We have examined the interaction between the ammonium cation and the aromatic face of toluene in water, by means of potential of mean force (PMF) calculations. Considering that (i) typical two-body additive molecular mechanical models cannot represent the energetics of pi-cation interactions accurately and (ii) employing nonadditive force fields increases the computational effort significantly, we have incorporated a short-range "10-12" term in our potential function, ensuring that the magnitude of the attraction between ammonium and toluene reproduces the value estimated from high-level quantum mechanical calculations. Interestingly, the PMF curve generated in water clearly demonstrates that association is favorable in a polar aqueous medium, with a minimum of the free energy equal to ca. 3 kcal/mol, and an association constant of 6.5 M(-1)-consistent with experimental data on related pi-cation systems. This association appears to be even stronger when the approach of ammonium toward the toluene ring is axially constrained, hence indicating that, in addition to non-negligible entropic effects, the magnitude and the directionality of ammonium-aromatic interactions might be intimately related. A comparison of the free energy profiles obtained in a vacuum and in water suggests that "contact" configurations should be stabilized in nonpolar environments. This observation concurs with the analysis of Phe-Lys interactions in several protein crystal structures.