How should I deal with Rigid Body in Gromacs ? For me ,I want to do a MD by Gromacs and I want to treat some residues of a protein as rigid bodies . But I want these rigid bodies would move as a group when MD is working. Should I use index file to define some groups , and what to do next with these groups ? And I  do not think I should use the options (freezegrps) , What options in mdp file should I use and how could I use them ? 

Thanks fou your reply !