  Using total energy calculations, based on interaction potentials from the
embedded atom method, we show that the presence of the tip not only lowers the
barrier for lateral diffusion of the adatom towards it, but also shifts the
corresponding saddle point. For a Cu adatom at a (100) microfacetted step on
Cu(111) this shift is 0.6 A. The effect of the tip geometry and shape on the
energetics of lateral manipulation was found to be subtle. In the case of
vertical manipulation of a Cu adatom on flat, stepped, and kinked Cu surfaces
we find an unusual but interesting result. It is found that as the tip
approaches the surface, it becomes easier to extract the adatom from the
stepped and kinked surfaces, as compared to the flat surface. This counter
intuitive result can be explained in terms of tip induced changes in the
bonding of the adatom to its low coordinated surroundings.
