Application of local-spin-density approximation to a-Si and tetrahedral a-C

We discuss the application of the local-spin-density approximation ~LSDA! and provide criteria to gauge the reliability of supercell models of a-Si:H and tetrahedral amorphous carbon (ta-C). We identify models of a-Si:H that exhibit a localization on dangling bond consistent with electron-spin-resonance ~ESR! experiments and show that a LSDA level description of the electron states is essential to describe these states. We offer an ab initio calculation of a well-isolated floating bond state and show that neither the charge nor spin is well localized. Finally, we suggest the origin of the ESR signal in ta-C is p-bonded pairs at the Fermi level. @S0163-1829~99!08639-5#