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## Re: Gamma in Langevin Thermostat

 From: Rudolf Weeber Subject: Re: Gamma in Langevin Thermostat Date: Fri, 24 Apr 2020 19:39:25 +0200 User-agent: Mutt/1.9.4 (2018-02-28)

```Hi,
On Fri, Apr 24, 2020 at 05:08:53PM +0000, Ahmad Reza Motezakker wrote:
> I hope you all are fine and healthy.
>
> I have been trying to understand how I can set gamma parameter in langevin
> thermostat. I have gone through the online documentation, github, and
> previously asked questions in espressomd archive with no clear answer. I will
> be really thankful if you open it up for me. I have a single particle with
> mass of 8*10^(-18) grams (which is not considered in the simulation but of
> course for doing unit conversions and  calculating other parameters such as
> time unit) at room temperature.
>
> Thank you again.
The equation of motion being solved is
mx.. = -gamma x. +F(x,x.) +F_random
Here, x = position, x.=vleocity, x..=acceleration.
F_random = Random kicks applied by the thermostat.
All terms in the equation are of unit force.
So, for the term including gamma we have [gamma_unit] * [velocity] = [force]
So, gamma would be in Netwon second / meter, if I didn't miscalculate.

All of this is only relevant, if you actually want to look at time-dependet
behavior. Otherwise, gamma can be chosen for stability of the integration.
For a Lennard-Jones system with LJ epsilon=1 and sigma=1 at a thermal energy
kT=1, values between 1 and 10 will do for gamma. Use 1 for the particle mass,
in that case.

Hope that helps.
Regards, Rudolf

```