GCMC - different number of atoms with number of MC exchanges


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?