Here’s a simple example input script for fix gcmc (see below).
Paul
units real
atom_style atomic
boundary p p p
lattice fcc 0.1
region box block 0 1000 0 1000 0 1000
create_box 1 box
mass 1 83.80
pair_style lj/cut 10.0
pair_coeff 1 1 0.337824 3.69 10.0
thermo 100
compute_modify thermo_temp dynamic yes
variable chempot equal -2000
variable density equal atoms/vol
variable specdensity equal v_density1.39156E-251e27
variable chempotkcal equal ${chempot}/503.219
fix 2 all gcmc 1 100 100 1 29494 300.0 ${chempotkcal} 0.5
thermo_style custom step atoms f_2[1] f_2[2] f_2[3] f_2[4] f_2[5] f_2[6] temp pe ke etotal press vol v_chempot v_specdensity
run 100000