[Chimera-users] rmsd of selected region: multiple chains

Hi, I want to get rmsd of an interface (constituted by 4 chains in heterotetramer protein) in bound and unbound conformations. so far I am trying to do this by using 'rmsd' command, but i am not sure if its the c-alfa or backbone or the full rmsd. I get a different rmsd value when I just open the sequences, perform structure association and then select sequence to get rmsd. Is there a command to select residues of different chains (the quaternary interface in my case) and perform either backbone or full rmsd??? thanks a lot. best, arvind.