I am modifying several ligand/protein complexes; usually with the “Build Structures” tool. I modify ligands by changing and deleting atoms, or by using “Join Models” after building or importing a structure with “Start structure”. Sometimes, minimizing the ligand works fine, but other times I get the error: “Attribute Error: ‘AtomType’ object has no attribute ‘amber_atom_type’”. I cannot figure out what is causing this error or how to fix it. It seems to happen when I begin working from a saved session or a pdb that was written with the ‘write’ command and not when I go through a whole series of steps in one shot. Any ideas on what is going on? Thanks,