Wannier function analysis of tetrahedral amorphous networks

Car-Parrinello molecular dynamics has been used to synthesise and characterise tetrahedral amorphous carbon (ta-C) and hydrogenated silicon carbide (a-SiC:H). Using maximally-localized Wannier functions we are able to unambiguously label bonding in the networks. These functions are considerably more reliable than the coordination sphere approach, where bonding statistics may strongly depend on the particular choice of radius. Application of the local spin density approximation to ta-C improves the agreement with experiment, giving an improved sp3 fraction and a small proportion of unpaired spins. The chemical order of stoichiometric a-SiC:H alloys is found to increase upon hydrogenation, a dramatic initial rise being followed by a gradual decrease with further addition of hydrogen.