Gap formation and defect states in tetrahedral amorphous carbon

We present a model of tetrahedral amorphous carbon (with density 3.0 g ${\mathrm{cm}}^{\mathrm{-}3}$) obtained from self-consistent plane-wave (Car-Parrinello) methods. This model is closely related to one we presented earlier [Phys. Rev. B 49, 16 415 (1994)], except for an even greater tendency toward ${\mathit{sp}}^2$ π pair and chain formation. We also discuss, in general terms, appropriate means for comparing models generated with different methods and emphasize the need for a proper relaxation of ionic coordinates. © 1996 The American Physical Society.