Molecular dynamics and free energy perturbation have been applied to study the complexation and the selectivity of a host molecule hexaanisole spherand (1) with its metal ion ligands, Two different potential function force fields were tested : a two-body interactive potential with the TIP3P water model and a recently derived polarizable and three-body interactive potential including a polarizable water model. Our results show that the two-body additive potential is able to provide useful information on the relative binding affinities of alkali metal ions, although for the Na+ --> Li+ case the agreement with experiment is only qualitative. The relative binding free energy of Li+ and Na with the hexaanisole spherand is calculated to be -2.7 kcal/mol, compared to the experimental value of < -3.8 kcal/mol. The agreement improved when the mole sophisticated polarizable and three-body interactive potential force field was employed, resulting in a relative binding free energy of Li+ and Na+ of -3.7 kcal/mol. We have also tested different electrostatic charge fitting protocols and models, and the results showed that the choice of protocol is of critical importance for obtaining quantitative agreement with experiment.