[lammps-users] using fix wall/region command

Dear LAMMPS users
I’m running a tensile test on CNT-Al alloy composite and I want to relax the atoms of the alloy before adding the CNT
So what I’m doing is Generating the structure of the alloy as a cylindrical cell with an empty space in the center for the later insertion of the CNT

I want to use the command fix wall/region to prevent the atoms of the alloy to move inside the hole in the center during relaxation but I’m not sure from where I should get the Lennard Jones parameters to use in the command fix wall/region
Can you please advice

Regards
Isra

Thank you for the fast response
I did as suggested but I’m getting the error " Particles outside surface of region used in fix wall/region"
my region is a hollow cylinder so I have created two cylinders, the inner one with side out and the outer one with side in and then took the intersect of both regions
I’m attaching the code for generating the regions and I highly appreciate the help

region whole cylinder z 28.15 28.15 28.15 0 103.3 units box

create_box 2 whole
create_atoms 1 region whole

region tube cylinder z 28.15 28.15 4.07 0 103.3 units box
delete_atoms region tube

region hollow cylinder z 28.15 28.15 4.07 0 103.3 side out units box #innercylinder
region alloy intersect 2 whole hollow

fix 1 all wall/region alloy harmonic 5 5 10

Thanks and regards
Isra