  We study colloidal ordering and disordering on two-dimensional periodic
substrates where the number of colloids per substrate minima is two or three.
The colloids form dimer or trimer states with orientational ordering, referred
to as colloidal molecular crystals. At a fixed temperature such that, in the
absence of a substrate, the colloids are in a triangular floating solid state,
upon increasing the substrate strength we find a transition to an ordered
colloidal molecular crystal state, followed by a transition to a disordered
state where the colloids still form dimers or trimers but the orientational
order is lost. These results are in agreement with recent experiments.
