[gmx-users] compressibility value for SPC water

David van der Spoel spoel at xray.bmc.uu.se
Thu Jan 11 09:17:56 CET 2007

chetana baliga wrote:
> Dear all,
> I read from the mailing list archives, that it is not just the value of 
> tau_p but the ratio of compressibility to tau_p which matters in 
> Berendsen presure coupling.  I am using SPC water as the solvent. Is 
> there any ideal value for compressibility to be used for SPC water? i 
> have used a value of 4.5e-5, and a tau_p of 1.0 ps, and my pressure 
> coupling is isotropic.  Should the tau_p value be increased or decreased 
> to get a pressure average closer to ref_p?
It doesn't matter. If you check the equation you see that it is indeeed 
tau_p/compressibility, so if you increase compressibility by a factor of 
ten that is the same as decreasing tau_p by a factor of ten. Using a 
value for the compressibility that is roughly correct (as you have done) 
means that tau_p relates directly to the relaxation time of the 
coupling. Note that the number you use is for real water, the 
compressibility for spc may be different. I'm sure it has been 
published, but it is left here as an exercise for the reader.

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://folding.bmc.uu.se

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