# [gmx-users] Question about phospholipid monolayer on a water/air interface...

paloureiro at biof.ufrj.br paloureiro at biof.ufrj.br
Wed Aug 2 15:27:00 CEST 2006

```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
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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