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  • 1
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2128-2138 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The proton-ordered ice VIII structure has been investigated by ab initio periodic Hartree–Fock calculations in the pressure interval from 0 to 30 GPa using a 6-31G** basis set. The structure was optimized by energy-minimization at different volumes, and from the resulting energy vs volume relationship, the equation of state of ice VIII was derived. The variation of the structure, intramolecular geometry, Mulliken charges, electron density, Raman spectrum, and infrared stretching vibrations with varying pressure were investigated. The agreement with existing experimental data is generally good. Nearest-neighbor hydrogen-bonded O...O distances decrease from 2.88 to 2.57 A(ring) as the pressure is increased from 0 to 30 GPa. For the same pressure range, the intramolecular OH bond increases from 0.951 to 0.955 A(ring) (giving a drOH/dP value of 0.000 14 A(ring)/GPA), the Mulliken charge on H increases from +0.386 to +0.452, the calculated bulk modulus increases from ∼25 to ∼160 GPa (corresponding experimental values are ∼25 at 2.4 GPa and ∼135 at 30 GPa), and the electron density redistribution is considerably enhanced. The frequency downshift of the OH stretching vibration varies from −200 cm−1 at 2.4 GPa to −500 cm−1 at 20 GPa; the corresponding experimental values are −300 and −650 cm−1. Electronic density-of-states diagrams are presented.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 2
    Digitale Medien
    Digitale Medien
    Springer
    Theoretical chemistry accounts 72 (1987), S. 277-289 
    ISSN: 1432-2234
    Schlagwort(e): Ab initio-Hartree Fock ; Perfect crystals ; Defective crystals
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Accurate Hartree-Fock LCAO calculations for moderately complex crystalline systems are now feasible; a number of important applications may be envisaged in the areas of material science and technology. Some critical aspects of the corresponding computer schemes are discussed which are of fundamental importance in determining the cost of the computation. Data are provided concerning actual computations which are indicative of the kind of periodic systems that can (or cannot) be treated at present. The result of a perfect-crystal ab initio HF study can be used as an input for treating with the same approximation local-defect problems, by use of suitable embedding techniques. A scheme of this kind is presented, and its computational implications are discussed: due to the intrinsic complexity of this problem, it may be foreseen that the study of defects in crystals will be a typical application of supercomputers in the area of quantum chemistry.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 1755-1774 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The symmetry properties of the mono- and bielectronic terms contributing to the Fock matrix in the ab initio Hartree-Fock treatment of periodic systems are discussed. A computational scheme which takes full advantage of the point symmetry is presented; in this respect, it represents a generalization of the scheme proposed in Int. J. Quantum Chem. 17, 501 (1980). Computational details and numerical examples are reported; it is shown that with respect to two of the bottlenecks of this kind of calculation, namely, computer time and backing storage required for the bielectronic integrals, it is possible to obtain saving factors as large as h and h2, respectively, where h is the order of the point group. Preliminary tests are reported which indicate that the study of relatively complicated systems, like quartz or corundum (9 and 10 atoms in the unit cell, respectively) at an ab initio Hartree-Fock level is now within reach.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 197-212 
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An ab initio Hartree-Fock method devoted to the study of polymers is presented, truncation criteria for the exchange and the Coulomb series are discussed, and it is shown that the error in the total energy evaluation can be reduced to below 0.0001 a.u./cell with little computational effort. Results for six polyacetylene isomers are presented and compared with previous ab initio data obtained using the same basis set. In agreement with the experimental findings, the alternating all-trans structure is the most stable.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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