Effect of Polarizability on the Potential of Mean Force of Two Cations. The Guanidinium−Guanidinium Ion Pair in Water

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The potential of mean force of two rigid guanidinium ions constrained to remain parallel is investigated in liquid water by means of free energy perturbation (FEP) molecular dynamics simulations, using various intermolecular potentials. The first simulation is carried out employing the Amber force field and the transferable intermolecular potential TIP3P water model. The second simulation is performed with the extended simple point charge SPC/E water model. In a third simulation, the polarizability of the water molecule is introduced via the use of the polarizable simple point charge model PSPC, whereas for the ions, distributed polarizabilities derived from the topological partitioning of electrostatic properties (TPEP) are incorporated on heavy atoms. For the last two simulations, atom−atom Lennard-Jones parameters and charges are derived from ab initio calculations on monomers and guanidinium−water pairs. The comparison with a previous simulation using the transferable intermolecular 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 Å, and a solvent-separated ion pair (SSIP) at about 6.5 Å, (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 respectively, and (iii) in all cases, the SSIP free energy minimum is very shallow and nearly disappears for the simulation using a polarizable model. Consideration of the distribution and the orientation of the solvent molecules around the ions for the non-polarizable (SPC/E) and the polarizable (PSPC) cases does not reveal any significant difference between the two models.