Perfect size matching and proper vdW interactions between C 18 and OPP induce relative rotation between them without separation, making C 18 @OPP and 2C 18 @OPP a novel ring-in-ring nanorotator. Although we have explicitly optimized the transition states of the rotational reaction of the complexes (Figure ), the intrinsic reaction coordinate (IRC) analysis of the reaction path was ultimately not realized due to the unusually flat PES (the relative energy of the transition states is calculated to be 0.66 and 0.70 kcal/mol, respectively, for C 18 @OPP and 2C 18 @OPP).