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

Sat Feb 28 18:03:11 CET 2009

Hi, 

The question is resolved by changing the pcoupltype from isotropic to semiisotropic. Correspondingly, the compressibility and ref_p are given for the x/y and z directions with the same original values, respectively. The test system includes 300 methane, 1725 water, and 300 methane, forming two interfaces parallel to the xy-plane. Now, the accurate average-total-pressure is obtained from a short run (0.2 ns) with the default tau_p (1 ps). 
 Thank you for your kind replies and useful clues.

Guang-Jun

From: gmx3 at hotmail.com
To: gmx-users at gromacs.org
Subject: RE: [gmx-users] Cannot obtain the expected pressure using the Parrinello-Rahman coupling in a gas-liquid system
Date: Sat, 28 Feb 2009 15:00:50 +0100

Hi,

Is 327 bar the averge total pressure or one the of components of the pressure tensor?
You will have a surface tension acting in the system causing a difference in the different components.

Also it is still not completely clear to me how long you equilibrate and average.
Also the results will depend very much on the size of the system.
In small systems the pressure fluctuations are enormous.
With Berendsen these fluctuations will be surpressed, with PR not.

With PR I would use a tau_p of around 10 ps.
You might need to average the pressure over several (tens of?) ns (after several ns of equilibration)
to obtain a good average.

Berk

From: guogj at hotmail.com
To: gmx-users at gromacs.org
Subject: RE: [gmx-users] Cannot obtain the expected pressure using the Parrinello-Rahman coupling in a gas-liquid system
Date: Sat, 28 Feb 2009 13:51:15 +0000

Hi, Justin,

Thanks for your rapid reply. Let me decribes the question in more details.
First ,two gaseous mathane systems and one liquid water system are equilibrated at 300 bar and 250 K in advance. Then, the gas systems are put at the both sides of liquid. The combined system is performed for 3ns with the Parrinello-Rahman barostat, which produces a average pressure of 327 bar. I am strange to the value and several tests are run, mentioned previously. All these test runs take the end gro file of the 3ns-run as the beginning. When the berendsen barostat is used, the pressure is OK. When the Parrinello-Rahman barostat is used, the questionary pressure occurs again. Although I will try to make a long run for equilibrium, I think the equilibrium has reached because the ~330 bar is always obtained no matter how long simulations are sampled, from 0.2 ns to 3 ns. The version of Gromacs I used is 4.0.4. By the way, how long did you equilibrate your membrane system and how many atoms in it? Thanks again.

Guang-Jun

> Date: Sat, 28 Feb 2009 07:43:29 -0500
> From: jalemkul at vt.edu
> To: gmx-users at gromacs.org
> Subject: Re: [gmx-users] Cannot obtain the expected pressure using the Parrinello-Rahman coupling in a gas-liquid system
> 
> 
> 
> GuoGuangjun wrote:
> > In fact, I once run the gas-liquid interface for 3 ns, the pressure is 
> > still as high as 327 bar.
> 
> There are two possibilities in my mind:
> 
> 1. 3 ns is still not long enough.
> 2. Your gas phase is causing the problem. If the pressure is right for water 
> alone, then the problem likely comes from the inclusion of your gas phase and 
> whatever parameters you're using for whatever species you have in this phase. 
> Examine your parameters and consider how they were derived and whether or not 
> they are adequate.
> 
> Also, what version of Gromacs are you using? There were some changes made to 
> the Parrinello-Rahman barostat between 3.3.x and 4.0.x.
> 
> -Justin
> 
> > Guang-Jun
> > 
> > 
> > > Date: Sat, 28 Feb 2009 07:19:30 -0500
> > > From: jalemkul at vt.edu
> > > To: gmx-users at gromacs.org
> > > Subject: Re: [gmx-users] Cannot obtain the expected pressure using 
> > the Parrinello-Rahman coupling in a gas-liquid system
> > >
> > >
> > >
> > > 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 sin! ! gle-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.
> > >
> > > -Justin
> > >
> > > > Thanks a lot.
> > > > Guang-Jun
> > > >
> > > > 
> > ------------------------------------------------------------------------
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> > ------------------------------------------------------------------------
> > > >
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> > > --
> > > ========================================
> > >
> > > Justin A. Lemkul
> > > Graduate Research Assistant
> > > Department of Biochemistry
> > > Virginia Tech
> > > Blacksburg, VA
> > > jalemkul[at]vt.edu | (540) 231-9080
> > > http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin
> > >
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> -- 
> ========================================
> 
> Justin A. Lemkul
> Graduate Research Assistant
> Department of Biochemistry
> Virginia Tech
> Blacksburg, VA
> jalemkul[at]vt.edu | (540) 231-9080
> http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin
> 
> ========================================
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