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

David van der Spoel spoel at xray.bmc.uu.se
Thu Sep 29 08:16:42 CEST 2016

```On 27/09/16 16:02, Surya Prakash Tiwari 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.
>
This is as it should be. Your system has a surface tension as you see
that you can compare to experimental data if you wish. Since you have
two surfaces you have to divide the number by 2 and correct for units.
Check J. Chem. Theory Comput. 11 pp. 2938-2944 (2015) for an up to date
discussion of surface tensions in simulations.

>
> 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.
>>

--
David van der Spoel, Ph.D., Professor of Biology
Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205.
spoel at xray.bmc.uu.se    http://folding.bmc.uu.se
```