This quantity is the variation of the Helmholtz free energy ∆F vacuum→CnAm (abbreviated later by ∆F CnAm for clarity) for bringing an electrolyte unit C n A m from the vacuum to the TIP4P liquid water. We divided the solvation process in two steps. First, water molecules are removed from vaccum and solvated in bulk water. Then, water molecules are alchemically transformed into ions as described in the previous section, so that the free energy of solvation reads: