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  • 1
    Electronic Resource
    Electronic Resource
    Electronic states of the copper, silver, and gold silicides and their ions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4654-4661 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of theoretical calculations for the ground state and low-lying excited states of SiCu, SiAg, and SiAu, and their ions SiCu+, SiAg+, SiAu+ and SiCu−, SiAg−, SiAu− are presented. Calculations were carried out with high-level correlated methods including relativistic corrections at the level of the Douglas–Kroll approximation. The ground state data are compared with the recent experimental findings and they differ in the assignment of the ground-state symmetry. All neutral silicides are predicted to have the electronic ground state of 2Π symmetry, in agreement with earlier theoretical data. The neutral species and both negative and positive ions of silicides are found to be quite stable in the ground electronic state and in several low-lying excited states. The relativistic effects bring significant contribution to the stabilization of the gold silicide and its ions in all electronic states investigated in this paper. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1288919
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  • 2
    Electronic Resource
    Electronic Resource
    The relativistic scheme for eliminating small components Hamiltonian: Analysis of approximations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4003-4007 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The derivation of the recently proposed one-component relativistic Hamiltonian, and the resulting relativistic scheme by eliminating small components (RESC) method of Nakajima and Hirao, are analyzed in terms of the Foldy–Wouthuysen transformation of the Dirac Hamiltonian. This approach reveals the meaning of different approximations used in the derivation of the RESC Hamiltonian and its close relation to approximate relativistic Hamiltonians resulting from the free-particle Foldy–Wouthuysen transformation. Moreover, the present derivation combined with what is called the classical approximation in Nakajima and Hirao's approach shows that there is a whole family of the RESC-type Hamiltonians. Some of them, including the original RESC Hamiltonian, are analyzed numerically. It is documented that neither of the RESC-type Hamiltonians offers variational stability. As a consequence the RESC methods may suffer from the variational collapse for heavier systems. On the other hand the energy differences (e.g., ionization potentials) computed within the RESC approach turn out to be close to the values obtained in the Douglas–Kroll scheme. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1286807
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  • 3
    Electronic Resource
    Electronic Resource
    Infinite-order two-component theory for relativistic quantum chemistry (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 2696-2704 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method for the iterative algebraic generation of the numerically accurate two-component Hamiltonian for the use in relativistic quantum chemistry is presented. The separation of the electronic and positronic states of the Dirac Hamiltonian is accomplished by the algebraic solution for the Foldy–Wouthuysen transformation. This leads to the two-component formalism whose accuracy is primarily limited by the choice of basis functions. Its performance is tested in calculations of the most sensitive 1s1/2 energy for increasing values of the nuclear charge. These calculations show that the electronic part of the Dirac eigenspectrum can be obtained from the two-component theory to arbitrarily high accuracy. Moreover, if needed, the positronic states can be separately determined in a similar way. Thus the present method can be also used for the evaluation of quantum electrodynamic corrections in the finite basis set approximation. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1436462
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  • 4
    Electronic Resource
    Electronic Resource
    Systematic treatment of relativistic effects accurate through arbitrarily high order in α2 (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 9315-9324 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A systematic method for the generation of two-component relativistic Hamiltonians for use in relativistic quantum chemistry is presented and discussed. The free particle Foldy–Wouthuysen transformation of the Dirac Hamiltonian is performed prior to the determination of the block-diagonalizing unitary transformation. The latter can be determined iteratively through arbitrarily high leading order with respect to α (fine structure constant). Certain freedom in the initialization of the iterative solution leads to the whole class of two-component Hamiltonians h2k which are exact through the order of α2k and differ in contributions of all higher orders in α2. The efficiency of different iterative schemes is analyzed. Also the relation between the present method and the Douglas–Kroll approximation is investigated. The performance of two-component Hamiltonians for k=2, 3, and 4 is studied numerically in calculations of energies of the 1s1/2 level in heavy hydrogen-like ions. Their performance in calculations of the valence-determined atomic and molecular properties is investigated by computing the ionization potential of Au and spectroscopic constants of the AuH molecule. The total energy of these systems strongly depends on the level of exactness with respect to α2. However, for moderately relativistic systems the α4-class Hamiltonian derived in this paper is found to be sufficient for accurate calculations of valence-determined properties. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1370532
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  • 5
    Electronic Resource
    Electronic Resource
