# [lammps-users] damping factor in fix langevin

Dear all

I want to calculate the Bead-spring polymer melt many chains and FENE bonds using
using fix langevin of lammps command.
This is the same simulation as Kremer, Grest, J Chem Phys, 92, 5057 (1990).

In fix langevin on the LAMMPS manual, equation and damping factor was described
as following:
F = Fc + Ff + Fr
Ff = - (m / damp) v
Fr is proportional to sqrt(Kb T m / (dt damp))

In the reference paper(Kremer, Grest, J Chem Phys, 92, 5057 (1990)), friction
factor was set 0.5.
I understand taht if the friction factior in the paper is 1/damp in above
equation,
the damp factor in lammps will be set 1/0.5=2 according to above equation.

Can I use this damping factor of 2 in this simulation?

In LAMMPS, you can use any damping factor you wish. If
you want to match the paper, then you need to read the LAMMPS
doc page and the paper carefully and figure out how to match
the units. The LAMMPS doc page is very explicit about the
units it uses, so you should be able to do that.

Steve

There is a more recent paper by Dobrynin and co-workers (PRL, 93, 037801, 2004) where they use:

Friction coefficient = m/damp = m/tau_lj

Where tau_lj= sigma*(m/epsilon_lj)^0.5

This seems to be give a pretty good scaling.