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

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
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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