Binding energies of carbon atoms adsorbed on a (2 x 1) reconstructed Si(100) surface were calculated as a function of sub-monolayer coverages. Also calculated were energies for C atoms deposited on the surface as small clusters, C-n (with n = 3 and 4). All calculations were conducted considering a model potential function developed recently for systems containing C and Si atoms. For the low coverage limit (representing coverages up to one monolayer), carbon adatoms were considered as occupying only low energy surface sites. Owing to relatively large separations among these sites, C-C interactions are negligible and only SI-C interactions are found to be contributing to binding energies. The lowest binding energy in this case corresponds to a coverage of theta = 0.25. In the case of small C clusters deposited on the surface, however, binding energies were found to be stronger because of the increasing importance of C-C interactions. From an energetic viewpoint, results obtained in this study indicate that adsorbing C atoms on a Si(100) surface are more likely to form clusters than a layer-by-layer growth leading to a smooth and uniform surface coverage. This outcome was found to be consistent with various experimental results.