[lammps-users] fix com and fix rigid

Dear LAMMPS users,

   I am running simulation of a rigid body (made of LJ Particles) immersed in
liquid also composed of LJ particles. I am using LAMMPS (12 Feb 2007)version
downloaded on 19 June (the latest package on that date). Please help me with
following questions:

1. Regarding "fix com":

  The online manual states that fix com is used to "Compute the center-of-mass
of the group of atoms every N steps, including all effects due to atoms passing
thru periodic boundaries".

  Does this mean that coordinates of the center of mass are unwrapped
coordinates or the coordinates in the simulation box? In the later case, is
there a way to output the image flags of the com coordinates?

2. Regarding restarting the fix rigid:

  As I mentioned earlier I ran a simulation of the rigid body in LJ liquid for
2.5 million steps and output the center of mass coordinates using fix com.
Restart files were written every 500,000 steps. Then I restarted the same run
from step 2,000,000 using one of the restart files. After 1040 steps the com
trajectory started diverging compared to original run which was up to 2,500,000
steps. Why is that happening?


Gaurav Pranami

Graduate Student
Chemical Engineering Dept.
Iowa State University

Yes, the coords are unwrapped. You can dump image flags for individual
atoms via the dump atom or dump custom command.

Restarts are not always perfect for many commands due to round-off.
Divergence after 1000 steps is not unusual.