[gmx-users] Pressure and NVT Simulations

Manik Mayur manik.mayur at gmail.com
Fri Jun 5 21:00:32 CEST 2009

On Fri, Jun 5, 2009 at 11:58 PM, Jussi Lehtola <jussi.lehtola at helsinki.fi>wrote:

> 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.

Thanks for the insight. So if the density of my water system was
corresponding to SPC/E water density at the given T, the pressure should
have been themodynamically consistent? (I mean the average value of course.)

> --
> ------------------------------------------------------
> Jussi Lehtola, FM, Tohtorikoulutettava
> Fysiikan laitos, Helsingin Yliopisto
> jussi.lehtola at helsinki.fi, p. 191 50632
> ------------------------------------------------------
> Mr. Jussi Lehtola, M. Sc., Doctoral Student
> Department of Physics, University of Helsinki, Finland
> jussi.lehtola at helsinki.fi
> ------------------------------------------------------
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Manik Mayur
Graduate student
Microfluidics Lab
Dept. of Mechanical Engg.
IIT Kharagpur
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