Binding energy of adsorbates on a noble-metal surface: exchange and correlation effects.

We discuss the adsorption of xenon and of PTCDA on the silver (111) surface within a first-principles approach, focusing on the adsorbate-substrate interaction energy as a function of distance. We combine exact exchange with correlation energy from the adiabatic-connection fluctuation-dissipation theorem. At a large distance Z from the surface, the correlation causes a van der Waals attraction [approximately -C3/(Z - Z0)3]. At a closer distance, the attraction deviates from its asymptotic form and, combined with the repulsive exact-exchange energy, yields an equilibrium in close agreement with experiment.