large variation in the pressure

marc marcvanh... at
Sun Jun 1 09:58:59 UTC 2008

thanks axel and teo for the info


On May 31, 6:52 pm, Axel <akoh... at> wrote:
> On May 31, 5:44 am, marc <marcvanh... at> wrote:
> > Hey,
> marc,
> > I've done a NPT-force field simulation on acetonitrile using CP2K.
> > I've noticed that the variation in the pressure is very large.  Does
> this question comes up regularly on any mailing list for an
> MD code that allows "constant" pressure MD. you have to realize
> that you have a very small system and that the compressibility
> of liquids is small. The smaller the system, the large the relative
> fluctuations in the kinetic energy (=temperature) and as a
> consequence
> of the limited compressibility even higher fluctuations in pressure.
> check your favorite text book on MD for information about pressure
> and virial.
> > anybody knows where this large variation comes from?  How I can avoid
> > it in the future?  I've uploaded my input files.
> as with "temperature" the instantaneous values of a small sample are
> of little use, what matters is the average over time (provided your
> system is in equilibrium).
> as teo already mentioned, those fluctuations become smaller
> with increasing system size. just consider the limiting case
> of a single harmonic bond. there your kinetic energy fluctuates
> between "all" and "nothing".
> cheers,
>    axel.
> > Thanks in advance,
> > cheers & beers,
> > marc

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