Re: Langevin thermostat question

[Ulf D Schiller]

David,

The factor of 24 is correct, provided a uniform distribution of random
numbers between 0 and 1 with variance of 1/12 is used. The theoretical
discussions of the Langevin thermostat typically assume a random
distribution with zero mean and unit variance, hence in practice the
actual variance of the random number generator has to be factored in.

By changing the factor, you effectively decrease the temperature of your
system which may not be what you want.

When using MASS, keep in mind that the units of langevin_gamma are
mass/time, so you may have to adjust it to get the desired ratio of the
relevant forces in your system (aka dimensionless numbers such as Re,
Ca, ..., you name it).

Hope this helps,
Ulf

On 1/21/20 10:37 AM, David Power wrote:
> I'm using a langevin thermostat in my simulation and I noticed that my
> test particle was bouncing around at a higher rate than I would have
> expected due to the noise term. I'm am trying to get some test particles
> to stick to a surface (the experimentalist tell me they should stick),
> but looking at the video output I see the particles just bouncing off
> the surface.
> 
> I noticed that the formula to calculate the noise coefficient in
> core/thermostat.cpp is:
> 
> *sqrt(24 * temperature * langevin_gamma / time_step)*
> 
> but from the description of the langevin thermostat in the documents
> (http://espressomd.org/html/doc/system_setup.html) it says that the
> noise coefficient is given by:
> 
> *sqrt(2 * temperature * langevin_gamma / time_step)*
> 
> A 2 instead of a 24. I changed this, recompiled and now I can see my
> particles sticking to the surface. I was just wanting to confirm if the
> 24  in the code is correct or should that be a 2?
> 
> I'm not sure if it's relevant but I have the MASS module enabled and my
> particle masses are around ~1000. I know that will change the units of
> time in my simulation, does the noise generation function account for
> this change?
> 
> Thanks,
> David

-- 
Dr. Ulf D. Schiller
Assistant Professor, Department of Materials Science and Engineering
Faculty Scholar, School of Health Research
Clemson University
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Clemson, SC 29634

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