Inquiry on Volume Expansion using fix GCMC

Dear Dr. Axel Kohlmeyer and LAMMPS Users,

Hello Dr. Kohlmeyer. Thank you for your advice.

I am currently trying to model a swelling simulation where I have a polymer network with water molecules (solvent particles) entering the polymer network. Observing the swelling literature and from the previous mailing lists, the fix gcmc seems to be the best tool.

I was curious if I could ask a question on fix gcmc that performs the grand canonical Monte Carlo exchange of atoms or molecules with a solvent reservoir at specified temperature and chemical potential.

According to Frenkel which the fix gcmc is based upon, fix gcmc will allow solvent particles to be created and inserted into my simulation box until the chemical equilibrium is reach which is governed my the user-defined mu.

If fix gcmc combined with fix npt does not enforce constant volume and constant pressure, then how is the inserted solvent molecules and the beads that make up the polymer network remapped when the volume of the simulation box is increasing?

The fix deform command has an input argument which I need to specify as remap yes in order to have the velocities and the positions of the atoms remap to the opposite side of the deforming box surface in case if the atoms cross the periodic boundary.

Once again, I would like to deeply thank you for your time.


Masato Koizumi

It is still not really clear to me what you are trying to do. It sounds like you are thinking of using fix gcmc together with a fix that tries to keep pressure constant. However, if none of N, V and U are constant, the free energy is unbounded and your simulation would grow forever, which does not make much sense.