  Recent theoretical studies of amorphous silicon [Y. Pan et al. Phys. Rev.
Lett. 100 206403 (2008)] have revealed subtle but significant structural
correlations in network topology: the tendency for short (long) bonds to be
spatially correlated with other short (long) bonds). These structures were
linked to the electronic band tails in the optical gap. In this paper, we
further examine these issues for amorphous silicon, and demonstrate that
analogous correlations exist in amorphous SiO2, and in the organic molecule,
b-carotene. We conclude with a discussion of the origin of the effects and its
possible generality.
