A brief review is given on density functional calculations performed for group-IV adatom diffusion on hydrogenated Si(100) surfaces. It is shown that the diffusion is a complex atomic process in which hydrogen capture and emission as well as adatom-exchange with subsurface atoms are involved. Step structures are determined and the modulation of the diffusion process near the steps is clarified. Relations between microscopic atomic processes and macroscopic thin-film morphology are also discussed.