[lammps-users] Cooling with fix rigid


I simulate triangular lattice potential function with cooling from k_BT=1.0 to k_BT=0.01. During the simulation I group some portion of particles as a rigid body using fix rigid command for a while and want to unfix the rigid body. The script looks like

fix 1 all nvt 1.0 0.25 0.025 drag 1.0 -> cooling using fix nvt
fix 2 all enforce2d

####### Run a simulation #######
run 1000

if rigid body exists

group rigid_body1 id 10 11 24 25 26 27 40 41 42 43 56 57 58 59
group rest1 subtract all rigid_body1
fix 3 rigid_body1 rigid single
neigh_modify exclude group rigid_body1 rigid_body1
unfix 1
fix 1 rest1 nvt 0.25 0.21 0.025 drag 1.0
reset_timestep 1000
run 1000

unfix 3
unfix 1
fix 1 all nvt 0.21 0.01 0.025 drag 1.0
reset_timestep 2000
run 1000

From time step 2000, the position of particles grouped as a rigid body looks to be wrong if I view it using xmovie. How can I cool the system from 1.0 to 0.01 with a rigid body and relaxing them?

Lee, Sun Hwan

If you unfix the rigid body fix, the particles won't stay rigid
after step 2000. To thermostat a system with rigid
bodies you either want to do Brownian motion, or if
you have solvent as in your case, then just thermostat
the solvent and let them equilibrate the rigid body.