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

We discuss the application of the local-spin-density approximation (LSDA) a nd 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-reson ance (ESR) experiments and show that a LSDA level description of the electr on states is essential to describe these states. We offer an ab initio calc ulation of a well-isolated floating bond state and show that neither the ch arge nor spin is well localized. Finally, we suggest the origin of the ESR signal in ta-C is pi-bonded pairs at the Fermi level. [S0163-1829(99)08639- 5].