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  • 1
    Electronic Resource
    Electronic Resource
    On the strength of electron correlations in high-T c superconducting copper oxides (1989)
    Springer
    The European physical journal 76 (1989), S. 143-151 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We analyze the strength of electronic correlations in the half-filled antibonding Cu−O orbitals of high-T c superconducting copper oxides by considering a Cu12O 17 n− cluster withn=8 or 10, respectively. The correlated ground state is calculated by the method of the local approach (LA) in a version which allows the treatment of stronger correlated electrons. As mean-field basis a semiempirical Hamiltonian of the ZDO (zero differential overlap) type has been adopted. It is found that the correlations are particularly strong in the Cu 3d x 2−y2 orbitals. The nonintegral orbital occupation allows for valence fluctuations between Cu+ and Cu2+ in spite of the remarkable correlations. According to the present model excess holes are located at the oxygen sites. The theoretical findings are compared with the results of spectroscopic investigations. The present electronic-structure analysis allows for a straighforward rationalization of previous experimental measurements. The strong connection between the importance of electronic correlations and the symmetry properties of the electronic wave function is emphasized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01312677
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  • 2
    Electronic Resource
    Electronic Resource
    A CNDO/INDO crystal orbital model for transition metal polymers of the 3d series—basis equations (1983)
    Springer
    Theoretical chemistry accounts 62 (1983), S. 351-372 
    Details
    ISSN: 1432-2234
    Keywords: Tight-binding formalism for 3d polymers ; Band structure approach in the CNDO/INDO approximation ; Basis equations and parametrization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The crystal orbital formalism in the tight-binding approximation is combined with a recently developed CNDO/INDO model for transition metal species of the 3d series in order to allow band structure calculations on the Hartree-Fock (HF) SCF level for one-dimensional (1D) chains with organometallic unit cells. The band structure approach based on the CNDO and INDO approximation can be used for any atom combination up to bromine under the inclusion of the 3d series. The matrix elements for the tight-binding Hamiltonian are derived for an improved CNDO and INDO framework. The total energy of the 1D chain is partitioned into one-center contributions and into two-center increments of the intracell and intercell type. Semiempirical band structure calculations on simple model systems are compared with available ab initio data of high quality.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00547893
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  • 3
    Electronic Resource
    Electronic Resource
    A Green's function approach to the photoelectron spectrum of bis(π-allyl)nickel[1] (1980)
    Springer
    Theoretical chemistry accounts 57 (1980), S. 315-321 
    Details
    ISSN: 1432-2234
    Keywords: INDO calculations ; relaxation and correlation contributions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The vertical ionization potentials of bis(π-allyl)nickel (see (1) in Fig. 1) are calculated by means of the Green's function approach within a semiempirical INDO extension to the first transition metal series. The computed ionization potentials are in good agreement with an experimentally deduced assignment. In contrast to earlier theoretical and experimental studies, the 7a u (π) level is predicted on top of the levels corresponding to the Ni 3d orbitals. Our approach leads to a complete assignment of the PE spectrum of (1) in the outer valence region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552743
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  • 4
    Electronic Resource
    Electronic Resource
    A CNDO/INDO crystal orbital model for transition metal polymers of the 3d series—the band structure of nickel(II)glyoximate (1983)
    Springer
    Theoretical chemistry accounts 62 (1983), S. 373-396 
    Details
    ISSN: 1432-2234
    Keywords: Tight-binding formalism for 3d polymers ; Band structure approach in the CNDO/INDO approximation ; Nickel(II)glyoximate polymer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The band structure of nickel(II)glyoximate has been investigated by means of an INDO crystal orbital formalism based on the tight-binding approximation. Calculations have been performed for unit cell dimensions of 3.223 Å which corresponds to the geometry of the partially oxidized chain and of 3.547 Å for the unoxidized polymer. The convergence of the lattice sums has been studied. It is shown that resonance and classical electrostatic integrals lead to a fast convergence while the exchange contributions are only slowly reduced with increasing intercell separation. The rotational profile of the title compound shows a pronounced minimum for the staggered 90 ° conformation; this is in line with experimental X-ray data. The conformation of the low-dimensional system is determined by the maximization of stabilizing intercell energies. The intracell stabilization is largest for an orientation where the intercell coupling is most inefficient (α = 50 °−60 °). Ligand π and lone-pair bands are predicted on top of the various Ni bands in the INDO band structure calculations. This sequence is conserved even if the creation of localized 3d states with trapped valences upon partial oxidation is considered. The ground state of the partially oxidized polymer corresponds to a system where charge density from the ligand framework has been removed leading to a typical organic metal or semimetal. The computational results are compared with experimental observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00547894
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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
    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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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    A CNDO/INDO molecular orbital formalism for the elements H to Br. applications (1981)
    Springer
    Theoretical chemistry accounts 59 (1981), S. 153-179 
    Details
    ISSN: 1432-2234
    Keywords: Semiempirical LCAO-method ; CNDO- and INDO-approximation ; Transition metal compounds ; Computational results
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The CNDO/INDO molecular orbital formalism introduced in the preceding paper has been applied to a large number of atom combinations up to bromine under the inclusion of the first transition metal series. The results are compared with experimental data (geometries, ionization potentials, dipole moments) or with the results of sophisticatedab initio calculations (one electron energies, net charges, atomic populations). The semiempirical model reproduces for a wide range of molecules the experimental andab initio data with remarkable success.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552537
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  • 9
    Electronic Resource
    Electronic Resource
    A CNDO/INDO molecular orbital formalism for the elements H to Br. theory (1981)
    Springer
    Theoretical chemistry accounts 59 (1981), S. 127-151 
    Details
    ISSN: 1432-2234
    Keywords: Semiempirical LCAO-methods ; CNDO- and INDO-approximation ; Transition metal compounds ; theory and parametrization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A CNDO and INDO formalism is presented that can be used for any atom combination up to bromine under inclusion of the first transition metal series. The semiempirical parameters were chosen to reproduce results ofab initio calculations on metalorganic compounds. The calculational results are invariant to rotations of the coordinate system but not to a general transformation into other basis functions. The one-center Coulomb-expressions were selected in order to include intraatomic correlation contributions. Within the CNDO model this could be achieved by the scaled monopole termF 0, while in the INDO framework the one-center Coulomb integrals are given as a sum of the monopole-contributionF 0 and higher multipole contributions expressed as a linear combination of Slater-Condon parameters. The invariance problem in the case of local rotations within the INDO approximation was solved by considering the combination of one-center Coulomb and exchange integrals as a function ofl but independent ofm. The two-center electron-electron interaction terms were calculated via the Dewar-Sabelli, Ohno-Klopman relation. Penetration effects were treated according to Fischer and Kollmar. For the resonance integralH μv AB parameters are used which carry information related to the directed nature of the chemical bond by using optimized Klondyke functions. The core-core repulsion is constructed as a superposition of a soft potential function, describing polarization effects of the atomic cores, and a hard repulsion function, avoiding the collapse of the atomic cores with decreasing distance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552536
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  • 10
    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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