[gmx-users] Meaningless pressure values: NVT and NVE ensembles

[Tsjerk Wassenaar]

Hi Francesco,

You have to consider the difference between microscopic and macroscopic
pressure, and also think of what it means to be stuck in a periodic system.
You have a mere 108 particles. What would the probability be that they move
close to each other and give a high pressure? And what would the reaction
be? Pushing outward again, giving a pretty high probability that they are
at larger distances, with the attraction being the main effect. This will
happen perpetually, giving very large fluctuations. Over time, your average
should converge to the reference pressure. But the fluctuation will remain
very large, because the system is so small.

Best,

Tsjerk

On Wed, Nov 5, 2014 at 1:17 PM, Francesco Mambretti <
francesco.mambretti at studenti.unimi.it> wrote:

>
> So you confirm me that pressure calculation in Gromacs is done through
> virial theorem and should provide meaningful results?
> I start from a perfect fcc crystal and launch simulations of 20 ps...is it
> a
> too short simulation? density and temperature should be already optimized
> for argon solid state. However, I am trying to run simulations of 2000 ps.
> If the problem persists, I'll let you know.
>
> Thanks
>
> Francesco
>
>
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-- 
Tsjerk A. Wassenaar, Ph.D.