A review is given of the pseudopotential total energy method and of its applications to the study of adsorption and diffusion processes of single adatoms on the Si(100) surface. The subjects covered include the description of the computer code CASTEP/CETEP used for the simulations, results on the equilibrium structure and reconstructions of the clean Si(100) surface, the potential energy surface for a single Ge adatom on the Si(100) surface, investigation of the possibility of the exchange mechanism for the Ge diffusion on this surface, the influence of the adatom on the buckling of Si dimers, and the structure of the S-B rebonded step.