[gmx-users] Reg; Surface tension of liquid-liquid interface

[vinothkumar mohanakrishnan]

Hi all

I had simulated water-ccl4 (ccl4-water-ccl4) interface and measured the
interfacial tension in NVT ensemble and i found a average value to be
97.0615 mN/m (correct me if iam wrong) and divided it by two (because i had
two surfaces) to get the interfacial tension of single liquid-liquid
interface and the value was found to be 48.530 mN/m which is close to the
experimental value of 44.3 mN/m.

Energy                      Average       RMSD     Fluct.      Drift
Tot-Drift
-------------------------------------------------------------------------------
#Surf*SurfTen               970.615    4684.12    4672.71   -3.77379
-1132.15

1. My question is in which ensemble one should measure the interfacial
tension NVT (or) NPT and why?

2. Can one use surface tension coupling option in gromacs to measure the
interfacial tension of liquid-liquid interface.If so, what should be the
parameters in the pressure coupling section. I have gone through the manual
but not able to understand it clearly such as what value one should give for
ref_p and other parameters in the pressure coupling section?


I used g_analyze to estimate the standard error (error estimate) in the
surface tension values by using the sf.xvg file obtained from NVT simulation
and it says fitted parameter is negative and invalid fit below is my error
message and i don't what is going wrong.


                                                 std. dev.    relative
deviation of
                              standard       ---------      cumulants from
those of
set      average       deviation       sqrt(n-1)   a Gaussian distribition
                                                      cum. 3   cum. 4
SS1   3.835787e+02   5.051521e+03   4.124550e+02       0.321    0.713

a fitted parameter is negative
invalid fit:  e.e. -nan  a 0.994955  tau1 1.50734  tau2 -8.62581e+08
Will fix tau2 at the total time: 300
a fitted parameter is negative
invalid fit:  e.e. -nan  a 1.00655  tau1 1.72207  tau2 300
Will use a single exponential fit for set 1
Set   1:  err.est. 529.855  a 1  tau1 1.6613  tau2 0


3. How to use g_analyze to get the standard error correctly?

any help regarding my questions are greatly appreciated.

Regards
Vinothkumar
Research scholar
IIT Madras
Chennai 600036
India
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