[lammps-users] Hydration free energy of a water molecule

I’ve done the calculation for SPC/E water hydration free energy (1 solute, 599 solvent, cutoff 12.0 A, tilmestep 1 fs, 300 K, 1 bar, NpT, 20 FEP stages, each 100000 steps) in 2 steps:

- first growing the uncharged LJ site: 2.41 kcal/mol == 10.1 kJ/mol
- second bringing up the charges: -9.78 kcal/mol == -40.9 kJ/mol

The sum is -7.4 kcal/mol == -30.8 kJ/mol, which seems ok.

The result you got doing the two transformations simultaneously still puzzles me.

in-fep01-lj.lmp (1.86 KB)

in-fep01-q.lmp (1.83 KB)

Thanks very much for your supporting.

Thank you for your test, separating the free energy calculation into two parts did get a better result. Now i want to try another water model, the OPC model, which is similar to the TIP4P water model. In my opinion, both the solute and solvent need to be defined as TIP4P type, but it seems that we can define only one of them to be TIP4P type in lammps. Do you have any idea to solve this problem?

in-fep01-lj.lmp (1.93 KB)

in-fep01-q.lmp (1.9 KB)