[gmx-users] Surface tension readings in NVT vs NPT simulations

[Denny Frost]

I have run NPT simulations using isotropic and semiisotropic coupling with
the same results.  I have never done coupling in just one direction though,
how do you do this?.  I have never used Dispersion corrections.  It seems to
me that this would help, rather than hurt though since, as Aldi said, it
will make the system closer to experimental values.  I will give this a try
and see what happens.  My question still remains - why do NPT and NVT
simulations give such different values for surface tension?
Denny Frost

On Sat, Mar 12, 2011 at 6:40 AM, aldi asmadi <aldi.asmadi at gmail.com> wrote:

> David,
>
> I have a question that is still related to your reply.  If the bulk
> liquid NPT and the interfacial liquid-vapor NVT simulations are
> performed using dispersion corrections to the pressure and energy,
> while the intefacial liquid-liquid NPAT simulation don't use any
> correction, can we say that all results are valid since we don't give
> the same treatment for all systems?
>
> In the NPT and NVT calculations, we apply corrections in order to
> reduce the discrepancy between the calculated and experimental
> properties (say density and surface tension) as small as possible.
> Here we have more confidence that our molecules in systems behave
> accordingly judging from the macroscopic values we obtain.  Meanwhile,
> in the NPAT calculation, we don't use such correction meaning that the
> property (say interfacial tension) is expected to deviate more from
> the experimental value? This indicates that the system behaves
> differently in comparison to the same simulation conducted with
> correction?
>
> Many thanks,
> Aldi
>
>
> On Sat, Mar 12, 2011 at 11:44 AM, David van der Spoel
> <spoel at xray.bmc.uu.se> wrote:
> > On 2011-03-12 06.09, Denny Frost wrote:
> >>
> >> I am running MD simulations on Liquid/Liquid interfaces and measuring
> >> the interfacial tension between them.  I have found that the readings in
> >> NVT simulations are close to experimental values, but have a lot of
> >> variation.  I run NPT simulations on the exact same system and find the
> >> results show very little variation, but the values are far from
> >> experimental results.  Does anyone know why this happens?
> >>
> > Please be more specific. How do you do NPT simulations? This may
> influence
> > the result. To get good result I would suggest to do pressure coupling
> only
> > in the normal direction and to turn off dispersion corrections to the
> > pressure.
> >
> > --
> > David van der Spoel, Ph.D., Professor of Biology
> > Dept. of Cell & Molec. Biol., Uppsala University.
> > Box 596, 75124 Uppsala, Sweden. Phone:  +46184714205.
> > spoel at xray.bmc.uu.se    http://folding.bmc.uu.se
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