[gmx-users] surface tension of monolayer

Sriprajak Krongsuk srikro at kku.ac.th
Wed May 18 06:37:48 CEST 2011

Dear Users,


I have perform the simulation of nonionic surfactant monolayer at the
water-air interface.  I started to run the system at the NPT-ensemble for 10
ns until it reached to an equilibrium.

After that I continued to run it at the NVT-ensemble for 8 ns (4 ns for
equilibrium and 4 ns for data collection). I have calculated the surface
tension of the monolayer by using g_energy 

and it yielded about 23 mN/m which this value is close to an experimental
data. However, I continued to run from the previous point for 4 ns and then
performed the surface tension 

calculation again. At this point the surface tension is negative value (-17
mN/m). Again, I tried to continue to run it for 4 ns and I got the surface
tension of 12 mN/m.  


My question is why the average surface tension is too fluctuation? In my
understanding if the system is in an equilibrium, the average surface
tension should be nearly the same value.


Please everyone suggest me how to handle such a problem.


Thank you 



Dr. Sriprajak Krongsuk

Physics Department, Faculty of Science

Khon Kaen University, Thailand

Tel. (Mobile) 081-2630095


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