To calculate the single-ion solvation free energy, a SO -2 4 ion is placed in a cubic box of size L = 2.5 nm containing 512 water molecules. A cut-off distance of r LJ = 1.0 nm is chosen for the Lennard-Jones interactions, and MD simulations are performed with a time-step of 1 fs. The solvation free energy is calculated from the integration of the Hamiltonian of the system H ϕ over a solvation path,