[gmx-users] dissolution of gases in liquid

[Vitaly Chaban]

Hi all,

We are trying to calculate the solubility of argon in water using the
partial densities function. The following traditional MD setup is
used. The water (=4000) is in the center of the box and is surrounded
by the argon atoms (=50) along Z direction. The electrostatics is
treated with PME, the LJ - with a shifted force method, the cut-offs
are 0.9nm. The Parrinello-Rahman barostat is applied in Z direction
only. Water and gas are coupled separately to 298K. The FFs are OK
both for water and argon separately.

In the NPT, a rather strange result is obtained  (please see a plot) -
http://yfrog.com/3mdissolj
The argon atoms and the water molecules are distributed uniformly in
the box and this results in the unphysically big solubility of argon.
We tried also NVT (please see the above plot,
http://yfrog.com/3mdissolj) and got a better situation. This one is
rather physical if one considers the pressure which occurs in such
setup.

The question is WHY NPT gives so weird results? I would like to get
the dependence of the solubility vs pressure, thus I am interested in
NPT mostly.

Does anybody have any insights?
Vitaly


Dr. Vitaly Chaban