Solvation free energies of molecules and ions: a first principles study

The solvation free energy of molecules or ions is a fundamental quantity to study the properties of aqueous solutions. There are various computational methods to calculate this quantity, but many of them reply on experimental inputs or empirical models.  Here, we introduce a first-principles method to calculate this quantity accurately and efficiently. Our method is based on ab initio molecular dynamics and thermodynamic integration. As an example, we calculated the solvation free energies of methane, methanol, water molecules and the Na⁺ ion in water at ambient conditions and compared them with experimental data. Because our method does not need any experimental input, it can be readily used to study supercritical aqueous solutions at elevated pressure and temperature conditions, where experimental data are scarce. Our method can be also easily implemented with machine learning force fields.