  We study the distribution of hydrogen and various hydride configurations in
realistic models of a-Si:H for two different concentration generated via
experimentally constrained molecular relaxation approach (ECMR) [1]. The
microstructure corresponding to low (< 10%) and high (> 20%) concentration of H
atoms are addressed and are compared to the experimental results with
particular emphasis on the size of H clusters and local environment of H
atoms.The linewidths of the nuclear magnetic resonance (NMR) spectrum for the
model configurations are calculated in order to compare to the experimental NMR
data. Our study shows the presence of isolated hydrogen atoms, small and
relatively large clusters with average proton-proton neighbor distance in the
clusters around 1.6 to 2.4 Angstrom that have been observed in multiple quantum
NMR experiments.
