About Damp in Langevin affects with velocity


Group was applied a Langevin thermostat to maintain temperature and a normal force to move it down in z direction.

With larger damp, it was moved faster.
With smaller damp, it was moved slower.

I think damp only affects with temperature, not with velocity. But it does.

Anyone has suggestions?


Are you thermostating and apply an external force to the same group of atoms? If yes, then the thermostat will equilibration the group and neutralize the force.

What you observed is expected: larger damp value, weaker damping, slower the neutralization, hence the atoms moved faster. The opposite is also true.

Bottom line: remove the thermostat on the group.


If you thermostat a group of atoms

moving collectively, then the thermostat

will affect the collective motion. Unless

you remove the collective velocity before

thermostatting. In LAMMPS that is called

removing a bias and there are several

temperature computes that do it in different ways. See

Section 6.16 of the doc pages for an overview.



@ Steve:

compute myTemp group temp/profile 1 1 1 xyz 1 1 1

fix 3 group langevin 300.0 300.0 10.0 699483

fix_modify 3 temp myTemp

Is that right?

@ Ray:

Many Refs applied both on the same group.

You’ll have to try it yourself, see

what it does, use your script and output

to debug what it is doing, read the doc
pages carefully. Asking the

mail list for reassurance is not a substitute

for that.