Application of local-spin-density approximation toa−Siand tetrahedrala−C

We discuss the application of the local-spin-density approximation (LSDA) and provide criteria to gauge the reliability of supercell models of $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ and tetrahedral amorphous carbon $(ta-\mathrm{C}).\mathrm{}$ We identify models of $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ 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 $ta-\mathrm{C}$ is $\pi$-bonded pairs at the Fermi level.