[gmx-users] What is the right way to get reasonable interfacial tension of surfactant at oil/water interface

[Klniu]

Dear Gromacs users,

I have do simulation of surfactant at oil/water interface for a long time.
But I find it too difficult to get a reasonable interfacial tension. The
key question is the pcoupltype.

My system:

Box size:  3.0 * 3.0 * 13
> Molecule Number: decane 230, water 2041
> pcoupltype = semiisotropic

Gromacs version: 4.5.5


1. When I set compressibility = 4.5e-5 4.5e-5.  Surfactant number is 32
. The box will be shrink at x and y. At last simualion will end for a fatal
error:

One of the box vectors has become shorter than twice the cut-off length or
box_yy-|box_zy| or box_zz has become smaller than the cut-off.
For more information and tips for troubleshooting, please check the GROMACS.

2. When I add the number of surfactants to 40. compressibility = 4.5e-5
4.5e-5. The interfacial tension is -20bar/nm for two interfaces.  It is not
reasonable because it should be positive.

3. When I compressibility = 0 4.5e-5.  Surfactant number is 32. The
interfacial tension is 794bar/nm for two interfaces.


My question is:

What is the right way to simulate for this system?  My work confused me too
much.  Very appreciate for you help.

Hugh
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