Hi JD, There is no built-in center of mass method in Chimera. If you've selected the atoms beforehand, then the below Python code will calculate the center of mass: from chimera import selection, Vector sum = 0 center = Vector(0, 0, 0) for a in selection.currentAtoms(): sum += a.element.mass center += a.element.mass * Vector(*a.coord().data()) print "center of mass:", center / sum I've also attached it as a file that you can just use File...Open (or "open") to run. --Eric On Nov 13, 2007, at 7:18 AM, Jean-Didier Maréchal wrote:

Hi everyone,

I have another question: what would be the easiest way to calculate the center of mass of a given set of atoms? In particular, is there a python module that already has this function implemented? I am interested in particular to find the center of mass of a ligand bound to a protein.

Thanks a lot, JD

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