The substitution of C into bulk Si was investigated by using the super-cell approach, combined with the periodic  ab initio  Hartree-Fock method. The convergence of the defect formation energy and the relaxed defect geometry, as a function of the super-cell size, was considered for super-cells which contained 8, 16, 32 or 64 atoms. It was found that convergence of the unrelaxed defect formation energy was rapid; in spite of a large local charge redistribution around the defect (the net charge on C was 1.2|e|). The relaxation effects were very large (about 2eV) and mainly involved the first- and second-nearest neighbors. However, relaxation of the fifth-nearest neighbors of the defect (which was possible only in the case of the largest super-cell) lowered the energy by a further 0.06eV. The defect formation energies and the atomic displacements which were found in the case of the 32- and 64-atom super-cells were similar, whereas the energy difference between the 16- and 32-atom cells was as large as 0.4eV.

R.Orlando, R.Dovesi, P.Azavant, N.M.Harrison, V.R.Saunders: Journal of Physics - Condensed Matter, 1994, 6[41], 8573-83