To further characterize the C-C bond formation in the SV, we have reported the ELF profile along the direction connecting C u2 and C u3 for each reactive structure (see insets in Fig. ). This is a good estimation of the evolution of the basin separatrix of the two attractors along the reaction. The basin separatrix is a local minimum in the direction linking two attractors basins that gives useful insights into the relation between them. The high ELF value of 0.58 of the basins separatrix between attr2 and attr3 (structure I), compared to those between attr1 and attr2/attr3 (around 0.30), indicates that those two basins are specially related, emphasizing the specific under-coordinated character of C u1 at the reactant state.