  The relaxed and unrelaxed formation energies of neutral antisites and
interstitial defects in InP are calculated using ab initio density functional
theory and simple cubic supercells of up to 512 atoms. The finite size errors
in the formation energies of all the neutral defects arising from the supercell
approximation are examined and corrected for using finite size scaling methods,
which are shown to be a very promising approach to the problem. Elastic errors
scale linearly, whilst the errors arising from charge multipole interactions
between the defect and its images in the periodic boundary conditions have a
linear plus a higher order term, for which a cubic provides the best fit. These
latter errors are shown to be significant even for neutral defects. Instances
are also presented where even the 512 atom supercell is not sufficiently
converged. Instead, physically relevant results can be obtained only by finite
size scaling the results of calculations in several supercells, up to and
including the 512 atom cell and in extreme cases possibly even including the
1000 atom supercell.
