Re: Constant pressure contril

From: Jérôme Hénin (
Date: Sat Feb 28 2004 - 17:50:46 CST

Hello Sichun,

The GPRESSURE field should fluctuate around the requested value (if you
specified the useGroupPressure option). However, the fluctuations are HUGE,
because (1) the system is microscopic and (2) it's in a condensed phase. So
don't worry if the pressure is 100 times too high at times (depending on
system size).
What's relevant is computing averages of this value over large times. Then you
should get *roughly* the pressure you asked for.

So as you can see, "constant-pressure" is a strange expression for what we're
simulating. But what does "pressure" mean in the first place, when you're
speaking of, say, 50000 atoms in a 10 ns trajectory? We can't ask too much
regarding macroscopic (or even mesoscopic) quantities...


Le dimanche 29 Février 2004 00:05, Sichun Yang a écrit :
> Dear All,
> I have used Nose-Hoover Langevin piston pressure ON.
> Now I want to double-check and make sure my configuration is ok.
> Which pressure should be constant in the output file, PRESSURE
> or GPRESSURER? It seems none of them is close to constant in my output.
> Any hint? Thanks,
> Sichun

Jérôme Hénin
Equipe Dynamique des Assemblages Membranaires
Université Henri Poincaré / CNRS    UMR 7565
B.P. 239        54506 Vandoeuvre-lès-Nancy Cedex
Tel : (33) 3 83 68 43 95        Fax : (33) 3 83 68 43 71
In principio creauit Linus Linucem

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