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  • American Institute of Physics (AIP)  (3)
  • Springer  (3)
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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 6540-6557 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structural and electronic properties of small sodium clusters are investigated using a distance-dependent extension of the tight-binding (Hückel) model and a Monte Carlo growth algorithm for the search of the lowest energy isomers. The efficiency and advantages of the Monte Carlo growth algorithm are discussed and the building scheme of sodium microclusters around constituting seeds is explained in details. The pentagonal-based seeds (pentagonal bipyramids and icosahedral structures) are shown to play an increasing role beyond n=12. Optimized geometries of Nan clusters are obtained in the range n=4–21 and for n=34. In particular, Na20 is found to have C3 symmetry, hardly prolate with all axial ratios almost equivalent, whereas Na34 has D5h symmetry and consists of a doubly icosahedral seed of 19 atoms surrounded by a ring of 15 atoms. Stabilities, fragmentation channels, and one-electron orbital levels are derived for the lowest isomers and shown to be characterized by a regular odd–even alternation. The present results are in generally good correspondence with previous nuclei-based calculations when available. The global shapes of clusters, as well as the shape-induced fine structure splitting of the spherical electronic jellium shell are found, with a few exceptions, to be also consistent with the ellipsoidal or spheroidal versions of the jellium model.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 10089-10089 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 4829-4836 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The new method, called effective group potential, which generalizes the concept of effective core potential by including in a potential the effects of many atoms ligand, is used to calculate transition energies. Three molecules containing a metal atom (Cu, Cr, Ni) surrounded by NH3 ligands have been studied. It is shown that by replacing the NH3 group by an effective group potential, the characteristic transitions energies are very well reproduced at various ab initio levels. The transitions studied are transitions between open shells on the metallic center but also ligand to metal transitions where, though unexpected, quite reliable results were also obtained. This method is thus very well suited for the study of the spectrum of a large molecule under the condition that a separation between an active part plus some spectator groups can be made, a condition satisfied in most organometallic molecules as well as in biological molecules. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-2234
    Keywords: Key words: Ab initio ; Effective group potentials
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract.  A new method for an increased numerical efficiency of ab initio calculations is proposed. It is based on the assumption that in most cases chemical properties of functional groups in molecules are mainly controlled by a few electrons. This statement allows one to distinguish between two classes of nuclei and electrons: active and inactive ones. The effective group potential (EGP) method presupposes that the effect of inactive electrons in a functional chemical group can be described by a pseudopotential, in the same way that core electrons are replaced by effective core potentials in atoms. It is shown that EGPs are able to predict chemical and structural features of the active part of a molecule and at a fraction of the ordinary computational cost. The preliminary results reported here concern the determination of EGPs for ammonia, the methyl radical and the cyclopendadienyl ligand, which represent different types of bonding.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 30 (1994), S. 57-68 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 64.70.-p
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na4, Na8, and Na20). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na4 and Na8, and two phase transitions for Na20. As concerns Na4 and Na8, the transition occurs at ≈200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na20, the two observed phase transitions can be described as a melting of the surface atoms (at ≈200 K) preliminarily to the fluctuation of an inner icosahedron seed (at ≈300 K).
    Type of Medium: Electronic Resource
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  • 6
    Publication Date: 1994-03-01
    Print ISSN: 0178-7683
    Electronic ISSN: 1431-5866
    Topics: Physics
    Published by Springer
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