GCMC unphysical pressures

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.

The pore atoms are moving because you are applying fix nvt to them. Remove that fix and you will be running pure GCMC on the gas molecules with rigid pure. If you want to do MD as well, apply fix nve or nvt only to the gas molecules.

The pressure is large because it is calculated using the forces on the pore atoms before they are set to zero. Bulk pressure in a rigid periodic matrix is not a well-defined quantity and there is no simple way to estimate the fluid pressure.