[gmx-users] pressure / compressibility

[m b]

der GMX-users,

I am somewhat confused about one issue in
NPT simulations and I'd very much appreciate
if anybody could enlighten me.

I understand that when doing a constant
pressure simulation one has to give not only
the pressure but also the compressiblity
as input parameter (at least in some cases,
in which ??)

I would have thought that the compressiblity
is a material property and hence ought to be
calculated from the output of a simulation
rather than given in the input ?
(doesn't giving both compr. and P as input
overdetermine the ensemble in some way ?)

In some parts of the docu it is suggested to
use 4.5e-05 as the compressibility when simulating
aqueous systems. 
What if I simulate other materials, e.g., a 
crystalline system ?
For many substances accurate experimental
values for the whole compressibility matrix
are not known.

Even if I know these values I still cannot be
sure if my simulation would be consistent
because I would expect the actual compressibility
of the system to depend VERY sensitively on
the forcefield used, and then the input
compressibility
and the compressibility corresponding to the
simulated system, geometry and forcefield might
differ considerably, possibly resulting in
all sorts of artefacts ??

grateful for any comments!

cheers
Michael


__________________________
Michael Brunsteiner
RDMCh, Agfa-Gevaert
Mortsel
Belgium





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