Simulations have been performed for dimers and diatomic molecules adsorbing on the (110) surface of GaAs. Al2usually remains together after adsorption, indicating a tendency toward cluster formation. In one run, this dimer broke up, but the energy was found to be substantially higher; i.e., the dissociated atoms are metastable with respect to the metallic cluster. On the other hand, O2tends to dissociate immediately upon adsorption, in ways that suggest the beginning of oxide formation. Molecular chemisorption of O2was observed in one run, but found to be metastable.