# [gmx-users] lipid monolayer isotherm

Andre Farias de Moura andre at qt.dq.ufscar.br
Mon Jan 17 16:07:38 CET 2005

```On Mon, 17 Jan 2005, Hector Mtz-Seara wrote:

> I've been running several test dynamics and I'm really confused. In
> general the surface tensions that gromacs gives me are much higher than
> experimental ones, many times more than 3 times (I know that to have in
> mN/m units I have to divide by 10 and by two for the two monolayers) and
> it means that they are higher than experimental water, 72mNm( someone
> knows the surface tension of the SPC water that is in gromacs??), but
> what is really annoying me is that in theory the surface tension of a
> fatty acid monolayer goes down when the concentration of the lipid in
> the monolayer goes up but the results seem to be the opposite, and I say
> "seems" because sometimes the same dynamic brings me to different
> equilibrium surface tension or area/lipid ( it happens in both ensembles
> I use TNAhz and TN(st)hz). If anyone can give me any clue why I been
> having all this problems and if there's any way to solve them, I'll be
> so pleased.

g_energy output gives surface pressure, not surface tension. dividing the
result by 20 just changes the units from bar nm to mN/m. to compute the
surface tension you have to use the standard expression:

gama = gama_0 - pi

where:

gama = surface tension

gama_0 = pure water surface tension

pi = surface pressure

anyway, if you're getting surface pressures 3-fold larger than that
of pure water, then your surface tension is negative, isn't it? if
that happens to be the case, then your systems may be still away
from mechanical equilibrium.

maybe you could give some more details of your simulations: how did
you assembled each monolayer? how long was each simulation run? if
you plot surface area along the simulation time, is it stable?

best regards,

andre'

/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/

Dr. Andre' Farias de Moura

/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/

Laboratorio de Quimica Teorica
Universidade Federal de Sao Carlos
Sao Carlos - SP - Brasil

/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/

Endereco Atual

Grupo de Quimica Quantica
Instituto de Quimica de Sao Carlos
Universidade de Sao Paulo
Sao Carlos - SP - Brasil
Telefone: 55-16-3373-8055

/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/

>
> thank you in avance, Hector.
>
> --------------------------------------------------------------------------------------------
>
> Hector Martinez-Seara Monne
>
> Universidad de Barcelona
> Dept. Quimica Fisica
>
> hseara at netscape.net
> --------------------------------------------------------------------------------------------
>
>
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```

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