The intermolecular interaction energy (E int ) is a decisive factor to measure the binding stability between the units of a complex. This physical quantity between C 18 and OPP in C 18 @OPP is expressed as E int = E(C 18 @OPP) -[E(C 18 ) + E(OPP)], where E(X) is the electronic energy evaluated at optimized geometry of molecule X. The E int estimated at the very accurate ωB97M-V /def2-QZVPP level is -20.3 kcal/mol, which is a significant negative value, implying that the complexation is strong. The result is very close to that between C 18 and [11]CPP (E int = -20.9 kcal/mol) reported in ref. [35], indicating that the binding capacity of OPP to C 18 is comparable to that of [11]CPP.