Alternatively, starting from the optimized transition structures, we performed highprecision scanning calculation on the rotational PES (Figure ). The rotational barrier of C 18 in OPP (about 0.13 kcal/mol) is comparable to that of the supermolecular rotator C 18 @[11]CPP (0.06 kcal/mol at the ωB97X-D/def2-SVP level). Furthermore, we also found that the rotational energy profile of C 18 circulates in a cycle of 40.0°, which coincides well with its D 9h -symmetric structure of alternating long and short bonds and is similar to the situation of C 18 @[11]CPP. The ultralow barrier of rotation is thought to be closely related to the high symmetry of C 18 ring and is also a typical feature of a system with π-π stacking. Considering that the angular velocity of molecular rotator depends decisively on its rotational barrier, such a small barrier may imply ultrafast nanorotator characteristics for complexes C 18 @OPP and 2C 18 @OPP.