Figure shows the potential energy surface of various dimerization pathways of 1,3-cyclohexadiene at 1 atm, calculated at the ωB97XD/def2-TZVP level of theory. Table reports the computed reaction barriers at ωB97XD, multireference perturbation NEVPT2, and coupled-cluster CCSD(T) levels. The barrier of the exo pathway is 1 kcal/mol higher, likely due to the absence of secondary orbital interactions in TS-exo-2. ωB97XD/def2-TZVP calculations also show that the activation enthalpy of the threo [6+4]-ene reaction going through TS-threo-4 is 3 kcal/mol higher than that of the endo [4+2] cycloaddition. However, CCSD(T)/def2-TZVP single-