[gmx-users] Problem with computing graphite-water surface tension

[WU Yanbin]

Dear GMXers,

I'm trying to compute the surface tension between water and graphite
surface, using the C-O interaction parameters as in Werder et. al. J. Phys.
Chem. B 2003. The expected contact angle, based on Young's equation, is 86
degrees (as reported in Werder's paper by droplet method). The SPC/E water
model was used.

The simulation setup is shown as follows:

~~~~~~water~~~~~~
~~~~~~water~~~~~~
------------graphite--------
~~~~~~water~~~~~~
~~~~~~water~~~~~~

The graphite surface is frozen in the middle of the simulation box. The
water baths at the top and bottom are connected through periodic boundary
conditions.
The height of the simulation box is allowed to fluctuate through constant
pressure ensemble of 1bar.

The simulation is done using GROMACS3.3.4. The key words to set up the
pressure coupling are as follows:

   Pcoul               = Parrinello-Rahman
   Pcoupltype       = semiisotropic
   tau_p               = 1.0 1.0
   compressibility = 0.0 4.5e-5
   ref_p                = 1.0 1.0

The surface tension was computed using g_energy. The resulting surface
tension is 68.9mN/m, which is  larger than water liquid-vapor surface
tension for SPC/E water model (~58.9mN/m).
Based on Young's equation, the computed contact angle is 180 degrees, which
is not reasonable (The solid-vapor surface tension is considered as zero,
since the surface atoms are frozen).

Is there anything wrong with my calculation or simulation setup? Any
suggestion is appreciated.
Do let me know if the question is not made clear.

Thank you.

Best,
Yanbin
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