# friction in langevian

Hello

I am going to simulate a system using Langevin thermostat by setting friction parameter to 11.8. What should be my damp factor to get my desired friction parameter?

thanks

What is 11.8? The fix langevin doc page explains how

to use it:

The damp parameter is specified in time units and determines how rapidly the temperature is relaxed. For example, a value of 100.0 means to relax the temperature in a timespan of (roughly) 100 time units (tau or fmsec or psec - see the units command). The damp factor can be thought of as inversely related to the viscosity of the solvent. I.e. a small relaxation time implies a hi-viscosity solvent and vice versa. See the discussion about gamma and viscosity in the documentation for the fix viscous command for more details.

A value of damp equivalent to ~100 timesteps (in appropriate units)

is typically good for a thermostat.

Steve

Dear Steve,
Thanks for your answer. Yes, you are right I have read the document on the langevin I just observe something wrong on my results and I thought maybe there is mistake in my calculation of damo factor. If I want to have a Friction of 11.8 then my Damp factor should be 1/11.8 based on what you mentioned right?

From the manual page (http://lammps.sandia.gov/doc/fix_langevin.html):

Apply a Langevin thermostat as described in (Schneider) to a group of atoms which models an interaction with a background implicit solvent. Used with fix nve, this command performs Brownian dynamics (BD), since the total force on each atom will have the form:

F = Fc + Ff + Fr
Ff = - (m / damp) v
Fr is proportional to sqrt(Kb T m / (dt damp))

Take also a look at the Wiki page for Langevin dynamics: https://en.wikipedia.org/wiki/Langevin_dynamics

I don’t know. You should read the Langevin paper(s) that are cited.

I think of the damp factor as just affecting the timescale over

which the thermostatting takes place, i.e. how rapidly it forces

the particles to be at the requested temperature.

Steve

I don't know. You should read the Langevin paper(s) that are cited.
I think of the damp factor as just affecting the timescale over
which the thermostatting takes place, i.e. how rapidly it forces
the particles to be at the requested temperature.

i would also suggest to pay attention to units.

axel.

Dear Alex,
My problem is based on a paper that Friction factor in the lj unit is 11.8 and I am doing my simulation in the lj units,so if my interpretation of Damp factor is right( which is 1/11.8, actully my system shoud be very viscose so considering a relaxation of 0.08 should be logical) I guess there should not by any problem.

I don’t know. You should read the Langevin paper(s) that are cited.
I think of the damp factor as just affecting the timescale over
which the thermostatting takes place, i.e. how rapidly it forces
the particles to be at the requested temperature.

i would also suggest to pay attention to units.

axel.

Dear Alex,
My problem is based on a paper that Friction factor in the lj unit is 11.8
and I am doing my simulation in the lj units,so if my interpretation of Damp
factor is right( which is 1/11.8, actully my system shoud be very viscose so
considering a relaxation of 0.08 should be logical) I guess there should not
by any problem.

if you believe that you can do proper science on the base of guessing,
rather than doing a careful validation, then i have nothing to add to
this.

axel.

Dear Alex,
I have read the related equations on the Friction parameter and relaxation time so I believe that my statement is correct, I just wanted to check my conclusion and ask if anyone can confirm that.
Thank you

Dear Alex,
My problem is based on a paper that Friction factor in the lj unit is 11.8
and I am doing my simulation in the lj units,so if my interpretation of Damp
factor is right( which is 1/11.8, actully my system shoud be very viscose so
considering a relaxation of 0.08 should be logical) I guess there should not
by any problem.

if you believe that you can do proper science on the base of guessing,
rather than doing a careful validation, then i have nothing to add to
this.

axel.