Effect of inclusion of electron correlation in MM3 studies of cyclic conjugated compounds

Electron correlation at the Møller‚Plesset second-order level was incorporated into the „-system portion of MM3 calculations for several … † … † … † conformers of 10 annulene, 18 annulene, bicyclo 5.3.1 undecapentaene, and … † bicyclo 4.4.1 undecapentaene. The conformers with ‘‘localized’’ C—C „ bonds Ž . strongly alternating bond lengths were found to be of lower energy than their Ž . counterparts with ‘‘delocalized’’ C—C „ bonds similar bond lengths before correlation energy was included. Correlation always lowered the energies of the delocalized conformation more than it did that of the localized conformation, such that often the latter was found to be more stable after correlation energy was included in the calculation. When a delocalized structure was not at a stationary point on the MM3 energy surface, such comparison could not be made. An example is the porphin molecule. Š 1998 John Wiley & Sons, Inc. J Comput Chem 19: 475‚487, 1998