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## Re: [ESPResSo-users] Choice of timestep and langevin-gamma

 From: Clemens Jochum Subject: Re: [ESPResSo-users] Choice of timestep and langevin-gamma Date: Tue, 19 Sep 2017 17:07:56 +0200 User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0

```Thanks! I will check it out.
Best,
Clemens

On 19.09.2017 16:43, Georg Rempfer wrote:
> Hello Clemens,
>
> > I read previously that for a system with Temp = 1. and mass = 1. a good
> > value for the timestep is 0.01. I have Temp  = 2.5 and mass1 = 20 and
> > mass2 = 660. Is it correct to assume that I can scale the timestep
> > inversely to the thermal velocity with a factor of (m / kT)^(1/2) ~ 3
> > (using the mass1)? I have been using a timestep of 0.05 = 5e-15 s so far
> > without any major problems.
>
> In theory, yes. If you don't have any trouble with dt=0.05, I see no
> problem with that.
>
> > The other parameter I am concerned about is gamma of the
> > langevin-thermostat. So far I just used gamma = 1. = 10 ps^-1, which as
> > I recently read is a little bit high. In my system I simulate a charged
> > macromolecule with counter-ions in an implicit solvent (water). Is a
> > value of gamma = 0.2 = 2 ps^-1 a good fit?
>
> You typically just want to be in the overdamped regime for the ion
> motion. How far in there you are doesn't really matter. Trying to set
> "the right" value will fail anyways, because you are using the same
> gamma for particles that have different sizes and hydration shells in
> reality. Read [1] for a more thorough discussion of the relevant time
> and length scales.
>
> [1] https://arxiv.org/abs/cond-mat/0603391
>
> Greetings,
> Georg

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