[gmx-users] What is the meaning of negative surface tension?

[gozde ergin]

Dear all,

I am simulating organic covered salty water system in order to estimate the surface tension. Force field is Charmm36 and water model in TIP3P.
Surface tension is -127 mN/m after 5 ns run. What is the meaning of negative surface tension?
I was expecting +30 ~ +40 mN/m but not negative. 
Maybe my system was not under equilibrium conditions. 
Will longer running solve the problem? Any help would be appreciated.

Thanks in advance.