Be and Ca substitutional impurities in bulk MgO are srudied using the periodic ab inirio Hartre?-Fock method and the considerably, simpler ionic model based on two-body forces and the dipole shell model. In both cases a supercell approach was used; in the Harkee-Fock calculations the Izgest unit cell conkCned 32 atoms whilst in the case of the ionic model supercells of up to 250 atoms were employed. The HartreeFock results for the Be and Ca defect f o d o n em-are -4.10 and 6.25 eV respectively, the COnEspondiog results Itom the ionic model being -3.88 and 6.37 eV, these data being defined with respect to ionic, raIher than atomic. substitutions. The effect of such defects an the Hmee-Fock charge distribution of the host is examined in terms of nuclear displacements (these being compared with the corresponding results of the ionic model) 'and eleckonic redistribution, the latter being analysed in terms of c-e density maps and induced atomic multipolcs. The convergeace pattern of the Harlree-Fock deF8ct Formation energies is discussed using data h m cells containing 8.16 and 32 atoms. The larger unit cells employed with the ionic model allow a more careful study of the convergence rate of the superceil method.
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