[gmx-users] surface tension calculation in g_energy

[David van der Spoel]

Bo Zhou wrote:
> 
> 
> Hi all,
> 
>    I use the option “#Surf*SurfTens” in g_engery to calculate the 
> surface tension, but I find that I really do not know how it works after 
> I get some inconsistent results compared with the calculation using the 
> formula “gamma = 0.5*(Pzz - (Pxx+Pyy)/2) * Lz” ( there are two surfaces 
> here and the surfaces in normal to the Z-axis).
> 
>  
> 
> Energy                      Average       RMSD     Fluct.      Drift  
> Tot-Drift
> 
> ------------------------------------------------------------------------------- 
> 
> 
> #Surf*SurfTen                1136.5    2557.05       2557 0.00852598    
> 51.1559
> 
>  
> 
> <======  ###############  ==>
> 
>         <====  A V E R A G E S  ====>
> 
>         <==  ###############  ======>
> 
>    Pressure (bar)
> 
>    -1.93061e+02   -6.88599e-01   -1.66823e-02
> 
>    -6.84564e-01   -1.93953e+02   -2.93803e-02
> 
> 7.79006e-03   -6.97915e-03   -9.87987e+01
> 
>  
> 
> The “#Surf*SurfTens” gives a value 568.25 bar*nm, and “gamma = 0.5*(Pzz 
> - (Pxx+Pyy)/2) * Lz” gives a value 473.54 bar*nm. Please give me some 
> suggestions about the inconsistency. Thanks in advance.

How did you calculate it? Did you divide the average pressures by the 
average box length? Or did you compute gamma for each step and average 
that (this is automatically done in the simulation and stored in the 
energy file). The two ways of computing are not the same.

-- 
David van der Spoel, Ph.D.
Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se