[gmx-users] Cannot obtain the expected pressure using the Parrinello-Rahman coupling in a gas-liquid system

Jussi Lehtola jussi.lehtola at helsinki.fi
Tue Mar 10 11:18:14 CET 2009

On Tue, 2009-03-10 at 09:46 +0000, GuoGuangjun wrote:
> Hi,
> According to Berk's instructions, I first run a 5 ns-long berendesen
> P-coupling simulation, and then change to Parrinello-Rahman coupling
> for another 5 ns. The parameters and results are listed in the
> following. My question is why the average pressures in the X- and
> Y-directions is not equal to the expected pressure.
> tau_p                        = 10

Your runtime is one half of the pressure relaxation time. I'd say you
need to run for a much longer time than that.

Also, the deviation compared to the fluctuations is small: 57 bar / 121
bar ~ 0.47 < 0.5 sigma. In x and y directions the deviation is even
smaller: 85/201 ~ 0.42
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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