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.asm...@gmail.com
<mailto:aldi.asm...@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
<sp...@xray.bmc.uu.se <mailto:sp...@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.
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