Hi! Since you are getting lists of atoms back, you only need to do some attribute access (rather than method calling)! Every chimera.Atom object has an attribute called molecule: a reference to its parent chimera.Molecule. chimera.Molecule objects have an attribute called 'name', but sometimes this is not very informative (depends on your input), so you need to resort to the pdb filename, stored in the first element of the openedAs tuple. In code, this looks like this: for atoms1, atoms2, rmsd, fullRmsd in match(CP_BEST, [ref, sims], defaults[MATRIX], "nw", defaults[GAP_OPEN], defaults[GAP_EXTEND]): molecule1 = atoms1[0].molecule # any atom from the list will do molecule2 = atoms2[0].molecule print molecule1.name, "\t", molecule2.name, "\t", rmsd # molecule1.openedAs[0] will also work here I don't know if atoms1 comes consistently from the reference molecule, but I'd guess it does. Hope it helps!