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

[Klniu]

Dear Chris:

Thank you for your help. Maybe I can try it as this. First set x and y
fixed by setting compressibility = 0 4.5e-0.5 until the system
reaching equilibrium and then release. If it sounds reasonable. I am
trying to do it.

Dear Hugh:

I can't answer your question, but I can address the error message that you see.

Basically, your semi-isotropic pressure coupling allows the surface
area of the water-decane

interface to get smaller as z increases and x and y decrease. In a
system with only water

and decane, this will continue until you have a single file of decane
aligned in z.

To avoid this, you need to use semi-isotropic pressure coupling but
set the compressibility

to zero in the z dimension (or, alternatively set the compressibility
to zero in both x and y).

This will get rid of the error message, as I see that you have already noted.

So I guess that all I can tell you is that your first option is not
going to be possible and

that your second option is giving you the wrong value because your
surfactants are fighting the

elongation of your box along z, but the effect persists.

Chris.

-- original message --

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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