[Chimera-users] how to calculate the core diameter of a trimer (the individual subunits of this trimer is a helix)

Hi, I have a transmembrane domain of a protein (20 amino acid long, single helix). The NMR derived structure comes out as a coiled-coil trimer. So can you please assists me how to calculate the core diameter of this trimer. Regards, Ashok -- thanks & regards, Ashok