Hi Tristan and chimerax-users, Some time ago I asked the list about modeling PTM and got several helpful answer, including this one: https://mail.cgl.ucsf.edu/mailman/archives/list/chimerax-users@cgl.ucsf.edu/... Now, I am working on a structure containing chlorophyll A molecules (residue code CLA) and realized that it won’t work for MDFF in ISOLDE because there is no MD template for it. After trying to manually fit one of these molecules rigidly, I concluded that I would rather spend the rest of the month learning how to make an MD template for it (than next month painstakingly fitting every single molecule manually rigidly). From the message linked above, I think this should be doable because this time it is not a covalent modification on an otherwise parameterized residue, but simply a new ligand (ok, maybe the coordinated Mg2+ makes it a bit more complicated?). I looked up ANTECHAMBER, and also some documentation from OpenMM, but that got me nowhere, mostly because I have little clue what the task really entails. My guess is that I need some program to take the CLA residue in PDB or mmCIF format (which I can easily save from a previous PDB entry) and write out an OpenMM XML file like the one you sent me back then for KCX. Is this correct? The ANTECHAMBER documentation mentions many programs and formats, and I got totally confused. If you have time to elaborate a bit on the specific practical steps to take (including how to supply the final XML file to ISOLDE), that would be super helpful! I can do with pointers to documentation, not afraid to read, but I need help getting oriented. Thank you in advance! Guillaume När du har kontakt med oss på Uppsala universitet med e-post så innebär det att vi behandlar dina personuppgifter. För att läsa mer om hur vi gör det kan du läsa här: http://www.uu.se/om-uu/dataskydd-personuppgifter/ E-mailing Uppsala University means that we will process your personal data. For more information on how this is performed, please read here: http://www.uu.se/en/about-uu/data-protection-policy