Ca and Be substitution in bulk MgO: 'ab initio' Hartree-Fock and ionic model supercell calculations

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Abstract
Be and Ca substitutional impurities in bulk MgO are studied using the periodic ab initio Hartree-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 Hartree-Fock calculations the largest unit cell contained 32 atoms whilst in the case of the ionic model supercells of up to 250 atoms were employed. The Hartree-Fock results for the Be and Ca defect formation energies are -4.10 and 6.25 eV respectively, the corresponding results from the ionic model being -3.88 and 6.37 eV, these data being defined with respect to ionic, rather than atomic, substitutions. The effect of such defects on the Hartree-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 electronic redistribution, the latter being analysed in terms of charge density maps and induced atomic multipoles. The convergence pattern of the Hartree-Fock defect formation energies is discussed using data from 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 supercell method.