GCMC unphysical pressure

Dear Lammps Users,

I am attempting to perform a GCMC simulation in order to fill a porous material with methane. Below is my input script in brief:

timestep 0.01
dump 1 all custom 75000 traj_ngpu.lammpstrj id mol type x y z ix iy iz

molecule MTN methane.txt #toff 54 boff 52 aoff 97

group pore type < 55
group GAS type > 55

compute mdtemp pore temp
compute_modify mdtemp dynamic yes
fix 1 pore nvt temp 300.0 300.0 10.0
fix_modify 1 temp mdtemp

#fix 2 pore recenter INIT INIT INIT units box
fix 2 pore setforce 0 0 0
fix 3 GAS gcmc 10 100 100 0 26828 300 0 0.01 mol MTN pressure 70

compute_modify thermo_temp dynamic yes

thermo_style custom step cpuremain temp press density atoms
thermo 1500
thermo_modify flush yes

restart 1500 sim_ngpu.restart sim_ngpu.restart

run 10000

write_data 70atm_equi.data

When I visualize the output, the pore molecules are fragmented and not at all “unmoved” as fix 2 should allow. I also tried using “recenter” but the GCMC simply won’t work when used with this fix. In addition, the system pressure is on the order of 5e+15 atm and I can’t seem to reduce it.

Does anyone know of a better way to maintain the structure of the pore atoms while the GCMC insertions are taking place AND maintaining a reasonable system pressure?

Thanks in advance,

Evan L.

Please do not post duplicate questions. See response to last duplicate.