Free Energy Perturbation of ion in water

Dear all,

I want to use free energy perturbation package of LAMMPS to calculate an ion’s free energy of solvation in water, but the the charge would be unneutral. I have checked the examples but all are about solvation of neutral molecules whose overal charge would be still neutral over the coarse of different lambdas. But how to deal with this in Lammps. Shall I use ewald for kspace style or shall I do this along with a counterion to counter-balance the net charge?


This is a problem of methodology and not LAMMPS-specific – anyone doing a free-energy perturbation of a single ion in GROMACS, OpenMM or any other MD package would also have to account for the non-zero charge.

Personally this seems to match up to physical intuition and macroscopic reality to me as well. In real life we can only measure the activities of solvated neutral mixtures of ions – never an individual ionic species.

Dear Dr. Shern Tee,

Thanks for your reply. Now it is clear for me what to do next. Also I use this opportunity to thank you for your contribution by implementing conp2.