In 2008, Ess et al. calculated the TS structures and reaction pathways for the dimerization of 1,3-cyclohexadiene in the gas-phase. Calculations at B3LYP, CASPT2 and CBS-QB3 levels show that the computed reaction barriers for the concerted [4+2]cycloadditions, concerted [6+4]-ene reactions, and the stepwise additions (Figure ) are within 5 kcal/mol. This small difference is consistent with the difference in experimental 𝐸 ! , confirming the competitive nature of the concerted and stepwise mechanisms.