Revisiting the C60ISO and iso-C60 datasets of relative energies for C60 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. Due to 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 inter-electronic distances just below 2 bohr on the description of intermediate-range correlation effects. While this does not signal multi-reference-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 to replace the older iso-C60 and C60ISO sets.