Problems with freezegrps

GROMACS version: 2016.4

Hi all,

I am trying to simulate a surface to study interactions between it and other molecules sitting on top. In my minimisation .mdp file I have

freezegrps = G
freezedim = Y Y Y

which I’m using to freeze the surface in its position. When I complete the minimisation step, half of the surface remains with its initial coordinates but the other half starts fluctuating, with some neighbouring molecules moving much further than their original positions. This has confused me since I thought freezegrps would stop any movement like this from occurring. Am I misunderstanding what this does or could there be some conflicting problem causing this movement?

Thanks in advance