Atomistic simulations of different hole centres (such as the self-trapped hole, V2-, V-and VMgcentres) have been performed in order to study theoretically their equilibrium geometry, charge distribution and optical absorption in corundum crystals ( alpha -Al2O3). Both the semi-empirical quantum-chemical intermediate neglect of differential overlap method and the atomistic simulation technique based on atom-atom pair potentials (as implemented in the CASCADE code) have been used, and the principal results from these two approaches turn out to be in good agreement with one another. It is shown that the most favourable defect configuration energetically is a hole shared equally between two neighbouring O atoms, and that this is the main factor responsible for the very similar absorption energies of these hole centres.
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