Fix GCMC - Number of inserted molecules different with changing insertion frequency


I am using fix gcmc to for the absorption of water into a polymer matrix. I’ve noticed that the equilibrium number of waters in the system is depended upon the number of GCMC exchanges, so the keyword “X” in the fix command.

I’m defining the fix like:

fix do_gcmc h2o gcmc 10000 {exchanges} 100 0 418531 {rm_temp} {mu} {disp} &
mol h2o_mol pressure {gcmc_pressure} tfac_insert {tfac} group h2o


For example, if the gcmc_pressure is set to 0.01 atm:

exchanges, Water-wt%, number of waters add
250, 0.2076
500, 0.4502

1000, 0.7351

The equilibrium number of waters added in each case is well below the number of exchanges.

Any ideas on why the equilibrium water uptake would be affected by the number of exchanges/deletions?


You are right to be concerned. A well-converged sampling of the Grand Canonical ensemble should not exhibit this behavior. This suggests some kind of unphysical coupling between the water absorption and the polymer dynamics. One way to explore this question is to ask what other properties are affected by X. For example, is the temperature and velocity distribution of the polymer and water atoms the same in all cases? Another idea would be to eliminate the notoriously long relaxation times of the polymer dynamics from the problem i.e. by replacing polymer with an equivalent amount of monomer.