[gmx-users] Surface tension calculation for organic monolayer

[gozde ergin]

Dear users,

I simulated three different systems in cubic box and calculated their surface tension as shown below;

1. Pure water , surface tension = 61.5 mN/m
2. Water with 3M NaCl salt, surface tension = 66.5 mN/m
3.Water surface covered with cis-pinonic organic, surface tension = 63.5 mN/m

For system 1 and 2, the results are almost correct however the result for system 3 is not correct.

Cis-pionic is an surface active molecule and should lower the surface tension. 
However in my simulations I could not capture this trend. 
I used GAFF force field and LINCS for the constraints. 
Does anyone has any idea? 
Or is there anyone that calculated the surface tension for organic coated surfaces with GAFF force field? 

Thanks