    On the ground state of NiH2 (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 3699-3700 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The uncertainty over the symmetry of the ground state of NiH2 is resolved, showing that it is a bent 1A1. The computations have been performed using a complete active space self-consistent field wave function (CASSCF), a second order perturbation method (CASPT2), and quasirelativistic corrections to the energy and geometry.© 1998 American Institute of Physics. [S0021-9606(98)30333-5]
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476967
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  • 6
    Electronic Resource
    Electronic Resource
    Electronic states of the copper silicide and its ions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2973-2977 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Potential energy curves and spectroscopic parameters of the ground and exited states of SiCu, SiCu+, and SiCu− are presented. The calculations were performed by high-level correlated methods including the relativistic correction for the lowest states. The present results are compared with recent theoretical and experimental studies of SiCu and its ions and support the earlier theoretical conclusions concerning the assignment of the electronic ground state of SiCu. According to calculations presented in this paper the lowest energy states of SiCu, SiCu+, and SiCu−, are 2Πr, 1Σ+, and 3Σ−, respectively. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479579
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  • 7
    Electronic Resource
    Electronic Resource
    Fock-space coupled-cluster method (1991)
    Springer
    Theoretical chemistry accounts 80 (1991), S. 483-507 
    Details
    ISSN: 1432-2234
    Keywords: Coupled-cluster method ; Fock-space formalism ; PPP model ; Electronic spectra calculations ; Intruder states
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A generalization of the single-reference coupled-cluster method, employing the algebraic properties of the fermionic Fock space, is presented. This Fock-space coupled-cluster (FSCC) method is capable of providing not only the ground-state energy of anN-electron system, but also an important fraction of system's excitation spectrum, including ionization potentials, electron affinities, and excitation energies corresponding toN-electron singlet and triplet states. The FSCC method is applied to study the electronic spectra corresponding to the Pariser-Parr-Pople model of butadiene, hexatriene, and benzene, with the full configuration-interaction results taken as the reference. The problem of intruder states is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01119667
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  • 8
    Electronic Resource
    Electronic Resource
    Expectation values of operators in approximate two-component relativistic theories (1997)
    Springer
    Theoretical chemistry accounts 97 (1997), S. 260-270 
    Details
    ISSN: 1432-2234
    Keywords: Key words: Relativistic quantum chemistry ; Expectation values ; Direct perturbation theory ; Two-component relativistic Hamiltonians ; Regular relativistic Hamiltonians
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. Two methods for the evaluation of expectation values with approximate two-component relativistic functions are analysed. The first of them is based on the change of picture for the operator whose expectation value is to be calculated and associated with approximations leading to the given two-component relativistic wave function. This method, though hardly used in numerical calculations, gives the expectation values that directly reflect the accuracy of the wave function used for their calculation. The second method, most commonly used in calculations, neglects the picture change and is shown always to introduce an error of the order of α2, where α is the fine structure constant. This error is present independently of the accuracy of the approximate two-component wave function. The perturbation formalism developed in this paper is illustrated by calculations of relativistic corrections to the expectation values of r −1 for arbitrary states of hydrogenic ions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002140050260
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  • 9
    Electronic Resource
    Electronic Resource
    Fock space multi-reference coupled cluster study of transition moment and oscillator strength (1995)
    Springer
    Theoretical chemistry accounts 90 (1995), S. 257-271 
    Details
    ISSN: 1432-2234
    Keywords: Coupled cluster method ; Fock-space formalism ; Excitation energies ; Transition moment ; Oscillator strength
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A method of calculating transition moment and oscillator strength within the framework of the Fock space multi-reference coupled cluster method is described. Diagrammatic technique is used to obtain coupled cluster equations. The general form of equations for the transition moment betweenN-electron ground and excited states is obtained. MBPT analysis of the final equations is done. The excitation energies, dipole transition moments and oscillator strengths for theCH + molecule are calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113471
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  • 10
    Electronic Resource
    Electronic Resource
    Fock space multi-reference coupled cluster study of transition moment and oscillator strength (1995)
    Springer
    Theoretica chimica acta 90 (1995), S. 257-271 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Coupled cluster method ; Fock-space formalism ; Excitation energies ; Transition moment ; Oscillator strength
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  A method of calculating transition moment and oscillator strength within the framework of the Fock space multi-reference coupled cluster method is described. Diagrammatic technique is used to obtain coupled cluster equations. The general form of equations for the transition moment between N-electron ground and excited states is obtained. MBPT analysis of the final equations is done. The excitation energies, dipole transition moments and oscillator strengths for the CH + molecule are calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113471
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    Paper (German National Licenses)
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