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

[Jussi Lehtola]

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
-- 
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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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