A calculation of the van der Waals contribution to the physisorption energy is performed for molecules interacting with small metallic spheres. The induction contribution is introduced by using the first field gradient susceptibilities of the sphere at zero frequency. In order to illustrate the non local effects as well as the importance of the curvature of the metal particle on the magnitude of the physisorption energy, we present numerical results for typical systems (HF, HCl on Ag, Cu and Al).