[gmx-users] Surface tension readings in NVT vs NPT simulations
David van der Spoel
spoel at xray.bmc.uu.se
Sat Mar 12 17:24:43 CET 2011
On 2011-03-12 17.17, Denny Frost wrote:
> Thanks for answering that question about dispersion, that makes sense.
> Also, The values I currently get with NPT are around 58 mN/m, while the
> average values I get for NVT are around 16 mN/m, but with a variance of
> nearly 100% of that value. I'm beginning to see why you only do
> pressure coupling in the z direction, but gromacs 4.5.3 won't let you
> specify tau_p = 0. Any other way to do pressure coupling in just the z
> direction?
check manual.
maybe there is only one tau_p value.
> Denny
>
> On Sat, Mar 12, 2011 at 8:59 AM, Denny Frost <dsfrost at cableone.net
> <mailto:dsfrost at cableone.net>> wrote:
>
> Is that using anisotropic pressure coupling?
>
>
> On Sat, Mar 12, 2011 at 8:55 AM, David van der Spoel
> <spoel at xray.bmc.uu.se <mailto:spoel at xray.bmc.uu.se>> wrote:
>
> On 2011-03-12 16.45, Denny Frost wrote:
>
> 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
>
>
> You don't give any values so it is hard to judge.
> - NVT may have completely wrong pressure
> - Dispersion correction assumes a homogeneous system as regards
> the average disperson constant per volume, which you probably do
> not have. E.g. in an ice/water surface dispersion correction may
> induce melting.
>
> The dispersion correction is *not* to bring your system closer
> to experiment but rather to correct for the use of a cut-off.
>
> - Coupling in one direction: specify e.g.
> ref-p = 0 0 1
> compressibility = 0 0 4e-5
> tau_p = 0 0 5
>
>
> On Sat, Mar 12, 2011 at 6:40 AM, aldi asmadi
> <aldi.asmadi at gmail.com <mailto:aldi.asmadi at gmail.com>
> <mailto:aldi.asmadi at gmail.com
> <mailto: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 <mailto:spoel at xray.bmc.uu.se>
> <mailto:spoel at xray.bmc.uu.se <mailto: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 <mailto:spoel at xray.bmc.uu.se>
> <mailto:spoel at xray.bmc.uu.se <mailto:spoel at xray.bmc.uu.se>>
>
> http://folding.bmc.uu.se
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> --
> 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 <mailto:spoel at xray.bmc.uu.se>
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--
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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