Hi Sergio,
Hi
My name is Sergio Garay, and I'm working on a simulation of a phospholipid
monolayer on a water/air interface. To mimic that system I have constructed a
rectangular box with a thick layer of water at the center, two monolayer of
lipids (with their polar heads pointing toward water) at each side, an
finally two thick vacuum layer a each side (I think, the air component could
not be considered here due to the size of the simulation system)
I would like to make a constant Area calculation, to obtain the surface
tension of the system.
I think, in the Langmuir experiment, we are measuring the film
pressure (the difference [surface tension of the pure solvent] -
[surface tension of the film/solvent system].
My questions is:
What is the more adecuate condition to do this?
Although the normal pressure would be 1 atm at the experimental condition,
this pressure is not compatible with the vacuum in the simulation box unless
the compressibility in the Z direction would be fixed at value 0.0 to keep
the Z box dimension fixed.
1) Is it convenient to simulate at fixed volume (NVT)
Yes, because as long as there is a vacuum "layer", the system will be
free to adjust its Z-dimension.
You can select the "semi-isotropic" option in the [].mdp and set the
compressibilities to zero (XY and Z).
Is there a more appropiate alternative?
There is another alternative: to simulate just one leaflet of lipids
plus water and add a restraining potential (a "wall") to prevent water
"evaporation".
After your simulation is done, I think you must not use the "surface
tension" option in g_energy, in order to calculate it. Remember the Z
dimension of your simulation box is greater than what is effectively
occupied by the molecules... How one would solve this is open to
discussion...
Regards,
Pedro.
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