# [gmx-users] surface tension in gmx

Elisabeth katesedate at gmail.com
Wed Mar 9 16:25:21 CET 2011

```Dear gmx users,

Since I am new to surface tension topic I need to ask very trivial

As a starting point I am going to calculate surface tension of a pure alkane
in a cubic box and compare with experimental values.

1- g_energy is giving #Surf*SurfTen by default. On the other hand surface
tension can be obtained by gamma = (Pzz - (Pxx+Pyy)/2) / Lz. i.e
Pres-XX-(bar),  Pres-YY(bar),  Pres--(bar)

Can anyone tell me what the difference between these two is?

2- In pressure coupling settings there is surface_tension option which I
guess is applicable where surface tension needs to be kept fixed. If one
want to calculate surface tension I dont think this option make sense. Am I
right?

3- I am using the following setting: I calculate the average for a 2ns run
and different start times as shown below. Although T, P and other quantities
are equilibrated after 200ps, surface tension is not giving a constant
value. Is that because I am not using berenden P coupling? (As mentioned in
the manual surface tension works with berendsen)

Pcoupl              =  Parrinello-Rahman
Pcoupltype          =  isotropic
tau_p               =  1      1
compressibility     =  4.5e-5
ref_p               =  40

time period for which average is calculated   Average       RMSD
Fluct.      Drift  Tot-Drift
-------------------------------------------------------------------------------
1-2000 ps run: #Surf*SurfTen                    6.43844    3588.74
3588.35   0.091406    182.721
500-2000 ps    #Surf*SurfTen                   12.8518    3605.72
3605.26   0.132126    198.189
1000-2000ps   #Surf*SurfTen                    18.8821    3610.97
3610.8    0.11819    118.191
1500-2000ps   #Surf*SurfTen                     23.0072    3585.51
3584.93  -0.444037   -222.019

4- Assuming I am getting surface tension for a cubic box, to compare this
with reported values in literature I need to divide by 6 (no. of surfaces)?

5- Does box six affect the results? (mine is 3.3 nm ).

Thank you,
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