Hello: I was unable to put to work define centroid radius 0.5 color red :ligand_residue_name or define centroid radius 0.5 color red #0&@serialNumber=1972 with a homodimeric protein (I am at CHARMM NAMD/XPLOR, so that the two subunits are only distinguished by either the atom sequential number or the segment name, the latter not being understood by chimera, as far as I know). In the first case, running the movie after the appropriate further steps, the centroid sequence obtained was for the centroid between the two protein ligands, as it was expected. In the second case (as I wanted to refer to the ligand in subunit A only), the command raised the error: AttributeError: 'NoneType' object has no attribute 'openState' File "/opt/UCSF/Chimera64-1.7/share/StructMeasure/Geometry.py", line 293, in _getRefInfo invXForm = lowest.openState.xform Please notice that the line command #0&@serialNumber=1972 correctly selects the ligand on subunit A (a diatomic ligand, where "1972" is the sequential number of one of the two atoms of the ligand. Here "#0" may be redundant, as it deals of a single model. However, using "@serialNumber=1972" raised the same error. I fear I am missing some obvious facet of define centroid, so that I thank you for your patience. francesco pietra