Structural parameters for chemisorption of atomic carbon above a Si(100) surface have been obtained through a Si35H32 cluster model and a MINDO/3 hamiltonian. The most stable position has been found to be the bridge one when considering the unrelaxed surface. The stability increases about 14 kcal/mol when relaxation of the surface is allowed. Further research has been carried out using a reduced cluster model (Si9H12) at the ab initio Hartree-Fock level of calculation. Results confirm the increase of stability of the relaxed system. At this level, the binding energy is 90 kcal/mol for the unrelaxed surface and the stabilization when the surface is relaxed is of about 20% with respect to the non-relaxed surface.