Session X19 - General Theory.
FOCUS session, Friday morning, March 22
117, Indiana Convention Center
Periodic boundary conditions are regularly employed in the calculation of point defects employing density-functional theory. However, the interaction of the defect with its images in neighboring cells is a possible source of error in this scheme. We have therefore studied the dependence of formation energies and charge transfer levels on the cell size for vacancies in bulk and on the surface of III-V semiconductors. For neutral defects we find that formation energies can be calculated accurately even for quite small cells provided that dispersion of defect states is taken into account when choosing the k-points and occupation. For charged defects we tested various compensation schemes (constant background, Gaussian charge distribution, dopants with integer and fractional valency). We find that in bulk systems, charge transfer levels calculated with constant background converge faster with cell size than predicted by prior estimates. For surface calculations, however, we find that the electrostatic potential diverges with the thickness of the vacuum layer. In this case, local compensation is more appropriate.