Several composite methods, namely, B97-3c, HF-3c, PBEh-3c, 31 r 2 SCAN-3c, 32 and ωB97X-3c were used to compute the single point energies of the seven known conformers of the pyrene homodimer 22 in the gas phase and in toluene solution; these previous geometries were determined at the ωB97X-D4 39 /def2-TZVP level of theory. The best performing method was B97-3c, see Section 3.1. To gauge the differences in optimal geometry and energy between B97-3c and ωB97X-D4, the seven conformers were reoptimized and Gibbs free energies of dimerization were determined using B97-3c. Finally, single point energies were determined using ωB97X-D4/def2-TZVP at the B97-3c geometries. The results showed that while the differences in geometry between conformers optimized using B97-3c and those using ωB97X-D4/def2-SVP were small (average all-atom RMSD was 0.02 Å in the gas phase, and 0.09 Å in toluene), the energy differences between the conformers, as evaluated using ωB97X-V/def2-TZVP single point energies were large enough (averaging 5 kJ/mol) to warrant further reoptimization using ωB97X-D4/def2-SVP, rather than using the B97-3c geometries directly. See Section 3.1 for further discussion.