[gmx-users] Problems with surface presure on bilayers and monolayers

[Hector Mtz-Seara]

Dear all,

I've been working the last 6 months on lipid monolayers and bilayers 
trying to calculate the isotherm (surface Pressure versus area per lipid 
molecule) using as solvent SPC waters.
The problem is that with all the systems I've been working on, give me a 
negative surface pressure when the system seems to be stabilized. The 
last runs has been done with dppc of Tieleman's home page using dppc.itp 
and lipid.itp as parameters of the Gromacs force field. I been working 
with constant volume, constant temperature and constant molecules 
assemble. Some of you have recommended me several times to couple better 
a surface tension bath but to be honest It seems that it fluctuates a 
lot and the system never gets stabilized in a area per molecule. My 
systems where at the beginning more or less 1000 lipids per monolayer 
but I've reduced the size for the unusual results that I have been 
obtaining. It can't be a stabilization problem because even with the 
bilayer that Tieleman's gives on his home page (supposed to be 
stabilized in a correct way), run without any change during 20ns the 
surface tension grow up from the good mark of 50 mN/m, more or less, to 
110mN/m as average of the final part of the simulation, for the surface 
tension. That means that even at the beginning we have a positive 
surface Pressure 72-50=22 mN/m more or less ( taking the surface tension 
of water at 300k as 72 mN/m, and as I know in a bilayer you only have to 
divide the number that gromacs give you by one, not as with two 
monolayers systems that you have to do by two), finally it gives you a 
negative surface pressure if you let the dynamics to run far enough.

-Is the Gromacs force field correct to simulate monolayer and bilayers 
if you are interested in the value of the surface tension?
-Has any one tried to make a surface tension or surface pressure 
calculations? If, yes give me the references of the articles and if you 
can the molecule and the force field to see what's is going wrong in the 
way I manage the system?
-Do you think that can be expected a good isotherm representation from a 
molecular dynamic studies as I have told you?
-Is the number that gromacs gives you as surface tension correct?

Any clue should be fine and HELPFULLY!!

Thank you any one in advance for your patient, If you need more 
information because something isn't clear enough just tell me, because 
to don't make this e-mail to long may be not clear enough.

Thanks a lot

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Hector Martinez-Seara Monne

Universidad de Barcelona
Dept. Quimica Fisica

hseara at netscape.net
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