[gmx-users] Compressibility issue - real concern

Erik Lindahl lindahl at cbr.su.se
Thu May 3 13:13:58 CEST 2007


On May 3, 2007, at 12:37 PM, Jones de Andrade wrote:

> Hi Berk! Thanks for your so fast answer!
> I'll try to look for a different tau_p. I would really feel 4 ps a  
> too high value, even for water systems, but will look around before  
> decision. Anyway, I'm still concerned about the compressibility value.
> Long time ago in this list I came to this question, and this is how  
> this ended:
> > Hi,
> >
> > Just use the water value for any liquid system (5e-5 or so).
> >
> > As the manual explains, it only affects the coupling time, not the
> > equilibrium pressure.
> >

This is valid for long times, and (as always for pressure) assumes  
the external coupling to be clearly slower than the internal  
dynamics. I have no idea how Amber or DLPOLY sets their tau_t with  
Nose or Parinello-Rahman, but as specified in the Gromacs manual we  
use the period of average fluctuations at equilibrium. With tau=0.1  
the system will very likely never reach that equilibrium.



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