a posteriori significant, effect, namely zero-point vibrational corrections. Using the density functional restricted-unrestricted approach, the zero-point vibrational corrections are evaluated for the allyl radical and four of its derivatives. In addition for establishing the numerical size of the zero-point vibrational corrections to the isotropic Hyperfine Coupling constants, we present simple guidelines useful for identifying hydrogens for which such corrections are significant. Based on our findings, we critically re-examine the computational procedures used for the determination of Hyperfine Coupling constants in general as well as the practice of using experimental Hyperfine Coupling constants as reference data when benchmarking and optimizing exchange–correlation functionals and basis sets for such calculations.
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Vibrationally induced carbon Hyperfine Coupling constants: a reinterpretation of the McConnell relation
2011Co-Authors: Xing Chen, Zilvinas Rinkevicius, Kenneth Ruud, Hans AgrenAbstract:Vibrationally induced carbon Hyperfine Coupling constants: a reinterpretation of the McConnell relation
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Restricted-unrestricted density functional theory for Hyperfine Coupling constants : vanadium complexes
2010Co-Authors: Xing Chen, Zilvinas Rinkevicius, Hans Agren, Olav Vahtras, Fuming Ying, Wei WuAbstract:Restricted-unrestricted density functional theory for Hyperfine Coupling constants : vanadium complexes
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density functional theory for Hyperfine Coupling constants with the restricted unrestricted approach
Journal of Chemical Physics, 2004Co-Authors: Zilvinas Rinkevicius, Lyudmyla Telyatnyk, Olav Vahtras, Hans AgrenAbstract:This work presents derivation, implementation, and the first applications of the restricted-unrestricted approach based on restricted Kohn-Sham formalism for evaluation of Hyperfine Coupling constants. By using the spin-restricted Kohn-Sham method the well-known spin contamination problem existing in the unrestricted Kohn-Sham formalism is avoided and a proper description of spin polarization is achieved via the restricted-unrestricted approach without introducing spin contamination into the evaluation of the Hyperfine Coupling constants. The performance of the proposed formalism is evaluated for a set of organic radicals and transition metal compounds. The results of this investigation indicate promising accuracy of the restricted-unrestricted approach for calculation of the isotropic Hyperfine Coupling constants in organic radicals as well as transition metal compounds.
Zilvinas Rinkevicius - One of the best experts on this subject based on the ideXlab platform.
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role of zero point vibrational corrections to carbon Hyperfine Coupling constants in organic π radicals
Journal of Chemical Physics, 2013Co-Authors: Xiao Chen, Zilvinas Rinkevicius, Kenneth Ruud, Hans AgrenAbstract:By analyzing a set of organic π radicals, we demonstrate that zero-point vibrational corrections give significant contributions to carbon Hyperfine Coupling constants, in one case even inducing a sign reversal for the Coupling constant. We discuss the implications of these findings for the computational analysis of electron paramagnetic spectra based on Hyperfine Coupling constants evaluated at the equilibrium geometry of radicals. In particular, we note that a dynamical description that involves the nuclear motion is in many cases necessary in order to achieve a semi-quantitatively predictive theory for carbon Hyperfine Coupling constants. In addition, we discuss the implications of the strong dependence of the carbon Hyperfine Coupling constants on the zero-point vibrational corrections for the selection of exchange-correlation functionals in density functional theory studies of these constants.
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zero point vibrational corrections to isotropic Hyperfine Coupling constants in polyatomic molecules
Physical Chemistry Chemical Physics, 2011Co-Authors: Xing Chen, Zilvinas Rinkevicius, Kenneth Ruud, Hans AgrenAbstract:The present work addresses isotropic Hyperfine Coupling constants in polyatomic systems with a particular emphasis on a largely neglected, but a posteriori significant, effect, namely zero-point vibrational corrections. Using the density functional restricted-unrestricted approach, the zero-point vibrational corrections are evaluated for the allyl radical and four of its derivatives. In addition for establishing the numerical size of the zero-point vibrational corrections to the isotropic Hyperfine Coupling constants, we present simple guidelines useful for identifying hydrogens for which such corrections are significant. Based on our findings, we critically re-examine the computational procedures used for the determination of Hyperfine Coupling constants in general as well as the practice of using experimental Hyperfine Coupling constants as reference data when benchmarking and optimizing exchange–correlation functionals and basis sets for such calculations.
