  We apply a new method "force enhanced atomic refinement" (FEAR) to create a
computer model of amorphous silicon (a-Si), based upon the highly precise X-ray
diffraction experiments of Laaziri et al. The logic underlying our calculation
is to estimate the structure of a real sample a-Si using experimental data and
chemical information included in a non-biased way, starting from random
coordinates. The model is in close agreement with experiment and also sits at a
suitable minimum energy according to density functional calculations. In
agreement with experiments, we find a small concentration of coordination
defects that we discuss, including their electronic consequences. The gap
states in the FEAR model are delocalized compared to a continuous random
network model. The method is more efficient and accurate, in the sense of
fitting the diffraction data than conventional melt quench methods. We compute
the vibrational density of states and the specific heat, and find that both
compare favorably to experiments.
