AB-INITIO AMMONIA AND CO LATERAL INTERACTIONS ON PT(111)

Massively parallel local density functional calculations, using a 91-a tom cluster to mimic the metal surface, yields the first computed pair interactions to 10 Angstrom. The NH3 results are in excellent agreeme nt with recently derived empirical interactions. The CO pair potential , while in close agreement between 3 and 4 Angstrom with an empirical potential used for high coverage simulations, becomes attractive at la rger distances, helping to explain the observed low coverage superstru ctures. The pairwise approximation to the total interaction works well for NH3 but fails badly for CO.