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Re: [ESPResSo-users] Grand canonical thermostat

From: Stefan Kesselheim
Subject: Re: [ESPResSo-users] Grand canonical thermostat
Date: Sun, 1 Mar 2015 09:47:06 +0100

just a quick question: What was it, that Sela Samin implemented a few years 
ago? Wasn't it something like GCMC?

On Feb 27, 2015, at 11:50 AM, Salim Maduar <address@hidden> wrote:

> Dear, Axel
> Thank you so much! for the comments
> Then I will try to use TCL grand-canonical version, but I would optimize it 
> in terms of number of MD and MC steps so that to achieve good speed and stay 
> at equilibrium.
> Thank you for the advice.
> -- 
>  Best Regards,
>  Salim Maduar
>  PhD student 
>  Faculty of Physics, 
>  Lomonosov Moscow State University
>  Moscow, Russia
>  Web:
> 27.02.2015, 12:33, "Axel Arnold" <address@hidden>:
>> Hi!
>>> Am 27.02.2015 um 09:26 schrieb Jakub Krajniak <address@hidden>:
>>> On 27.02.2015 07:02, Axel Arnold wrote:
>>> (...)
>>>> Note also that you need to be careful when adding particles so often.
>>>> The Langevin thermostat needs a while to establish the desired
>>>> temperature, namely roughly 1/gamma/dt time steps. So, for gamma=1 and
>>>> dt=0.01 you need about 100 steps to “heal” the overall temperature. You
>>>> can accelerate that by increasing gamma, however, also the product of
>>>> gamma and dt can’t be too large. 20 time steps is already critical, and
>>>> 10 time steps is from my experience not enough to get the temperature
>>>> correct.
>>> I'm sorry for off-topic but that sounds very interesting. So it is possible 
>>> to calculate how fast Langevin thermostat will bring system to desired 
>>> temperature? Do you know any reference about that?
>> Well, of course that depends on how far you are off equilibrium. But what 
>> gamma (or gamma/m, if you have masses compiled in) tells you is the 
>> relaxation time of the resulting dynamics. That is, you can determine gamma 
>> from the decay constant of the velocity autocorrelation function. I don’t 
>> know any reference on that (nor on the Langevin thermostat, actually), but 
>> it is immediately obvious from the Langevin equation, which the Langevin 
>> thermostat approximates.
>> As Peter already pointed out, that does not guarantee that the system has 
>> equilibrated yet, but before the velocity autocorrelation has decayed, there 
>> is no chance that the thermostat has equilibrated even a single degree of 
>> freedom.
>> Best,
>> Axel
>> ------------------------------------------------
>> Dr. Axel Arnold
>> ICP, Universität Stuttgart
>> Allmandring 3
>> 70569 Stuttgart, Germany
>> Email: address@hidden
>> Phone: +49 711 685 67609

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