Solvation free energies of methane and alkali halide ion pairs: An expanded ensemble molecular dynamics simulation study

Solvation free energy for methane, dissolved both in pure water, water/methane mixture (14 mol % methane) and in aqueous NaCl solution, is calculated using the expanded ensemble molecular dynamics method. Dependencies due to system size and potential model are investigated. Results, using a simple one-site methane model, together with large enough system size, are found in very good agreement with experimental data, while calculations using a flexible five-site methane model give too high free energies. Also, the solvation energy for 20 different ion pairs of alkali halides is calculated in a systematic study. Very good overall agreement is found for the solvation energies of all the ion pairs. Calculations of solvation free energies provide a sensitive test of the used potential models.