Dear GMXers,<br><br><font size="2"><span style="font-size: 10pt;">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.<br><br>
The simulation setup is shown as follows:<br><br>~~~~~~water~~~~~~<br></span></font><font size="2"><span style="font-size: 10pt;">~~~~~~water~~~~~~<br>------------graphite--------<br></span></font><font size="2"><span style="font-size: 10pt;">~~~~~~water~~~~~~<br>
</span></font><font size="2"><span style="font-size:10pt;">~~~~~~water~~~~~~</span></font><br><font size="2"><span style="font-size:10pt;"><br>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.<br>
The height of the simulation box is allowed to fluctuate through constant pressure ensemble of 1bar. <br><br>The simulation is done using GROMACS3.3.4. The key words to set up the pressure coupling are as follows:<br>
<br>
Pcoul = Parrinello-Rahman<br>
Pcoupltype = semiisotropic<br>
tau_p = 1.0 1.0<br>
compressibility = 0.0 4.5e-5<br>
ref_p = 1.0 1.0<br>
<br>
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).<br>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).<br>
<br>Is there anything wrong with my calculation or simulation setup? Any suggestion is appreciated.<br>Do let me know if the question is not made clear.<br><br>Thank you.<br><br>Best,<br>Yanbin<br></span></font>