Ab Initio studies of the atomic structure and electronic density of   states of pure and hydrogenated a-Si
  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. 
Ariel A. Valladares, Fernando Alvarez, Zhihua Liu, Juergen Stitcht and   John Harris