“…The CDs are computationally tractable [9,16,23,61,73,92,107,108,154,156,190,191,202], and thus can be used for "hard" benchmark sets to test force fields (Section II A 1 c). One complicating feature for calculations, relative to CB7, is that the two entryways to a CD are not equivalent, and asymmetric guest molecules may prefer to bind "head-in" or "head-out".…”