We calculated defect energies by using a Hartree-Fock method to model an inner region that includes the defect site and oneor two shells of lattice ions. This is surrounded by an embedding region that is described by a classical Mott-Littleton calculation. Electrostatic multipole consistency between the two jegions is maintained throughout.
The results of defect calculations on MgO are examined as a function of basis set. We find that a sophisticated basis set is required before a reliable defect formation energy can be guaranteed. This experience has allowed us to develop a new model for the off-center relaxation of the exciton in LiC1. These calculations have required the use of pseudo potential cores for anions and floating functions to model the diffuse electron. Lastly, we report on recent calculations concerning the stability of hole states in La2CuO4. We find considerable delocalization of the hole over both the Cu2+ ions and the 02− ions that form the Cu-O planes.