The graph displays potential surface calculations for a 4-Cu-atom chain. R is the total length of the chain. Each ribbon in the graph represents the potential profile (U) at a particular R with different center bond length rc. It can be seen that as we pull the chain longer and longer, the bond breaks. This is represented by two energy minima along the energy profile at longer Rs. One of those minima corresponds to the breaking of the center bond; the other one corresponds to the breaking of the side bonds.
This graph was submitted by Xi-yong “Sean” Fu, graduate student, Department of Materials Science and Engineering, Ohio State University.