[gmx-users] Pressure control in Gromacs

Mark Abraham Mark.Abraham at anu.edu.au
Tue Nov 22 03:32:59 CET 2005

Rongliang Wu wrote:
> Hello gmx-users,
>   I've tried the demos in the folder tutor, the pressure also fluctuate rather fiercely and its value is too high i think, does it mean that there is something wrong with the force field or pressure units of GROMACS?

No. As I posted in reply to a misplaced thread on gmx-developers last

"An instantanous pressure (however you define it) will necessarily vary
quite a lot during pressure-coupled simulation. A positive pressure
value just means the system has a tendency to expand, and a negative
value means the system will tend to contract. This is exactly what you
should expect - the coupling is (effectively) a dynamic equilibrium with
an infinite body with the reference pressure.

After a proper equilibration period, a P-coupled MD simulation should
have an average pressure that is close to the reference pressure for the

Much of the problem is in defining an instantaneous, rather than average
pressure. The apparently ferocious variation is totally normal.


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