large variation in the pressure

Axel akoh... at
Sat May 31 16:52:31 UTC 2008

On May 31, 5:44 am, marc <marcvanh... at> wrote:
> Hey,


> 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
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".


> Thanks in advance,
> cheers & beers,
> marc

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