Hi, I am trying to obtain the surface tension for the hexane-water interface. I used Gromacs 4.0.7.
A simulation where I keep the X-Y dimensions fixed and let z fluctuate yields the following output: Energy Average RMSD Fluct. Drift Tot-Drift ------------------------------------------------------------------------------- Box-X 8 0 0 0 0 Box-Y 8 0 0 0 0 Box-Z 9.90151 0.00902362 0.00902139 1.00596e-07 0.000694869 Volume 633.697 0.577512 0.577369 6.43816e-06 0.0444716 Density (SI) 790.105 0.720108 0.71993 -8.03015e-06 -0.0554683 Pres-XX (bar) -93.007 111.668 111.668 -5.85703e-05 -0.404574 Pres-YY (bar) -93.8345 111.464 111.461 -0.000388743 -2.68524 Pres-ZZ (bar) 1.04823 102.795 102.795 7.14103e-05 0.493267 #Surf*SurfTen 935.415 1174.75 1174.74 0.00292727 20.2201 In order to test this value, I have done a constant surface simulation. I expect that this run should also give me a stable system. In this case I set the X-Y dimensions free, fixed the Z dimension by setting the compressibility to zero. I set the surface tension to 935, which I obtained from the first simulation. This simulation yields completely bizarre results: Energy Average RMSD Fluct. Drift Tot-Drift ------------------------------------------------------------------------------- Box-X 7.22688 0.478216 0.0850938 -0.00115819 -1.63015 Box-Y 7.22688 0.478216 0.0850938 -0.00115819 -1.63015 Box-Z 9.90248 0 0 0 0 Volume 519.449 68.6696 12.4766 -0.166195 -233.919 Density (SI) 980.851 128.888 23.3119 0.311983 439.117 Pres-XX (bar) 8178.74 7814.17 2469.25 18.2466 25682.1 Pres-YY (bar) 8189.38 7827.51 2458.45 18.29 25743.2 Pres-ZZ (bar) 8284.24 7819.63 2462.7 18.2661 25709.6 #Surf*SurfTen 992.022 1516.44 1516.41 -0.0216172 -30.4263 The section from the .mdp file I used is as follows: ; Pressure coupling is on Pcoupl = berendsen tau_p = 1.0 1.0 Pcoupltype = surface-tension compressibility = 4.5e-5 0 ref_p = 935.605 1.0 Some how, the system shrinks in X-Y dimensions rather than expand, and this yields strange pressure values. I does maintain the target surface-tension more or less, but the density is completely unrealistic. I also tested letting the z-dimension also free, which gave more reasonable results. The options are set to: ; Pressure coupling is on Pcoupl = berendsen tau_p = 1.0 1.0 Pcoupltype = surface-tension compressibility = 4.5e-5 4.5e-5 ref_p = 935.605 1.0 Energy Average RMSD Fluct. Drift Tot-Drift ------------------------------------------------------------------------------- Box-X 7.88739 0.0533823 0.0365402 -0.000116266 -0.134811 Box-Y 7.88739 0.0533823 0.0365402 -0.000116266 -0.134811 Box-Z 10.1838 0.13604 0.0933104 0.000295756 0.342929 Volume 633.462 0.92414 0.917408 -0.000332644 -0.385701 Density (SI) 790.398 1.15266 1.14426 0.000414983 0.481174 Pres-XX (bar) -90.6569 105.24 105.186 0.010099 11.7098 Pres-YY (bar) -91.2499 104.286 104.267 0.00588557 6.82433 Pres-ZZ (bar) 1.79822 95.6962 95.6725 0.00636455 7.37971 #Surf*SurfTen 944.344 1095.81 1095.8 0.010786 12.5064 However in this case also the system does not seem to be stable, but slowly the z dimension increases. Any ideas? Am I doing a mistake in the .mdp file, or is there a problem with the surface tension coupling? Thanks in advance, Mehmet -- gmx-users mailing list gmx-users@gromacs.org http://lists.gromacs.org/mailman/listinfo/gmx-users Please search the archive at http://www.gromacs.org/search before posting! Please don't post (un)subscribe requests to the list. Use the www interface or send it to gmx-users-requ...@gromacs.org. Can't post? Read http://www.gromacs.org/mailing_lists/users.php