Force field for monovalent, divalent, and trivalent cations developed under the solvent boundary potential

Presented are parameters for mono-, di-, and trivalent cations compatible with the CHARMM additive force field and the TIP3P water model. Thermodynamic perturbation molecular dynamics simulations were performed for the cations located at the center of a TIP3P water sphere under a solvent boundary potential. A series of perturbations generated free energies of hydration indexed by the two Lennard-Jones parameters, ε and R(min). Interpolating the experimental free energies of hydration showed that multiple combinations of ε and R(min) values reproduced the free energies of hydration for each ion. To overcome this nonunique parameter problem, the hydration shell model in combination with an empirical scaling parameter was applied to assign values for each ion. R(min) values were then identified via interpolation of the calculated free energies of hydration. The presented parameters are anticipated to be of utility for simulations of ions, including ions complexed to proteins.