[lammps-users] centre of mass velocity in polymer glass simulation

Hello,
I am running simulations of bead-spring polymers in the glassy phase (LJ
interactions, FENE bonds) and I find that my simulation box acquires a
constant centre of mass velocity upon cooling. I don't know if this is a
bug exactly, but it is certainly very annoying. I've so far been
modifying the fix_msd code every time I download a new version, but I'm
hoping you can help me find a more permanent solution. I've attached the
data file I use, and the input file. Let me know if there is anything
else you need. Any suggestions would be much appreciated.
Cheers,
Mya
p.s. It might be useful to put a link to this mailing list in the
documentation on how to report bugs!

in.test (1.04 KB)

T03.data (76.6 KB)

Hi, Mya. Why not use fix_recenter? Something like

fix 4 all recenter INIT INIT INIT units box

That should eliminate the COM velocity. Although even if you use it, I don't think it makes much sense to measure the msd in a npt quench where the volume is changing. Wouldn't you really be interested in the nonaffine part of the displacement? That (I think) can only be calculated in postprocessing.

Best,
Rob

Quoting Mya Warren <[email protected]>:

There are various commands in LAMMPS to deal with this:

fix momentum, fix recenter, velocity zero linear, etc

You might want to figure out why you are getting drift. Using
fix langevin can be one source. It adds a random force to
atoms which does not conserve global momentum.

p.s. It might be useful to put a link to this mailing list in the
documentation on how to report bugs!

Done - see doc/Section_errors.html#9_2

Steve