GCMC with immobile walls

LAMMPS LAMMPS General Discussion
1

Hello MATSCI Community,

I am trying to perform a GCMC/MD simulation with a membrane and some fixed (immobile) walls at the box boundaries. I am “fixing” the walls at the boundaries by excluding them in my fix nvt command; I only include in the fix nvt the two other groups (MFI, CH4) I have in the system. One of these two groups is the molecule (CH4) that I am adding by GCMC. However, when I am running the simulation the group (CH4) that I am adding is also immobile, as if no velocities are generated for the molecules that are added. As far as I understand it, when I define a group including all the molecules that should be integrated (MFI and CH4) during the NVT run they should be mobile, and only the group (walls) should stay immobile.

I define the groups as follows:
group MFI type 1:4
group CH4 type 5 6
group move type 1:6
group walls type 7

and my nvt and gcmc commands are:
fix mynvt move nvt temp 300 300 100.0

fix my_gcmc CH4 gcmc 2000 100 0 0 212433 300 -10 0.5 pressure 1.0 mol ch4 group CH4 region feed tfac_insert 1

I can run the simulation without issues using reflective walls, so at the end the final results should be practically the same. However, I saw that using walls significantly makes the simulation slower. So, I would like to have physical barrier and make the simulation periodic in all directions.

Obviously, I am doing it wrong or I misunderstood some commands, but I am unable to see what is wrong in this case. I am attaching the input files. Thanks!
combined_box_new.data (2.3 MB)
ch4.mol (477 Bytes)
gcmc.in (2.4 KB)

2

I see that you wrote

compute         mdtemp move temp
compute_modify  mdtemp dynamic/dof yes

but you never used a fix_modify in the fix nvt. I think this could be at least contributing to your problem.

PS: I am not sure what these lines below are doing, since you never defined a compute with the ids thermo_temp and mynvt_temp (or maybe I misunderstood something?)

compute_modify  thermo_temp dynamic/dof yes
compute_modify  mynvt_temp dynamic/dof yes

I am not sure you need to write “group CH4” in the fix gcmc command since in fact the added molecules should be added to the group specified in the command already (but I am not sure it causes any harm or double counting in specifying it again either).