Through investigation of the solvation thermodynamics of the Na + and Cl -ions in the molten NaCl liquid, we have presented and validated an efficient and general methodology to calculate the solvation free energy of ionic species in the molten salts. The methodology incorporates ab initio molecular dynamics simulations, interatomic potentials based on deep neural network models, and quasichemical theory. Efficient molecular dynamics sim-ulations with the NNIP model reduces the calculation uncertainty significantly to roughly 1 kcal/mol. Due to the over-estimation of the magnitude of the attractive interaction energy of the solute ion with the solvent ions at the DFT level (with D2 dispersion corrections), a high-level quantum chemical correction of roughly 15 kcal/mol to the free energy is required to make a quantifiable prediction of the excess free energy. These results highlight the importance of dispersion and polarization interactions in the molten salt liquids.