Calculations of the potential of mean force from molecular dynamics simulations using different methodologies: an application to the determination of the binding thermodynamic properties of an ion pair

We report here the calculation of the potential of mean force (PMF) of the Ca2+…SO ion pair in the gas phase and in water using molecular dynamics simulations. The PMF calculation is performed using different free-energy perturbation (FEP) and force constraint (FC) methods. We pay attention to the FEP method for which we propose a new development specially adapted to systems with solvent molecules. The gas-phase interionic PMFs are compared with that calculated from the primitive model. We compare the shapes of the calculated PMFs in water and the resulting thermodynamic properties (, and ) of complexation with the experimental ones. We also tend to give a molecular description of the environment of the ion pair for some regions of the PMFs.