[EXTERNAL] Modify pressure

I did not try running your script, but I am guessing that the small volume:

region zone1 block 0 10 0 10 0 10 units box

and the small GCMC target pressure

pressure 0.88 fugacity_coeff 0.99

result in a very small average particle count. If you want to see the time-average pressure reported by LAMMPS converge to your target pressure, you will need to do one of the following:

  1. Increase target pressure

  2. Increase box volume

  3. Increase duration of your simulation

I suggest that pick the pressure and volume so that you get a few hundred argon atoms on average, and then invoke the fix many times i.e. nsteps/N should be large. Note that invoking the fix 1000/100=10 times with large values of X and M is not a good idea, because you only measure the particle count 10 times, even though you performed 10^7 exchange moves.