The potential of mean force of 2 rigid guanidinium ions constrained to remain parallel is studied in liq. water by free-energy perturbation (FEP) mol.-dynamics simulations, using various intermol. potentials. The 1st simulation is carried out using the Amber force field and the transferable intermol.-potential TIP3P water model. The 2nd simulation is performed with the extended simple point-charge SPC/E water model. In a 3rd simulation, the polarizability of the H2O mol. is introduced via the use of the polarizable simple point-charge model PSPC, whereas for the ions, distributed polarizabilities derived from the topol. partitioning of electrostatic properties (TPEP) are incorporated on heavy atoms. For the last 2 simulations, atom-atom Lennard-Jones parameters and charges are derived from ab initio calcns. on monomers and guanidinium-water pairs. The comparison with a previous simulation using the transferable intermol. potential TIP4P, by Boudon, et al. (J. Phys. Chem., 1990, 94, 6056-61), reveals that (i) all the models predict a stable contact ion pair (CIP) at a distance of 3.0-3.4 .ANG., and a solvent-sepd. ion pair (SSIP) at about 3.8 .ANG.; (ii) the stabilization energy of the CIP is strongly model-dependent, varying from 10.0 kcal.mol-1, for the TIP4P model to 4.7 and 2.7 kcal.mol-1 for the SPC/E and PSPC models, resp.; and (iii) in all cases, the SSIP free-energy min. is very shallow and nearly disappears for the simulation using a polarizable model. Consideration of the distribution and the orientation of the solvent mols. around the ions for the non-polarizable (SPC/E) and the polarizable (PSPC) cases does not reveal any significant difference between the 2 models.