Hi Tim, I added a descriptive subject line, which is helpful for people searching the archive. The colon means residue name or number. So I'm guessing your Ca ion has residue name CA. You can also use element symbols, but they would not be in a comma-separated list of residue names or numbers. Maybe this would work (all element symbols): size Ca | Mg | Mn atomRadius +0.5 ....using the vertical bar as an OR (union) operator, as per combinations: <https://rbvi.ucsf.edu/chimerax/docs/user/commands/atomspec.html#combinations> That page also covers hierarchical specifications (model number, chain ID, residue names/numbers, which can be given in comma-separated lists) and other specifications (element, functional group, etc. which would not be in comma-separated lists), zones, ... <https://rbvi.ucsf.edu/chimerax/docs/user/commands/atomspec.html> 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

On Feb 9, 2026, at 10:17 AM, Timothy Springer via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:

This works for Ca and Mg but not Mn

size :CA, Mg,Mn atomRadius +0.5 - Tim Timothy A. Springer, Ph.D. Latham Family Professor, Harvard Medical School and Boston Children's Hospital Founder, Institute for Protein Innovation