We study by means of first-principles pseudopotential method the coordination
defects in a-Si and a-Si:H, also in their formation and their evolution upon
hydrogen interaction. An accurate analysis of the valence charge distribution
and of the ``electron localization function'' (ELF) allows to resolve possible
ambiguities in the bonding configuration, and in particular to identify clearly
three-fold (T_3) and five-fold (T_5) coordinated defects. We found that
electronic states in the gap can be associated to both kind of defects, and
that in both cases the interaction with hydrogen can reduce the density of
states in the gap.Comment: To appear in Philos. Ma