J. Phys.: Condens. Matter 8 (1996) 1123--1133. Printed in the UK

The substitution of carbon in bulk silicon is investigated at the Hartree--Fock level using a cluster model, and the results compared directly with periodic `supercell' calculations based on the same Hamiltonian, basis set and computational scheme. To study variations in calculated properties with cluster size, hydrogen-saturated clusters containing five, thirty-five and eighty-seven Si atoms are considered, with relaxation of up to the second shell of neighbours surrounding carbon impurities. The calculated atomic relaxations and charge distributions in supercells and large clusters are reasonably similar. In relaxed clusters however, boundary effects lead to appreciable differences in calculated defect formation energies