A simple, parameter-free method for computing solvation free energies of ions.

Solvation free energies of ions are difficult to determine from molecular dynamics simulations due to the long-range Coulomb interactions. Various approximations and corrections are introduced to enable their calculation in small systems, which, however, raises issues of robustness. We show that solvation free energies of ions can be calculated using the spherical boundary conditions without introducing any corrections at the boundary via a buffer zone. The results are shown to converge for a droplet size of 21 Å and are independent of the parameters used for confining water or restraining the ion. The proposed method thus resolves the robustness issues in solvation free energy calculations of ions and can be used with confidence to determine force field parameters from such calculations. We apply the method to calculate the solvation free energies of the side chain analogs of charged amino acids. Tests using periodic boundary conditions show that similar results are also obtained in that case.