  We enumerate all local minima of the energy landscape for model rigid
adsorbates characterized by three or four equivalent binding sites (e.g., thiol
groups) on a close-packed (111) surface of a face-centered-cubic crystal. We
show that the number of energy minima increases linearly with molecular size
with a rate of increase that depends on the degree of registry between the
molecule shape and the surface structure. The sparseness of energy minima and
the large variations in the center-of-mass positions of these minima vs
molecular size for molecules that are incommensurate with the surface suggests
a strong coupling in these molecules between surface mobility and shape or size
fluctuations resulting from molecular vibrations. We also find that the
variation in the binding energy with respect to molecular size decreases more
rapidly with molecular size for molecules with a higher degree of registry with
the surface. This indicates that surface adsorption should be better able to
distinguish molecules by size if the molecules are incommensurate with the
surface.
