Although our choice of reference is ultimately still subjective, we believe the BEBOP "nonaromatic" resonance stabilization energies in Table are a realistic measure of the specific effects of delocalization in these species. The BEBOP model will accommodate whatever reference one finds appropriate. With the cycloalkanes as a starting point, we have developed a procedure analogous to our treatment of resonance stabilization for a BEBOP analysis of ring strain. We treat the C-C bond order in cyclohexane (0.7336) as a reference for a strain-free cyclic C-C bond, except in three-membered rings. The (1, 3) antibonding interaction between alternant carbons in cyclohexane (-0.0465) must be included in the reference bond order for both adjacent carbons in a 3-membered ring, since these directly bonded carbons are also geminal in a 3-membered ring. We therefore employ 0.6406 as a reference for a strain free C-C bond in a 3-membered ring. The resulting "unstrained" energies in Table give energy changes, ∆E = E strained -E unstrained in reasonable agreement with the energetically derived strain energies for the cycloalkanes. They also provide a plausible estimate of the ring strain in species such as spiropentane and tetrahedrane. The BEBOP model facilitates a detailed analysis of the relative energies of valence isomers (Figure ). The relative energies of eight C 4 H 6 isomers are qualitatively correct, but the range is about 1/3 too large (Table ).