Massively parallel local density functional calculations, using a 91-atom cluster to mimic the metal surface, yields the first computed pair interactions to 10 Å. The N${\mathrm{H}}_3$ results are in excellent agreement with recently derived empirical interactions. The CO pair potential, while in close agreement between 3 and 4 Å with an empirical potential used for high coverage simulations, becomes attractive at larger distances, helping to explain the observed low coverage superstructures. The pairwise approximation to the total interaction works well for N${\mathrm{H}}_3$ but fails badly for CO.

• Received 26 July 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.4828