# [gmx-users] determine surface tension

Sun Dec 31 16:12:19 CET 2017

```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?

Incorporation of Surface Tension into Molecular Dynamics Simulation of
an Interface: A Fluid Phase Lipid Bilayer Membrane

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.

what should I do?

Any suggestion will be appreciated.

Thanks