Theory of diamondlike amorphous carbon

First-principles methods are used to study diamondlike amorphous carbon. Several structural models are introduced, and the topological, electronic, and vibrational properties are discussed. We explore the nature of defects in the amorphous network, and particularly the atomistic origin of the virtually defect-free optical gap. We observe that threefold-coordinated defects pair, and that this is important for obtaining a gap free of localized defect states as seen in experiments.