[gmx-users] Surface tension II

[paloureiro]

On Thursday, February 07, 2002 at 08:29:42 AM, David van der Spoel wrote:

> On Wed, 6 Feb 2002, paloureiro wrote:
> 
> >is it possible to use the surface tension option with the Parrinello algorithm (the manual says it is not) ?
> >I have been using the surface-tension with the Berendsen p couple in a DPPC bilayer, but my system behaves in a strange way:
> >the use of -200 bar.nm or +200 bar.nm leads to a contraction in the XY plane (I would expect -200 bar.nm to contract and +200 bar.nm to expand the xy area).
> >I have used either cut-off and PME electrostatics. I have set the compressibility to 9.8e-5 bar-1 in  xy and z.
> >
> >Any suggestion will be very helpful.
> I don't know whether something could be wrong with the algorithm, but I
> suggest you check the initial conditions: run without P coupling and
> measure the pressure (average over > 100 ps). Chances are that your
> pressure is very negative already, so the system should contract. Also,
> since your compressibility is low you will have only small differences in
> area with -200 and +200 bar. Measure (using g_energy) the area and surface
> tension after quilibration and then compare again.
> 
> Groeten, David.
> _________________________________________________________________

Dear David,

once more, thank you for your suggestions!
1)my system bears a pressure of 400 atm in the XY plane (the membrane plane) in a constant volume simulation (as you suggested) - area per lipid 62.2 A**2.
2)in another simulation (area per lipid 60 A**2) the pressure fluctuates around - 20 atm in the membrane plane.
(The smaller the area the greater the intermolecular interactions between the headgroups - the greater the virial the smaller the pressure.)
3)if I use the semiisotropic option in the .mdp file and set the XY-plane pressure to -100 atm (and Z pressure to 1 atm), there is an expansion of the membrane (increase in the area per lipid).
4)with a value of +100 atm, there is a decrease in the area.
5)when I use the surface-tension option (with a value of 200 bar.nm - which is equivalent to a mean lateral pressure of -100 atm) the area decreases!

Cheers,

Pedro.

Pedro Loureiro
e-mail: paloureiro at biof.ufrj.br


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