[gmx-users] Questions about Thermostats

[Dommert Florian]

On Tue, 2012-05-15 at 19:47 +0100, Lara Bunte wrote: 
> Hello
> 
> To make better energy minimization procedures I read about thermostats and barostats. I understand the physical concepts and differences between global and local thermostats and the difference between Berendsen and Nose-Hoover thermostat. 
> 
> 1.) 
> Maybe this question is a little bit hairsplitting, but: This concepts of thermostats and barostats, is this Thermodynamics, is this Kinetics, is this statistical physics? What is it if I want to give this a name.
> 
> 2.)
> I read, that local thermostats, i.e. stochastic dynamics produce a NVT ensemble, which is a canonical ensemble and that this alway fulfills Ergodicity Theorem. About this I have following question: In my literature they said, that this means, that all degrees of freedom in the system are coupled strong enough each other. This confuses me. I learned, that ergodicity means, that the complete phase space is passed by a trajectory, if we wait long enough. This means, that ensemble average is equal to time average of the system. Where is in my statement about ergodicity the meaning of "all degrees of freedom in the system are coupled strong enough" Do this in fact mean, that the complete phase space is passed?
> 
> 3.)
> I ask myself what I should use. First question: Local or global thermostat? I guess (not knowing, guessing), that global is better for energy minimization, because as far as I understand, it is more stable than local description. From a physical point of view I think Nose-Hoover shoul be always better, because it produces a real canonical ensemble, while Berendsen is microcanonical ensemble, which is totaly unrealistic?! In an microcanonical ensemble, energy is not changed with the enviroment. This makes no sense? 
> 
> Thanks for helping me
> Greetings
> Lara
> 

Hi, there has been recently a discussion about this topic on this
mailing list. Check the archives for the information you look for.
However, Berendsen is not producing any kind of known ensemble, and
therefore only applicable for equilibration.

/Flo

-- 
Florian Dommert
Dipl. - Phys.

Institute for Computational Physics
University Stuttgart

Pfaffenwaldring 27
70569 Stuttgart

EMail: dommert at icp.uni-stuttgart.de
Homepage: http://www.icp.uni-stuttgart.de/~icp/Florian_Dommert

Tel.: +49 - (0)711 - 68563613
Fax.: +49 - (0)711 - 68563658
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