UBC Theses and Dissertations

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UBC Theses and Dissertations

Theoretical calculations on molecules and clusters composed of some heavier elements Head, John David


In this thesis calculations have been made with the Xα scattered wave (XαSW) and complete neglect of differential overlap (CNDO) methods. A common theme involves the parametrisation of the CNDO method for the heavier elements (e.g. atomic numbers greater than 10), although an initial study concerned the re-parametrisation of the CNDO method for systems based on LiF-, It is indicated that the conventional CNDO/2 parameters appear inadequate for ionic systems which involve substantial re-allocation of charge from the normal atomic states. For this application, the CHDO method was reparametrised by direct reference to a previously published near-Hartree-Fock calculation on diatomic LiF, and the suitability of the new CNDO parameters was assessed by calculations on (LiF)₂ and on clusters consisting of up 18 atoms. In independent calculations, Evarestov and Lovchikov (1977) showed these new parameters have advantages over the other sets when using CNDO-type calculations for investigating the band structure of LiF. Chapter 3 reports new XαSW calculations for some clusters formed by silver atoms; these are for Ag₇ and Ag₁₀. which simulate the (111) surface of silver, Ag₁₃ and Ag₁₉, which simulate bulk silver and two forms of Ag₇I which are designed to investigate the adsorption of iodine atoms on the (111) surface of silver. Local and total density of states curves are reported, and high coordinate atoms have been found to exhibit strong localisation OB the electronic structure. So far CNDO parametrisations are not well developed for So far CNDO parametrisations are not well developed for clusters of transition metal atoms, and in chapter 4 new CNDO parameters for silver have been obtained by comparing local and total density of states and charge distributions from CNDO calculations with those from the XαSW method for the Ag₇ cluster- The new CNDO parameters have then been used for making CNDO calculations on farther silver clusters, namely Ag₆, Ag₁₀, Ag₁₃ and Ag₁₉, and the results are compared with data from the XαSW calculations. These CNDO calculations give d-band widths in broad agreement with those from the XαSW method. The most significant difference is that the CNDO method gives less localisation on central atoms with high coordination numbers than is found from the XαSW calculations. It is suggested that this apparent deficiency of the CNDO calculations may be less serious when the clusters are being used for modelling part of a solid metal rather than for specifically investigating the properties of small metal particles. The final chapter of this thesis presents calculations on systems composed of the elements aluminium to sulphur. New calculations have been made with the XαSW method for the molecular clusters Al₇, Si₅H₁₂ and P₄, and comparisons made with experiment and with other calculations where possible. However the main reason for making these calculations has been to use the charge distributions and transition-state energies, along with information obtained previously by Salahub et al. for S₃, for deriving new CNDO parameters for the elements aluminium to sulphur. Calculations using these parameters have been tested against a new XαSW calculation made here for an Al₁₀ cluster (which simulates the (111) surface of aluminium), and against XαSW calculations made previously for P₈, P₄S₅, SiH₄, PH₃, H₂S and S0₂. This way of getting CNDO parameters seems broadly successful in these cases, and this work thereby provides a firmer basis for using results from XαSW calculations for extending the CNDO procedures systematically to a wider range of the heavier elements.

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