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Vibrationally induced carbon Hyperfine Coupling constants: a reinterpretation of the McConnell relation
2011Co-Authors: Xing Chen, Zilvinas Rinkevicius, Kenneth Ruud, Hans AgrenAbstract:Vibrationally induced carbon Hyperfine Coupling constants: a reinterpretation of the McConnell relation
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Restricted-unrestricted density functional theory for Hyperfine Coupling constants : vanadium complexes
2010Co-Authors: Xing Chen, Zilvinas Rinkevicius, Hans Agren, Olav Vahtras, Fuming Ying, Wei WuAbstract:Restricted-unrestricted density functional theory for Hyperfine Coupling constants : vanadium complexes
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density functional theory for Hyperfine Coupling constants with the restricted unrestricted approach
Journal of Chemical Physics, 2004Co-Authors: Zilvinas Rinkevicius, Lyudmyla Telyatnyk, Olav Vahtras, Hans AgrenAbstract:This work presents derivation, implementation, and the first applications of the restricted-unrestricted approach based on restricted Kohn-Sham formalism for evaluation of Hyperfine Coupling constants. By using the spin-restricted Kohn-Sham method the well-known spin contamination problem existing in the unrestricted Kohn-Sham formalism is avoided and a proper description of spin polarization is achieved via the restricted-unrestricted approach without introducing spin contamination into the evaluation of the Hyperfine Coupling constants. The performance of the proposed formalism is evaluated for a set of organic radicals and transition metal compounds. The results of this investigation indicate promising accuracy of the restricted-unrestricted approach for calculation of the isotropic Hyperfine Coupling constants in organic radicals as well as transition metal compounds.
Rodney J Bartlett - One of the best experts on this subject based on the ideXlab platform.
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Hyperfine Coupling constants of organic radicals
Journal of Chemical Physics, 1997Co-Authors: Ajith S Perera, Lynn M Salemi, Rodney J BartlettAbstract:The isotropic Hyperfine Coupling constants of several organic radicals including CH3, CH2, CH2−, C2H5, C2H3, H2CN, C6H7, and C3H5 are calculated analytically using the coupled cluster (CC) “relaxed density’’ matrix approach. We employ three different commonly used basis sets with CCSD and CCSD(T) in order to calibrate expected accuracy. The Chipman basis set combined with the CCSD(T) method performs best for carbon isotropic Hyperfine Coupling constants with a mean absolute deviation within 8% compared to experiment. The corresponding mean absolute deviation for hydrogen isotropic Hyperfine Coupling constants from experiment is 12%. We show that the UHF, ROHF, and quasi (QRHF) reference function CCSD spin densities are effectively numerically equivalent in the notorious case of the allyl radical.
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a theoretical study of Hyperfine Coupling constants
Journal of Chemical Physics, 1994Co-Authors: Ajith S Perera, John D Watts, Rodney J BartlettAbstract:Isotropic Hyperfine Coupling constants of first‐row atoms from B–F and the BH2 radical are calculated analytically from the coupled‐cluster (CC) relaxed density with a variety of extended basis sets. We employ both restricted and unrestricted Hartree–Fock reference functions, with the CC singles and doubles (CCSD), CCSD with noniterative triples [CCSD+T(CCSD) and CCSD(T)] methods. The latter provide excellent agreement with experiment. We also consider the role of orbital relaxation and atomic basis functions in accurate predictions.
Ajith S Perera - One of the best experts on this subject based on the ideXlab platform.
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Hyperfine Coupling constants of organic radicals
Journal of Chemical Physics, 1997Co-Authors: Ajith S Perera, Lynn M Salemi, Rodney J BartlettAbstract:The isotropic Hyperfine Coupling constants of several organic radicals including CH3, CH2, CH2−, C2H5, C2H3, H2CN, C6H7, and C3H5 are calculated analytically using the coupled cluster (CC) “relaxed density’’ matrix approach. We employ three different commonly used basis sets with CCSD and CCSD(T) in order to calibrate expected accuracy. The Chipman basis set combined with the CCSD(T) method performs best for carbon isotropic Hyperfine Coupling constants with a mean absolute deviation within 8% compared to experiment. The corresponding mean absolute deviation for hydrogen isotropic Hyperfine Coupling constants from experiment is 12%. We show that the UHF, ROHF, and quasi (QRHF) reference function CCSD spin densities are effectively numerically equivalent in the notorious case of the allyl radical.
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a theoretical study of Hyperfine Coupling constants
Journal of Chemical Physics, 1994Co-Authors: Ajith S Perera, John D Watts, Rodney J BartlettAbstract:Isotropic Hyperfine Coupling constants of first‐row atoms from B–F and the BH2 radical are calculated analytically from the coupled‐cluster (CC) relaxed density with a variety of extended basis sets. We employ both restricted and unrestricted Hartree–Fock reference functions, with the CC singles and doubles (CCSD), CCSD with noniterative triples [CCSD+T(CCSD) and CCSD(T)] methods. The latter provide excellent agreement with experiment. We also consider the role of orbital relaxation and atomic basis functions in accurate predictions.