Hello everyone,

I am not sure if this is the right place to post, maybe Science Talk is more appropriate.

I’m trying to predict the equilibrium water uptake of different ion exchange membranes using LAMMPS. These membranes are not rigid, they generally swell according to the generic law \rho = \phi_{\text{membrane}}\rho_{\text{membrane}} + \phi_{\text{water}}\rho_{\text{water}}, with \phi volume fractions.

I already followed the approach of this article [Prediction of equilibrium water uptake and ions diffusivities in ion-exchange membranes combining molecular dynamics and analytical models] which is purely MD but I wonder if the MD/GCMC approach would not be more elegant and rigorous.

My idea is to:

- Use
`fix/widom`

to obtain the \mu_{ex} of the water model I’m using. - From a weakly hydrated membrane, use
`fix/gcmc`

to add water molecules to the system. - Perform NPT integration to allow the membrane to swell.
- Loop points 2 and 3 until the number of water molecules in the system converges.

I’ve determined mu correctly, at least I think as I have double check that I obtain correct density for pure water, but I’m having trouble afterwards. I don’t see convergence but a constant increase in the number of water molecules in the system, so I am surely missing a point.

Could fluctuations in the dimensions of the box during NPT integration lead to a dilute system as I seem to be observing?

Is my approach feasible with LAMMPS?