Revisiting the C60ISO and iso-C60 Data Sets of Relative Energies for C ₆₀ Isomers

We revisit reference data for the widely used relative energies of different isomers of C₆₀, previously used in the iso-C60 test set and in the C60ISO subset of the GMTKN55 suite. Using a carefully constructed composite approach, we provide improved isomerization energies of approximate CCSD(T)-cf/CBS quality. Because of the observed changes of up to more than 10 kcal/mol compared to the previous C60ISO data, we re-evaluated many density functional approximations. While double hybrids improve, long-range-corrected hybrid functionals deteriorate even further. This can be traced back to detrimental effects of large exact-exchange admixture at interelectronic distances just below 2 bohr on the description of intermediate-range correlation effects. While this does not signal multireference-type static-correlation effects, it can be addressed efficiently by modern strong-correlation-corrected range-separated local hybrid functionals. We collected our final combined isomerization energies in the C60iso-17 data set and suggest it should replace the older iso-C60 and C60ISO sets.