[gmx-users] surface tension of monolayer

[Justin A. Lemkul]

Sriprajak Krongsuk wrote:
> 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. 
> 
>  

Please define what "previous point" means.

> 
> 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.
> 
>  

Any pressure-related quantity is subsequent to massive fluctuations, which will 
scale with system size.  You usually need fairly long, robust simulations to 
obtain properly converged quantities.  There is some discussion of this fact in 
relation to surface tension in the list archive.

-Justin

> 
> 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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Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
MILES-IGERT Trainee
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

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