We discuss a set of atomistic calculations of the structure of Si geometries with substitutional carbon atoms, involving the (100) surface or bulk features related to thin films grown in the (100) direction. We use both quantum mechanical density functional theory and empirical potential calculations at finite temperature and constant pressure to study the local structure, bonding characteristics and overall distribution of the carbon atoms in the host silicon lattice. These calculations reveal a strong nearest neighbor repulsion between substitutional carbon atoms, to the point where these atoms prefer to have fewer bonds than normally in order to avoid each other. This effect still holds for high temperatures and high carbon concentrations. As a result, bulk ordering of the type observed in Si–Ge alloys is unlikely to occur.