The independent gradient model based on Hirshfeld partition (IGMH) is a very intuitive method proposed by one of us to graphically depict the main interaction regions between specific fragments of compounds by using the isosurface of δg inter function. The two C 18 rings and the OPP unit in 2C 18 @OPP are respectively defined as fragments for calculation. The fairly extended δg inter isosurface in green color between C 18 and OPP indicates that the interaction between host and guest molecules covers a very wide spatial region (Figure ) and the electron density on it is very small of about 0.005 a.u. Since these observations are general characteristics of π-π stacking, and both C 18 and OPP have extensively delocalized π electrons , the interaction between C 18 and OPP can be unambiguously considered to be a typical π-π stacking. Especially noteworthy is that the C 18 ring utilizes its in-plane π electrons to laterally form a π-π stacking with the π system of OPP in its equatorial plane, which is a very unique feature of the sp-hybridized cyclocarbons.