# [gmx-users] calculation of surface tension

Wed Jan 3 14:25:00 CET 2018

```Hi all
I want to calculate surface  tension in lipid bilayer. I read manual and mailing list message and I have some question:
for calclation surface tension we should use Berendsen barostate with semiisotropic coupling , in mdp file:
; Pressure coupling is on
pcoupl         = Berendsen     ; Pressure coupling on in NPT
pcoupltype         = semiisotropic             ; uniform scaling of box vectors
tau_p         =  2             ; time constant, in ps
ref_p         = -100 1.0             ; reference pressure, in bar
compressibility     = 4.5e-5  0             ; isothermal compressibility of water, bar^-1
How should  I find lateral pressure in xy plane?
we should set comressibility zero in Z direction, becase the length of box is constant, is that right?
&#39;Incorporation of Surface Tension into Molecular Dynamics Simulation of an Interface: A Fluid Phase Lipid Bilayer Membrane&#39;
lateral pressure is calculated by this:
boundary lateral pressure
= 1 - surface tension/55 A
= 1 - (56 dynes/cm)/55 X 10 - 8cm
= _-108 dynes/cm2
= - --100 atmospheres.
but in that equation I need surface tension, whereas  I want to calculate it.
should I run system under NPT condition and when the pressure converges ,I can calculate correctt surface tension?
does the pressure converge to 1 bar?
Any suggestion will be appreciated.
Thanks