DFT-computed free-energy diagram for the reaction between a pair of molecules of (a) C₆H or (b) C₆H₂. The most energy-favorable reaction mechanism is highlighted in black color, while the second favorable one is in red. All energy values are the Gibbs free energies at 15 K in units of kcal/mol. Details of the optimized structures and their atomic coordinates are provided in supplemental materials at [doi:10.5281/zenodo.3963376].