[gmx-users] Pressure at constant volume

[Juliette N.]

On 17 May 2012 05:10, Erik Marklund <erikm at xray.bmc.uu.se> wrote:
>
> 17 maj 2012 kl. 05.00 skrev Juliette N.:
>
> Hello everyone,
>
> I am trying to estimate the thermodynamic expression, dP/dw at
> constant V and T, for my polymer-solvent system. Where P is the
> pressure, w is the mass fraction, V volume and T temperature. I guess
> this task can not be done by MD, as for constant Volume, pressure is
> meaningless. Since what g_energy reports is not the actual pressure
> which corresponds to that fixed Volume. Am I correct?
>
>
> Hi, I lost you there. Why is pressure meaningless at constant volume?

Thanks. Because in NVT pressure is not controlled by thermostat. I
have seen this issue discussed many times on the list archive that in
NVT pressure values has no significance. I have seen this myself in
simulations too, when I fix the pressure by NPT, get the density
corresponding to that P. But when creating a box size of that density
and running NVT, the pressure that NVT produces is far away from what
it should be (Sometimes large negative values).

>
> Best,
>
> Erik
>
>
> (I mean building different binary systems with different mass
> fractions of w, and measuring average P (dP/dw) while keeping T and V
> constant does not produce meaningful results by MD)
>
> Please comment if this makes sense in MD or not.
>
> Thanks,
> J.
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>
> -----------------------------------------------
> Erik Marklund, PhD
> Dept. of Cell and Molecular Biology, Uppsala University.
> Husargatan 3, Box 596,    75124 Uppsala, Sweden
> phone:    +46 18 471 6688        fax: +46 18 511 755
> erikm at xray.bmc.uu.se
> http://www2.icm.uu.se/molbio/elflab/index.html
>
>
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-- 
Thanks,
J. N.