  We determine the electronic density of states for computationally-generated
bulk samples of amorphous chalcogenide alloys As$_{x}$Se$_{100-x}$. The samples
were generated using a structure-building algorithm reported recently by us
({J. Chem. Phys.} ${\bf 147}$, 114505). Several key features of the calculated
density of states are in good agreement with experiment: The trend of the
mobility gap with arsenic content is reproduced. The sample-to-sample variation
in the energies of states near the mobility gap is quantitatively consistent
with the width of the Urbach tail in the optical edge observed in experiment.
Most importantly, our samples consistently exhibit very deep-lying midgap
electronic states that are delocalized significantly more than what would be
expected for a deep impurity or defect state; the delocalization is highly
anisotropic. These properties are consistent with those of the topological
midgap electronic states that have been proposed by Zhugayevych and Lubchenko
as an explanation for several puzzling opto-electronic anomalies observed in
the chalcogenides, including light-induced midgap absorption and ESR signal,
and anomalous photoluminescence. In a complement to the traditional view of the
Urbach states as a generic consequence of disorder in atomic positions, the
present results suggest these states can be also thought of as intimate pairs
of topological midgap states that cannot recombine because of disorder.
Finally, samples with an odd number of electrons exhibit neutral, spin $1/2$
midgap states as well as polaron-like configurations that consist of a charge
carrier bound to an intimate pair of midgap states; the polaron's
identity---electron or hole---depends on the preparation protocol of the
sample.
