[gmx-users] freezegroups and pressure

[Berk Hess]

>Hi,
>
>I'm having a strange problem (both in 3.05 and 3.1) with freezegroups. With 
>freezegroups turned on (eg on a group of water molecules) I get strange 
>(high) pressures. The problem arises both with and without pressure 
>coupling, the only difference is that the pressure rises faster with
   pressure coupling on. Pressures are OK if I release the >freezegroup,
>both with constant volume and pressure. Pressures are also OK if I exclude 
>all non-bonded interactions with the freezegroup (using energygrp_excl), 
>but I'm actually interested in the interactions with the freezegroups, so I 
>don't want to use energygrp_excl.

Using pressure coupling is tricky and will result in high pressures.
A few years ago I have thought a lot about how to implement pressure
coupling with freeze groups. One can choose to scale or not to scale
the frozen particles. In Gromacs they are not scaled, but in many
cases it might be better to do so.

But if you don't have pressure coupling the pressures should be ok.
You should energy exclude the freeze group with itself,
this also saves cpu time.

What is the size of your freeze group compared to the rest of the
system?

Berk.



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