[gmx-users] Negative pressure in an interface NVT simulation

[André Farias de Moura]

Hi Surya,

I don't think you have a gas phase of any kind, your model of water should
not produce any appreciable amount of vapor above the liquid slab (as a
matter of fact, I think that if you see water molecules in the gas phase
using SPC/E and other similar models at 298 K you most likely did something
wrong). Mind that force fields are parametrized to reproduce very specific
properties, and vapor pressure is not usually one of them (you cannot
expect force fields to reproduce properties unrelated to whatever
properties were used to set their parameters).

So, as a matter of fact, your system should be regarded as a vacuum-liquid
interface, not a liquid-gas interface.

Just a personal opinion: I would not worry about pressure components other
than the surface pressure. As long as both sides of the slab have equal
pressures, the system should be regarded as being in mechanical equilibrium.

But if you're actually interested in liquid-vapor coexistence curves, there
are better suited approaches (for instance, take a look at:
https://www.nist.gov/mml/csd/chemical-informatics-research-group/sat-tmmc-liquid-vapor-coexistence-properties-spce-water
)

best

Andre

On Tue, Sep 27, 2016 at 11:02 AM, Surya Prakash Tiwari <sptiwari at gmail.com>
wrote:

> Hello again,
>
> Can someone take my question. You don't need to fully answer my question.
> If you could just show me the direction, that would be more than enough.
>
> Thanks in advance.
>
> Surya Prakash Tiwari
>
> On Wed, Sep 21, 2016 at 12:11 PM, Surya Prakash Tiwari <sptiwari at gmail.com
> >
> wrote:
>
> > Dear Gromacs users,
> >
> > I am doing a water liquid-gas interface simulation in NVT ensemble (at
> 298
> > K, 1000 SPC/E, box size is described in the end). The pressure calculated
> > (after equilibration) using gmx energy shows negative values:
> >
> > Energy                      Average   Err.Est.       RMSD  Tot-Drift
> > ------------------------------------------------------------
> > -------------------
> > Pressure                   -151.627       0.67    190.708    2.20173
> (bar)
> > Pres-ZZ                    -31.7913     0.0072    262.697 -0.0293928
> (bar)
> > #Surf*SurfTen               1151.56        6.5    1806.55   -21.4399
> (bar
> > nm)
> >
> > Since liquid and gas phases are in equilibrium, shouldn't pressure (at
> > least Pzz) be equal to the saturation pressure of water. A large negative
> > value is not making sense. Can someone help me with understanding this.
> One
> > of my colleagues used NAMD, and he said that he is getting the right
> > pressure for the interface system!
> >
> > Though the pressures, I am getting, are negative; calculated surface
> > tension value matches well with those available in the literature. It
> seems
> > that something is getting cancelled out, and I am getting the correct
> > surface tension. Am I right?
> >
> > I tried to look how the pressure is calculated in Gromacs.
> Mathematically,
> > my pressures are negative because the virial energies are larger in the
> > case of interface simulation compared to the case without interface,
> other
> > parameters remain almost same.
> >
> >
> > The starting configuration for the NVT interface simulation was obtained
> > using the following procedure:
> > 1. 1000 water molecules were equilibrated using NPT ensemble (298 K, 1
> > bar).
> > 2. Find the average box lengths using gmx energy tool.
> > 3. Obtain a gro trajectory file from the xtc/trr file using:
> > gmx trjconv -f md -s md.tpr -o conf.gro -pbc whole -b 1000
> > 4. A snapshot configuration from the above gro file was chosen such that
> > its box lengths matches with the average box lengths obtained in step 2
> (to
> > get the pressure right).
> > 5. Double the Z length of the gro file obtained in step 4. Use this as
> the
> > starting configuration for NVT interface simulation.
> >
> >
> > I have tried to explain as much as possible to get the help, but if I am
> > missing some information, please let me know and I will provide that.
> >
> >
> > Thanks a lot. I look forward for some help.
> >
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-- 
_____________

Prof. Dr. André Farias de Moura
Department of Chemistry
Federal University of São Carlos
São Carlos - Brazil
phone: +55-16-3351-8090