It was noted that large unit cell calculations of the properties of charged point defects in insulators largely neglected dielectric polarization of the crystal, because the periodically repeated cells were so small. Embedded quantum cluster calculations with shell-model crystals, representing a single defect in a large crystal, were able to represent the polarization more realistically. For such embedded quantum clusters, the optical excitation energy was evaluated for the N vacancy in charge state (+3): vN3+ in AlN. This was done with and without dielectric polarization of the embedding crystal. A discrepancy of a few % was found, when both ground and excited state orbitals were well-localized within the vacancy. It was shown that the discrepancy rose rapidly as the excited state became more diffuse. It was concluded that an embedded cluster approach would be required for transitions that involved even somewhat diffuse states. The investigation was based upon a new model for AlN that showed promise with regard to quantitative accuracy.

Effect of Dielectric Polarization on the Properties of Charged Point Defects in Insulating Crystals - the Nitrogen Vacancy in AlN. J.M.Vail, D.Schindel, A.Yang, O.Penner, R.Pandey, H.Jiang, M.A.Blanco, A.Costales, Q.C.Qiu, Y.Xu: Journal of Physics - Condensed Matter, 2004, 16[20], 3371-8