How to set rigid for a carbon nanotube

Dear lammps users,

Now I am working on a problem about the movement of a carbon nanotube in fluid Ar. The fluid Ar is confined between walls which have steady velocities. In this way, a Couette flow can be built and I care about how the CNT moves in the fluid. If I use a real CNT model which has C-C potentials, the timestep should be set around 1fs. In order to accelerate the computation, I attempt to treat the CNT as a rigid body. Then the timestep may be set to around 10fs which is decided by the Ar atoms, I guess (Am I wrong on this point?). But when I run the input file below, the Error of “lost atoms” occur. By the way, everything is fine when I consider the C-C potentials and the timestep is set as 1fs and do not use “fix rigid” for CNT. Can you help me find out which part is wrong with the input file?

I’m looking forward to having your help.

Best wishes
Ruo-Yu Dong

3-d LJ flow simulation

dimension 3
boundary s p p
atom_style atomic
atom_modify map array
units metal

read_data data.ljcnt


pair_style hybrid lj/cut 7.7 rebo
pair_coeff * * rebo …/…/…/…/lammps/potentials/CH.airebo C NULL NULL NULL
pair_coeff 1 2* lj/cut 0.00369 3.573
pair_coeff 24 24 lj/cut 0.0102 3.405

define groups

group lower type 4
group upper type 3
group boundary union lower upper
group ar type 2
group cnt type 1
group nrigid subtract all cnt
neighbor 2.0 bin
neigh_modify delay 5 exclude group cnt cnt

main settings

fix 6 cnt rigid single
compute mobile ar temp/partial 1 0 1
velocity ar create 300.0 341233 temp mobile
fix 1 nrigid nve
fix 2 ar temp/rescale 100 300.0 300.0 5.0 1.0
fix_modify 2 temp mobile


fix 3 boundary setforce 0.0 0.0 0.0
velocity upper set 0.0 1.0 0.0 units box
velocity lower set 0.0 -1.0 0.0 units box


timestep 0.01
thermo 10
thermo_modify temp mobile

dump 3 all image 10 …/position/image..jpg type type view 0 0 zoom 1.6 adiam 1.2
dump_modify 3 first yes pad 7
dump 1 cnt custom 1 …/position/dump.cnt.
type id x y z
dump 11 ar custom 1 …/position/* type id x y z

run 5000

Too big a timestep I would guess. You can't go to 10 fs with
an atomistic model.