[gmx-users] How to restrain/constrain the center of mass of a protein?

[zcwang]

Hello
I'm currently running a SMD,
and according to the standard strategy by Grubmuller H, 
I should restrain/contrain the center of mass of the protein A while I
pull away the ligand or another protein B binding to it, so that the
average position of A is kept in place in the process, while rotation
and internal movement of A is still allowed.
I wonder if it's possible to do this in gromacs.
Anyway, the pricinple and algorithm is extremely simple, but I just
can't find a detailed description of implementing it in gromacs.
Thanks in advance.