hello: "centroid" works fine to me in getting the trace of a moving molecule in and out of a protein, using command define centroid radius 0.2 color red :residuename when the residue is unique. In fact, the method has produced extraordinarily illustrative (I am using the words of the referee) in a paper that will appear shortly on Chemistry & Biodiversity. Now I have three diatomic molecules, only one of which is made moving around. In this case define centroid radius 0.2 color red :residuename:residuenumber for the moving residue does not work. What happens is that "red balls" for the specified residue are being accumulated at the place of origin of the residue, while the residue is leaving the protein as a short bar, not a ball. Could that be fixed in the above command? thanks francesco pietra