[gmx-users] Temperature coupling of water with ions

David van der Spoel spoel at xray.bmc.uu.se
Fri Dec 9 08:57:31 CET 2005

sunita at chem.iitb.ac.in wrote:
> Dear gmx-users,
> Solvent and ions when coupled together, exchange of heat with the
> temperature bath takes place together (or velocities scaled for both the
> groups coupled simultaneously). But when solvent and ions coupled
> separately with the temperature bath, exchange of heat of the tc_groups
> with bath takes place separately (velocities scaled separately).
> Are the above statements correct?
> I performed two test simulations where the only difference is in the
> tc_grp. In one case it is sol_CL and in other it both sol and CL coupled
> separately. But I am getting similar kind of temperature statistic.
That is good.

Separate coupling is a trick necessary in some case where heat exchange 
does not work very well, usually when mixing hydrophobic and 
hyodrophilic molecules (e.g. a protein in water). This is really a 
problem with the potential function, as the same happens when using Nose 
Hoover. Since ions and water have strong interactions they also are able 
to exchange heat, and on average the temperatures will be correct. It 
might be less good if you use a cut-off, however.

> Could you please suggest which one is the correct method of temperature
> coupling.
> Thanks for any suggestion.
> With regards,
> sunita
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David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://xray.bmc.uu.se/~spoel

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