What Are the Ground State Structures of C20 and C24? An Explicitly Correlated Ab Initio Approach

A new benchmark study has been performed for six isomers of C20 and four isomers of C24 using explicitly correlated methods, together with coupled cluster theory with large basis sets and DFT with advanced functionals. The relative energy trends obtained are extremely sensitive to the methods used. Combining our best CCSD(T)-MP2 difference with our best MP2 basis set limit, the dehydrocorannulene bowl is found to be the most stable for C20, followed by the cage at about 8 kcal/mol, and the ring at about 46 kcal/mol. For C24, the D3d cage is found to be the most stable isomer, followed at only a few kilocalories per mole by dehydrocoronene, and at larger separations by then octahedral cage and the ring, respectively. This makes C24 the smallest classical fullerene. The estimated residual basis set error of the estimated CCSD(T) basis set limit is conservatively expected to be ±1 kcal/mol. In general, DFT exhibits large errors for relative energies with RMSD values in the 8-34 kcal/mol range. However, among the DFT functionals, the DSD-PBEP86-D3BJ double hybrid comes close to our best ab initio results, while the ωB97X-V range-separated hybrid is in semiquantitative agreement.