[gmx-users] Minimal box vectors chosen by the pressure integration

[Mirco Wahab]

For testing purpose, I simulated SPC (gromos53a5) at 300K
around it's liquid/gas line at pressures between 0.001,
and 0.2 bar (using verlet integration and PME electro-
statics, r's set to 1nm).

I'm somehow able to drive a 1000-water (mixed phase-)system,
starting from a ~50nm^3 box into a gas phase (0.001 bar,
resulting in a 230nm^3 box. This does work so far. When
increasing the applied pressure (Parrinello-Rahman) into
the liquid zone (eg. 0.2 bar), the system shrinks eventually
until a box size of 7.39492nm is reached. Then, the shrinking
stops, the box contains a water drop + vacuum.

How does gromacs determine the minimal box size
that it  allows the pressure algorithm to induce?
Is this based on the (in this case) PME grid or
on the domain decomposition?

I know that liquid-phase models like SPC are not
parametrized to the dilute gas phase. I'm only
interested in some hints to understand the volume
change on pressure coupling.

Thanks & regards

M.