[gmx-users] Surface tension readings in NVT vs NPT simulations
Denny Frost
dsfrost at cableone.net
Sat Mar 12 16:59:09 CET 2011
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>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>> 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>> 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>
>>
>> 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 http://folding.bmc.uu.se
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