Dear GMXers, I'm computing the liquid-vapor surface tension for SPC/E water using GROMACS 4.5.3. The initial water box dimension is 3nm*3nm*3nm, containing 1807 water molecules, which is extended in z direction to a length of 12nm, to create two liquid-vapor interfaces. The ensemble is NVT. The system was equilibrated for 1ns and ran for 60ns for data collection.
Below is the pressure and virial output using g_energy: Energy Average Err.Est. RMSD Tot-Drift ------------------------------------------------------------------------------- Vir-XX 4809.7 1.3 721.805 -5.05917 (kJ/mol) Vir-YY 4810.68 1 723.077 -3.29302 (kJ/mol) Vir-ZZ 4536.49 0.1 715.838 -0.434853 (kJ/mol) Pres-XX -97.9413 0.39 223.717 1.4721 (bar) Pres-YY -98.2664 0.33 224.085 1.21738 (bar) Pres-ZZ -0.204398 0.012 221.921 0.0134426 (bar) If I compute the surface tension using gamma = (pzz-0.5*(pxx+pyy))*Lz/2, the resulting surface tension is 58.7mN/m. But if the surface tension is computed using gamma=(PIzz-0.5*(PIxx+PIyy))/Axy, the resulting surface tension is -25.3mN/m, which is negative. Is there anything wrong with my calculation? I will be happy to provide any further information regarding the simulation setup if needed. Thank you in advance. Best, Yanbin
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