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  • 1
    ISSN: 1432-2234
    Keywords: Key words: Nature of bonding for Fe(CO)n(n = 1–3) – Complete-active-space self-consistent-field method – Multireference configuration interaction – Mulliken population analysis – Charge density difference map
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. Geometry optimization was performed for the ground states of FeCO, Fe(CO)2, and Fe(CO)3 at various levels of ab initio calculations, and the bond lengths and dissociation energies obtained were in reasonable agreement with experimental results. The nature of bonding was studied for these molecules using a complete-active-space self-consistent-field method. From the Mulliken population analysis, it was found that the traditional donation and back donation mechanism is valid for these molecules, including Fe(CO)3, which has a pyramidal structure.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-2234
    Keywords: Key words: Basis sets – Valence functions – Correlated functions – Contracted Gaussian-type functions – First-row transition atoms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. Contracted Gaussian-type function sets are developed for the valence 4s and 3d orbitals and for correlated functions of the first-row transition atoms from Sc to Zn. A segmented contraction scheme is used for its compactness and computational efficiency. The contraction coefficients and exponents of the valence and correlated sets are determined by minimizing the differences from weighted averages of accurate atomic natural orbitals for the 4s 23d n −2 and 4s 13d n −1 atomic states. The new basis sets give a well-balanced description for these configurations at the Hartree–Fock level and yield more than 97% of the atomic correlation energies predicted by accurate natural orbitals of the same size. Molecular tests of the present basis functions are performed for the FeCO molecule at complete-active-space self-consistent-field and at single and double excitation configuration interaction levels. The present sets show an accuracy similar to that of the averaged atomic natural orbital sets in spite of 3–5 times shorter computation time in the generation of two-electron integrals.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report here on the electronic structures of [(Mo6Cl8)Cl6]2− and [(Re6S8)Cl6]4− obtained by the Tamm–Dankoff configuration interaction approximation, where Darwin and mass-velocity terms are taken into account through the model core potentials, and the spin–orbit interaction term is included in a semiempirical manner. These complexes have absorption spectra ranging from the visible to the near-ultraviolet energy region and have the long-lived luminescence. The calculated results of the transition moments roughly reproduce the experimental absorption and emission spectra. Especially for the absorption spectra, the theoretical strong intensity for the region higher than 4.5 eV in the two complexes is in good agreement with the experimental results. The obtained intense peak at 3.9 eV may correspond to the experimental broad peak at approximately 3.6 eV for the Mo complex. For the Re complex, the calculated intensity distribution gives one broad peak at around 3.1 eV. This peak may correspond to the experimental peak at 2.9 eV. In the experimental emission spectra, the maxima are located at 1.62 eV for the Mo complex and approximately 1.8 eV for the Re complex. Theoretically, 12 and eight states are found in the energy range between 1.5 and 2.0 eV for the Mo and Re complexes, respectively. At least one, if not more, of these states would be responsible for each emission maximum. To determine the character of the above transitions, the absorption and emission transitions were experimentally shown to be ligand-to-metal charge-transfer transitions and metal-localized transitions, respectively. Theoretically, absorption transitions were characterized as a mixture of metal-localized transitions and ligand-to-metal charge-transfer transitions, and emission transitions a mixture of metal-localized transitions and metal-to-ligand charge-transfer transitions. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 7503-7512 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio calculations are performed to elucidate the mechanism of the photoisomerization of azobenzene. We obtain the excitation energies of the S1(n→π*), S2(π→π*), and S3(n2→π*2) states by complete active space self-consistent field (CASSCF) and multireference single double configuration interaction (MRSDCI) calculations. Two-dimensional potential surfaces of the ground- and excited states are obtained at the CASSCF level in order to investigate the isomerization pathways. A conical intersection between the ground state and the S1 state is found near the midpoint of the rotation pathway, and causes a radiationless transition. On the other hand, the S2 state has local minima at the cis and trans structures, so that the isomerization proceeds at the S2 surface following the deexcitation. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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