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Re: [ESPResSo-users] Grand canonical thermostat


From: Axel Arnold
Subject: Re: [ESPResSo-users] Grand canonical thermostat
Date: Sun, 1 Mar 2015 11:22:52 +0100

Sela implemented Ghmc, a hybrid MC/MD scheme, and I don't think it is grand 
canonical.

Axel

Dr. Axel Arnold
Email: address@hidden

> Am 01.03.2015 um 09:47 schrieb Stefan Kesselheim <address@hidden>:
> 
> Hi, 
> just a quick question: What was it, that Sela Samin implemented a few years 
> ago? Wasn't it something like GCMC?
> Cheers
> Stefan
> 
>> 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: http://nanofluidics.phys.msu.ru/maduar.htm
>> 
>> 
>> 
>> 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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