The ground-state geometry of C 18 Br 6 at the ωB97XD/def2-TZVP level is displayed in Figure , and the corresponding Cartesian coordinates are given in Table . The C 18 B r6 optimized in the gas phase is observed to be a strictly planar structure with D 3h symmetry. It is worth mentioning that the single crystal structure of C 18 Br 6 has been grown in CS 2 solution. X-ray analysis showed that C 18 Br 6 molecule has only a lower C 2 symmetry in the stacking environment, which is different from its gas-phase molecular geometry. The calculated energy difference between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO), namely the HOMO-LUMO gap, is 6.50 eV, which is slightly smaller than that of C 18 (6.75 eV) calculated at the same level. The energies of σ-MOs are much lower than those of the π-MOs, while the energy levels of the π in -and π out -MOs are comparable. The height of spikes in the DOS diagram indicates that quite a lot of MOs are doubly degenerate, which is due to the high structural symmetry of the C 18 Br 6 .