Summing up the total correction (PBC/Ewald and dispersion) of the solute ions, we see that the correction from the cluster calculations with 17 solvent ion pairs dominates 89.3% of that with 256 solvent ion pairs. As a result, the fluctuation term in Eq. 12 is assumed to be primarily captured by the interaction energy correction of 17 solvent ion pair cluster. This leads to a -2.5(0.1) kcal/mol correction for each of the Na + and Cl -solute ions. Inserting these results into Eq.12, we obtain the total correction (in the DFT→MP2 direction) as +13.7(1.9) kcal/mol for the solute ions. Compared to the experimental reference value of -163.5 kcal/mol, the calculated total excess chemical potential of -164.8(2.2) kcal/mol provides strong initial validation of the methodology.