Electron correlation effects in isomers of C20

Abstract

The C20 molecule exists as three low-energy isomers: the monocyclic ring, a corannulene-like bowl structure, and the cage-the smallest possible fullerene. The curious structures of these isomers, along with the valuable properties and possible applications of fullerenes more generally, mean that C20 has attracted interest both experimentally and computationally. Unfortunately, previous theoretical studies have found these C20 isomers present a significant computational challenge, and widely used methods such as Density Functional Theory with different functionals have been unable to agree on even the relative ordering of the isomer energies. Even accurate high-level methods such as coupled-cluster with singles, doubles, and perturbative triples (CCSD(T)) and Diffusion Monte Carlo (DMC) have previously disagreed on the energetic ordering of these isomers. Here we re-examine the ring, bowl, and cage isomers of C20 using a more accurate DMC technique. We employ a novel method to go beyond the single-determinant DMC approaches previously used, and instead use more accurate multi-determinant trial wavefunctions. Our results show that the fullerene cage exhibits different electron correlation effects than the ring and bowl, which when taken into account leads to a reordering of their relative energies. This finally establishes agreement between DMC and relatively recent complete-basis CCSD(T) results, thereby resolving a long-standing disparity between these two high-level descriptions of the C20 isomers. The approach we use is generalisable, and could be used to provide insight into even larger systems in future.