[gmx-users] Pressure and NVT Simulations

[Jussi Lehtola]

On Fri, 2009-06-05 at 23:28 +0530, Manik Mayur wrote:
> Hi,
> I am trying to equilibrate a box of SPC/E water using NVT simulations.
> After 2 ns of equilibration my pressure is still <0 bars. Is it an
> expected behavior for NVT simulations or the pressure should converge
> to the thermodynamic equilibration value corresponding to T and
> density after a finite time of equilibration? Can anyone point me to
> fundamental theory pertaining to this behavior as I couldn't find any
> upon googling.

The pressure fluctuates quite a bit during the simulation and it is only
the average value you should look at. In your case the simulation length
is of course long enough to see what happens.

Generally speaking, if your density is too small, the system wants to
increase it and thus you have negative pressure. If it is too large,
then the pressure is positive. Now you are running NVT, meaning that the
pressure can't be adjusted (it's done by scaling the simulation box), so
having a negative pressure is normal.
-- 
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Jussi Lehtola, FM, Tohtorikoulutettava
Fysiikan laitos, Helsingin Yliopisto
jussi.lehtola at helsinki.fi, p. 191 50632
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Mr. Jussi Lehtola, M. Sc., Doctoral Student
Department of Physics, University of Helsinki, Finland
jussi.lehtola at helsinki.fi
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