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  • 1
    Electronic Resource
    Electronic Resource
    An analysis of the zero differential overlap approximation. Towards an improved semiempirical MO method beyond it (1995)
    Springer
    Theoretical chemistry accounts 92 (1995), S. 13-47 
    Details
    ISSN: 1432-2234
    Keywords: Zero differential overlap ; Semiempirical MO methods ; Löwdin transformation ; Overlap density ; Mulliken approximation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Some systematic errors of the zero differential overlap (ZDO) approximation in semiempirical molecular orbital (MO) methods are discussed. In π electron methods, a power series expansion of the inverse square rootS −1/2 of the overlap matrix and application of the Mulliken approximation to the two-electron integrals show that the ZDO Hamiltonian coincides with the Hamiltonian obtained by explicit performance of the Löwdin transformation up to first-order terms of diatomic overlap densities. Higher than first-order terms lead to a systematic up-shift of the canonical MO energies. Although a power series expansion ofS −1/2 is no longer possible in all-valence-electron methods, the MO levels resulting from the ZDO approximation are also systematically placed at too low energies, especially the low-lying occupied and the virtual MOs. A method based on explicit performance of the Löwdin transformation and retaining the simplicity of the ZDO approach for the calculation of Fock matrix elements is developed. The parameters of this method are obtained by very simple manipulations of the original ZDO parameters. Numerical calculations show that a considerable improvement of the MO energy spectrum in the inner valence region can be obtained in this way
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01134202
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  • 2
    Electronic Resource
    Electronic Resource
    Electron correlation in weakly coupled transition metal dimers: Bimetallocenylenes and bimetallocenes (1981)
    Springer
    Theoretical chemistry accounts 60 (1981), S. 233-268 
    Details
    ISSN: 1432-2234
    Keywords: Hartree-Fock instabilities ; Electron correlation in binuclear transition metal compounds ; INDO calculations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The Hartree-Fock (HF) instabilities in a series of bimetallocenes (1) and bimetallocenylenes (2) with Fe, Co, Ni and Cr as 3d centers have been investigated by means of a semiempirical INDO Hamiltonian. The HF picture is only valid in the case of the iron dimers. Strong correlation effects are encountered in the Co, Ni and Cr complexes. The necessary conditions for singlet, non-singlet (triplet) and non-real variations of the HF orbitals are discussed in detail. Singlet fluctuations are the result of intraatomic angular correlation (short-range) at each 3d center. The violation of the spin symmetry corresponds to a long-range interaction between the transition metal centers. Only for MOs with large 3d xz amplitudes there exists a channel for the interatomic spin decoupling. Consequences for polymetallocenes are shortly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02394726
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  • 3
    Electronic Resource
    Electronic Resource
    The electronic structures of some extended conjugated electron acceptors within the Hartree-Fock approximation and beyond the mean-field approach as verified by the Hartree-Fock instabilities (1984)
    Springer
    Theoretical chemistry accounts 66 (1984), S. 51-64 
    Details
    ISSN: 1432-2234
    Keywords: Extended conjugated electron acceptors ; electronic structure in the INDO and π-electronic approximations ; Hartree-Fock instabilities
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The electronic structures of the extended conjugated electron acceptors1–3 were investigated within the framework of a semiempirical INDO and a π-electronic Hartree-Fock Hamiltonian. Utilizing Koopmans' theorem, predictions are made of the magnitude of the ionization potentials and the electron affinities and their dependence on the molecular geometry (“olefinic” vs. “aromatic”) and the nature of the acceptor functions. The validity of the Hartree-Fock approximation has been studied by means of the Thouless stability conditions. The INDO and π-SCF wave functions turn out to be adequate descriptions of the ground states of1–3. A single particle hole pair, due to a small energy separation between the highest occupied and lowest virtual orbital and a large Coulomb integral with respect to these orbitals leads to non-singlet (triplet) instabilities or near instabilities of the wave functions indicating the importance of spin correlation in1–3.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00577139
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  • 4
    Electronic Resource
    Electronic Resource
    Electron correlation in weakly coupled transition metal dimers: Bimetallocenylenes and bimetallocenes (1981)
    Springer
    Theoretical chemistry accounts 60 (1981), S. 233-268 
    Details
    ISSN: 1432-2234
    Keywords: Hartree-Fock instabilities ; Electron correlation in binuclear transition metal compounds ; INDO calculations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The Hartree-Fock (HF) instabilities in a series of bimetallocenes (1) and bimetallocenylenes (2) with Fe, Co, Ni and Cr as 3d centers have been investigated by means of a semiempirical INDO Hamiltonian. The HF picture is only valid in the case of the iron dimers. Strong correlation effects are encountered in the Co, Ni and Cr complexes. The necessary conditions for singlet, non-singlet (triplet) and non-real variations of the HF orbitals are discussed in detail. Singlet fluctuations are the result of intraatomic angular correlation (short-range) at each 3d center. The violation of the spin symmetry corresponds to a long-range interaction between the transition metal centers. Only for MOs with large 3d xz amplitudes there exists a channel for the interatomic spin decoupling. Consequences for polymetallocenes are shortly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00553747
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  • 5
    Electronic Resource
    Electronic Resource
    An analysis of the zero differential overlap approximation. Towards an improved semiempirical MO method beyond it (1995)
    Springer
    Theoretica chimica acta 92 (1995), S. 13-47 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Zero differential overlap ; Semiempirical MO methods ; Löwdin transformation ; Overlap density ; Mulliken approximation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  Some systematic errors of the zero differential overlap (ZDO) approximation in semiempirical molecular orbital (MO) methods are discussed. In π electron methods, a power series expansion of the inverse square root S -1/2 of the overlap matrix and application of the Mulliken approximation to the two-electron integrals show that the ZDO Hamiltonian coincides with the Hamiltonian obtained by explicit performance of the Löwdin transformation up to first-order terms of diatomic overlap densities. Higher than first-order terms lead to a systematic up-shift of the canonical MO energies. Although a power series expansion of S -1/2 is no longer possible in all-valence-electron methods, the MO levels resulting from the ZDO approximation are also systematically placed at too low energies, especially the low-lying occupied and the virtual MOs. A method based on explicit performance of the Löwdin transformation and retaining the simplicity of the ZDO approach for the calculation of Fock matrix elements is developed. The parameters of this method are obtained by very simple manipulations of the original ZDO parameters. Numerical calculations show that a considerable improvement of the MO energy spectrum in the inner valence region can be obtained in this way
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01134202
    Location Call Number Expected Availability
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    Paper (German National Licenses)
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