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

Justin A. Lemkul jalemkul at vt.edu
Sat Feb 28 13:19:30 CET 2009

GuoGuangjun wrote:
> Hi, All,
> I am simulating an interface system composed of methane gas and liquid 
> water in the NPT ensemble. The temperature is well controlled by using 
> either the Berendsen or the Nose-Hoover T-couplings. As for pressure, 
> the Berendsen P-coupling also works well. However, the Parrinello-Rahman 
> P-coupling always produces a higher pressure than the expected. For 
> example, I set ref_p=300 bar in the mdp file but obtain an average 
> pressure as ~330 bar (over 0.5 ns). I test the tau_p in the range 0.1 ~ 
> 10 ps, the case does not change. When I test the single-phase system, 
> i.e., pure water, the Parrinello-Rahman P-coupling can indeed produce 
> the expected pressure with the deviation less than 2 bar. What is the 
> matter in the two-phase system?

When I've used the Parrinello-Rahman barostat for membrane systems, it works 
just fine.  Perhaps 500 ps is too short of a timeframe to arrive at the average. 
  Heterogeneous systems often take longer to equilibrate.


> Thanks a lot.
> Guang-Jun
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Justin A. Lemkul
Graduate Research Assistant
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080


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