CONVERGENCE PROPERTIES OF THE CLUSTER MODEL IN THE STUDY OF LOCAL PERTURBATIONS IN IONIC SYSTEMS - THE CASE OF BULK DEFECTS IN MGO

A finite MgO cluster embedded in the electrostatic field generated by the distributed point multipoles of the perfect crystal is used to eva luate the energy, geometry and charge distribution modifications due t o the substitution of the central Mg2+ ion with a Ca2+ or Be2+ ion, us ing Hartree-Fock theory. Results are compared with well converged peri odic supercell data; the three kinds of calculations, namely cluster, perfect periodic crystal and periodic supercell, have been performed w ith the same program, CRYSTAL, with the same basis set and using simil ar computational conditions. Good agreement is obtained between the cl uster and supercell results when a large cluster is considered. A key condition for such agreement is the homogeneous treatment of the infin ite perfect crystal and the cluster.