We show how local density approximation ab initio calculations are applied to study the chemisorption of single adatoms on semiconductor surfaces. The binding energies of Al, Si and P adatoms on the Si(100) surface are calculated, as well as the barriers for diffusion of these atoms along the surface and a clear chemical trend is established. In particular, we find that the size of the adatom has a dramatic effect on the diffusion barriers. In a recent STM study the migration of clusters of Si adatoms has been observed directly. On the basis of our calculations we argue that these clusters consist of dimers, which we find to form stable structures on the Si(100) surface. The binding energy and geometry of a number of Si ad-dimer structures are established, and the effect of thermal motion is investigated by means of ab initio molecular dynamics simulations.