The MD simulation intuitively illustrates the rotational characteristic of C 18 rings in OPP ("Rotator characteristics animation" in Supplemental Material). To facilitate visual inspection, the thermal noise of atomic instant motions in the animation has been largely suppressed by taking the average coordinates of adjacent snapshots. It is evident that the C 18 rings rotate smoothly in their respective cavity, indicating that 2C 18 @OPP does have the expected dual-rotator feature. Statistical signed angular velocity of the two C 18 rings displayed, however, that the rotation is not always unidirectional, nor is it completely frictionless (Figure ). Due to the frequent exchange of atomic kinetic energy between C 18 and OPP, the rotational speed of C 18 alternates and the rotational direction sometimes changes. Since the two C 18 in 2C 18 @OPP are sufficiently separated and thus there is little interaction between them, no correlation is detected between their rotational behaviors.