[gmx-users] pressure coupling for lipid bilayer

[Erik Lindahl]

David L. Bostick wrote:

>I use anisotropic with
>
>compressibility         = 4.5e-5 4.5e-5 4.5e-5 0.0 0.0 0.0
>
>It seems to work quite well.  Since the 0.0 compressibilities of the
>off-diagonal elements keep the box from deforming.
>
That's the way I do it, but it should definitely be combined with 
anisotropic or semiisotropic scaling. In the
liquid crystalline phase it should be perfectly OK to allow cell 
deformations too (they won't be large).

>>Hi!
>>
>>There has been some discussions about proper pressure coupling for lipid 
>>bilayer. My experience is that the anisotropic or semiisotropic coupling 
>>with compressibility of 6e-5 or 4.5e-5 along the membrane normal easily 
>>deforms the bilayer in < 20ps and eventually the bilayer is flattened!! 
>>On the other hand, with isotropic or without any P-coupling, the system 
>>is running quite well.
>>
But what kind of surface tension do you get then? The isotropic coupling 
will per definition constrain
your system so the membrane cannot thin out or get thicker, but that is 
usually not the normal
conditions a biological membrane exists under.

>>
>>
>>Surface tension coupling method is suggested to be a nice approach (J. 
>>Chem. Phys. 111(3):1281), but the parameters are not trivial. Without 
>>experimental data on hand, it is difficult to estimate surface tension 
>>(in my case, POPC bilayer). In addition, the incorporation of other 
>>molecules may alter the surface tension again!
>>
>>How should I setup up such a system properly???
>>
Surface tension coupling won't help you; the only reason to do that is 
basically to get a non-zero surface tension.
The equilibrium state of biological membranes is the one with lowest 
free energy, and unless the cell is pressurized
or something this corresponds to zero surface tension.

I would guess that your topology, parameters or simulation setup has an 
error somewhere!

Cheers,

Erik