[gmx-users] heat exchanges

[Berk Hess]

Hi,

Strictly speaking you should never have thermostats for separate parts
of the system when those parts are coupled through potentials.
But in practice you can have integration errors which heat or cool different
parts of the system in different ways.
I think that for most systems only the electrostatics treatment will have
a significant effect. Plain Coulomb cut-off and reaction-field both produce
large integration errors. With PME you should never need multiple t-coupl groups.

The other issue is slow heat exchange between different parts of the system.
For a protein in solvent I think this is never an issue. But for other systems
it could be. Here it is the question if your simulation is much longer
than this exchange time or not.
The Berendsen thermostat can produce artifacts when you have weakly
coupled modes. but with Gromacs 4 there is no reason to use Berendsen,
since we have v-rescale and Nose-Hoover.

I think you can always use PME, v-rescale or nose-hoover and a single
t-coupling group.

Berk


> Date: Thu, 5 Feb 2009 13:07:21 +0100
> From: spoel at xray.bmc.uu.se
> To: gmx-users at gromacs.org
> Subject: Re: [gmx-users] heat exchanges
> 
> XAvier Periole wrote:
> > 
> > On Feb 5, 2009, at 11:02 AM, Carlo Camilloni wrote:
> > 
> >> Dear Gromacs users,
> >>
> >> I am doing some tests with thermostats and I would like to know if 
> >> someone has just done it. In particular:
> > I have not done systematic tests but here are my impressions
> >>
> >> 1) How big has to be a protein to be coupled with a separate bath?
> > a few residues ...
> >>
> >> 2) Do you know over which time scales a flux of heat is observed 
> >> between protein and solvent using only one temperature group?
> > few 10-100 ps.
> This depends a lot on how the rest of the setup is, I would think. I had 
>   some correspondence on the computational chemistry list about this 10 
> years ago, and there people boldly stated that with Nose Hoover there is 
> no need for coupling separate groups at all. At the time I could not 
> reproduce that with GROMACS, but I did not persevere it. It could also 
> have to do with flexibility, and/or using united atoms without charges, 
> i.e. that there is little energetic coupling between e.g. water and a 
> hydrophobic substance.
> 
> It would be worthwhile to do systematic tests with different thermostats 
> and coupling schemes. If it turns out that the same problems exist in 
> Nose Hoover thermostats then it is probably publishable as well.
> 
> 
> 
> >>
> >>
> >> Thanks,
> >>
> >> Carlo Camilloni
> >>
> >> -- 
> >> Dr Carlo Camilloni
> >> Department of Physics, University of Milano
> >> via Celoria, 16 - 20133 Milano, Italy
> >> phone: +39-02-50317654
> >> carlo.camilloni at mi.infn.it
> >> http://merlino.mi.infn.it/~carlo
> >>
> >>
> >>
> >>
> >>
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> 
> -- 
> David van der Spoel, Ph.D., Professor of Biology
> Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
> Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
> spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
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