[gmx-developers] More about the Ekin issue.

hessb at mpip-mainz.mpg.de hessb at mpip-mainz.mpg.de
Thu Aug 11 19:11:09 CEST 2005

Quoting David <spoel at xray.bmc.uu.se>:

> On Thu, 2005-08-11 at 18:50 +0200, hessb at mpip-mainz.mpg.de wrote:
> > I think the whole NPT REMD issue (which is no longer
> > related to the subject) is caused by the, probably,
> > incorrect CVS implementation of David.
> > 
> > I think the highly fluctuating, meaningless instantaneous
> > pressure in the criterion should be replaced by the reference
> > pressure, as one considers an constant NPT ensemble.
> > Just like a constant T is used in the criterion.
> > The pV term then only depends on the instananeous volume,
> > which fluctations do have meaning.
> > 
> > BTW I think there is also a factor 1/PRESFAC missing in the exchange
> criterion.
> Any obvious bugs apart from that then?

I don't think so.

> I will replace the instantaneous pressure by the constant trace of the
> reference pressure.

1/3 of the trace I guess?

> > 
> > Then there is still the barostat issue.
> > 400 waters at 300K, p=1 bar for 10 ns give:
> > Berendsen volume: 11.968 rmsd 0.089
> > Parrinello-Rahman: 11.977 rmsd 0.150
> This is not relevant but the derived volume fluctuations are of course.

What is not relevant?
You mean the difference between the average volumes?

For the fluctuations I am not sure what is correct.
I hope the current Parrinello-Rahman implementation is correct,
but I have not really used it myself.

Can one analytically derive what the volume fluctuations should be?

I was also wondering if a Langevin type of barostat exists.


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