Fig. 4. (a) A schematic diagram of the C18 molecule. θ is the angle between the two interleaving C9 nonagons, one of which is indicated with orange dashed lines. R is the radius of the regular nonagons. d1 and d2 are the two sets of C–C bond lengths, respectively. (b) and (c) Comparison of the energies obtained with UHF, B3LYP, CCSD and DMET-ESVQE for the potential energy curve of the C18 molecule within (b) STO-3G and (c) cc-pVDZ basis sets. The relative energy Erel is defined as E − Ecumu where Ecumu is the energy for the θ = 20° cumulenic structure. The DMET-ESVQE results suggest that the bond-length alternating structure is favoured, which agrees with experimental observation.
