  We propose a method to simulate a-Si and a-Si:H using an ab initio approach
based on the Harris functional and thermally amorphisized periodically
continued cells with at least 64 atoms, and calculate their radial distribution
functions. Hydrogen incorporation was achieved via diffusive random addition.
The electronic density of states (DOS) is obtained using density functional
theory with the aid of both the Harris-functional and Kohn-Sham-LDA approaches.
Two time steps are used, 2.44 and 10 fs for the pure, and 0.46 and 2 fs for the
hydrogenated, to see their effect on the topological and DOS structure of the
samples. The calculated long time-step radial features of a-Si are in very good
agreement with experiment whereas for a-Si:H the short time-step partial and
total radial features agree well; for the long time-step simulation molecular
hydrogen appears during annealing.The long time-step a-Si has a well defined
gap with two dangling bonds, that clears and increases upon hydrogen addition
and relaxation, as expected. The short time-step structures have more defects,
both dangling and floating bonds, that are less characteristic of a good
sample; however the radial structures of a-Si:H are in better agreement with
experiment indicating that the experimental work was done on defective samples.
