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

[Denny Frost]

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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