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

[Christopher Neale]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://maillist.sys.kth.se/pipermail/gromacs.org_gmx-users/attachments/20120601/8dd5d503/attachment.html>