  We present a first-principles study of the structural, electronic, and
optical properties of hydrogenated amorphous silicon (a-Si:H). To this end,
atomic configurations of a-Si:H with 72 and 576 atoms respectively are
generated using molecular dynamics. Density functional theory calculations are
then applied to these configurations to obtain the electronic wave functions.
These are analyzed and characterized with respect to their localization and
their contribution to the density of states, and are used for calculating
ab-initio absorption spectra of a-Si:H. The results show that both the size and
the defect structure of the configurations modify the electronic and optical
properties and in particular the value of the band gap. This value could be
improved by calculating quasi-particle (QP) corrections to the single-particle
spectra using the G$_0$W$_0$ method. We find that the QP corrections can be
described by a set of scissors shift parameters, which can also be used in
calculations of larger structures.
