  For a set of fcc metals our total energy calculations, based on many body
potentials, show that activation barriers for lateral manipulation of an adatom
at a step edge depend on the tip/substrate composition. Of the six homogeneous
systems studied, manipulation on stepped Ag(111) showed the lowest energy
barrier for adatom hopping towards the tip, although the relative probability
for this process was largest on Cu(111). For a representative Cu/Pt
heterogeneous system we find lateral manipulation of a Pt adatom along a step
on Pt(111) by a Cu(100) tip to be energetically much less favourable than the
reverse case of a Cu adatom manipulated by a Pt(100) tip. In the case of
vertical manipulation, atomic relaxations of the tip and its neighbouring atoms
are found to be prominent and tip induced changes in the bonding of the adatom
to its low coordinated surroundings help explain the relative ease with which
an adatom next to a step edge or a kink site, may be pulled, as compared to
that on a flat surface.
