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Re: Gamma in Langevin Thermostat

From: Ahmad Reza Motezakker
Subject: Re: Gamma in Langevin Thermostat
Date: Fri, 24 Apr 2020 18:08:42 +0000

Dear Rudolf,

Thank you for the response. As you mentioned, the unit will be 
Newton.second/meter or kg/second.
Unfortunately, I come up with really big numbers for gamma.

Best Regards,
Ahmad Reza
From: Rudolf Weeber <address@hidden>
Sent: Friday, April 24, 2020 7:39 PM
To: Ahmad Reza Motezakker
Cc: address@hidden
Subject: Re: Gamma in Langevin Thermostat

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(x,x.)=forces from your potentials
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

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