Distance restraint on the center of mass distance between 2 monomers in my dimer

GROMACS version: 2021.1
GROMACS modification: Yes/No
Here post your question

I have to run steered dynamics, for which I have to keep position restraint on one monomer and distance restraint on the center of mass distance between 2 monomers of the dimer such that the other monomer doesn’t have any restraint…how do I keep restraint on center of mass distance ?