Dear Maja, Apologies if you are getting multiple copies of this answer from me. I keep getting error messages that my reply could not be sent to the list! So, here is my third try. If you've been getting my previous messages (even though I don't think the rest of the list is), please ignore and pardon me for the spam! Adding a bond only works when the atoms are in the same model as each other (typically a structure opened from one PDB file is one model). Also simply adding a bond won't move anything. It will just make a long bond between the atoms in their current positions, or if you choose "reasonable" only, it won't add any bond because the bond would be too long to be reasonable. https://rbvi.ucsf.edu/chimerax/docs/user/tools/buildstructure.html#bonds You can see what model(s) you have by looking in the Model Panel: https://rbvi.ucsf.edu/chimerax/docs/user/tools/modelpanel.html If the two atoms are in two different models (typically opened from two different input files), the "join models" Peptide Bond option should work if you really have exactly two atoms selected, the peptide terminal "N" in one model and the peptide terminal "C" in the other model. As far as I can tell these two are selected in your image, so I couldn't say why you get an error message. If the atoms are in the same model as each other, however, it will not work. You would need to split the model so that they are in separate models first. You can try command "info atoms sel" to list in the Log what you have selected. In my "join peptide" test case I get the following, which confirms I have the correct atoms from two different models selected: info atoms sel atom id #1/A:29@C idatm_type Cac atom id #2/A:1@N idatm_type N3+ However, join models is not going to change the conformation of the peptide, it will just move one model (e.g. the entire green one if it has more atoms than the purple one) as a rigid body to form the specified bond with the other model. https://rbvi.ucsf.edu/chimerax/docs/user/tools/buildstructure.html#join I hope this helps, Elaine ----- Elaine C. Meng, Ph.D. UCSF Chimera(X) team Resource for Biocomputing, Visualization, and Informatics Department of Pharmaceutical Chemistry University of California, San Francisco