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

We discuss the physisorption of atoms (xenon) and molecules (PTCDA) to a noble-metal surface (silver) within a first-principles approach, focusing on the adsorption energy as a function of distance. Instead of density-functional theory (which fails to describe physisorption) we employ a combination of exact exchange and correlation energies, which we evaluate within the adiabatic-connection fluctuation-dissipation theorem. Correlation accounts for non-local dispersion energy, which is crucial in the present cases. At large distance {$Z$} from the surface the correlation causes van der Waals attraction with a characteristic $-C_3/(Z$$-$$Z_0)^3$ asymptotic form. At closer distance the attraction deviates from the asymptotic form and, in combination with the repulsive exact-exchange energy, yields an equilibrium of xenon and of PTCDA on the Ag(111) surface in close agreement with experiment.