We describe the derivation of a set of empirical interaction potential functions for various metal cations, by the free energy perturbation approach. The experimentally observed free energies of hydration and radial distribution functions are used for calibration and the resulting potentials reproduce these quantities. These potential energy functions are used to evaluate free energies in a number of test cases, involving metal ion catalysed proton transfer reactions in solutions and in carbonic anhydrase in addition to ion binding to host molecules.