[chimerax-users] Selection of the organic ligand

Dear ChimeraX users! Normally I use the selection name "ligand" to select all atoms of the organic ligand. For example: define centroid ligand name lig_com

sometimes depending on the pdb file, this command also selects one residue (located in the pdb file just near the ligand), considering it also as a part of the ligand.. May I use another selection to explicitly exclude all 20 natural amino-acids from the ligand selection? Many thanks in advance! Enrico