The potential energy surface for the Ar/Ag(111) system is studied within the local density approximation. Results for N atom clusters, where N=4, 10, and 19, are compared with the potentials obtained with slabs with summation of pair potentials. This last method predicts the threefold hollow site as equilibrium site. However, the larger clusters find the top site as adsorption site. On the top site, the adsorption height and energy are calculated as 3.2 Å and 0.10 eV, respectively. The physisorption energy is in reasonable agreement with experimental data, in spite of the LDA level of theory used. Thus the Ag(11 I)-Ar system is a new example where LDA appears to describe a van der Waals system with reasonable accuracy. A correction to the sum of pair potentials is given that effectively reproduces the calculated potentials at threefold hollow site and top site.