[gmx-users] monolayer and ensembles

[Andre Farias de Moura]

On Fri, 10 Sep 2004, Ramon Reigada wrote:

> Dear users of GROMACS,
>
>    two days ago I sent a question related to the appropriate procedure
> to simulate lipid monolayers, keeping both the normal pressure and the
> surface x,y area constant (instead of the constant normal pressure and
> constant surface tension algorithm that is implemented in GROMACS).
> Andre Farias de Moura kindly answer my question, but we still have some
> doubts about constructing the monolayer.
>
> The procedure I am thinking to built the initial configuration of the
> monolayer is the following. We place the lipids above a water slab, and
> below the water slab and above the lipid array, some vacuum is left.
> Then, we want to keep a constant normal pressure (so, z-box will vary)
> and a constant surface (x,y)-area (so, surface tension will vary).
> Andre (and other gromacs users), is this the correct way to built a
> langmuir monolayer? is my proposed ensemble NPAT equivalent to your
> proposed NVT ensemble? these are my two questions.


ensembles are different. in principle, if you set the area per
molecule equal to the equilibrium value of your system then the NVT
ensemble should give nearly the same results as a simulation using
surface tension coupling. but be sure to set compressibility to zero
in z direction to keep it constant.

I've already tried surface-tension coupling to simulate langmuir films but
the area took too long to relax. in fact, it didn't equilibrate until 1
ns, so I began to use NVT ensemble instead and it works just fine.

best regards,

andre'

>
>    thank you very much,
>
>      RAMON REIGADA
>
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