[gmx-users] a question about ensemble

[Patrick Fuchs]

Hi Florian,

>> I remark that Langevin method is used also for explicit water system!
>
> But there a big question arises to me. The thermostatting by Langevin is
> achieved due to random kicks. If I simulate all atoms explicitly, there
> is only vacuum between the atoms. Where do the random kicks come from
> and how do I set gamma, which is actually related to the viscosity of
> the medium I am simulating in? If my medium is vacuum, then gamma should
> be zero, shouldn't it, and gamma=0 means no coupling, and hence,
> Newton's equation of motion are recovered.
>
> I am not an expert with the Langevin thermostat, so this are serious
> questions that arise to me now. Furthermore I also thought, that
> Langevin dynamics were exactly established for a description of a system
> within a medium.
The use of Langevin dynamics (SD) to mimic solvant implicitely or as a 
thermostat in explicit systems depends on the friction coefficient you 
choose. It has to be chosen with care, see for example 
http://dx.doi.org/10.1007/b99427 or the GROMACS manual. In explicit 
simulations, SD with the appropriate choice of friction coefficient can 
be seen as a local thermostat. The advantage is that it samples the 
canonical ensemble (in the old versions of GROMACS it was the only way 
to get the canonical ensemble before the implementation of Nose-Hoover 
or velocity rescaling).
Ciao,

Patrick
-- 
_______________________________________________________________________
Patrick FUCHS
Dynamique des Structures et Interactions des Macromolécules Biologiques
INTS, INSERM UMR-S665, Université Paris Diderot,
6 rue Alexandre Cabanel, 75015 Paris
Tel : +33 (0)1-44-49-30-57 - Fax : +33 (0)1-43-06-50-19
E-mail address: patrick.fuchs at univ-paris-diderot.fr
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