Abstract: Aprocedure for separating out of π, π-like and σ fragment molecular orbitals(FMO) was established, and it provides for cyclopentadiene a complete FMObasis set in which all FMOs have correct electron occupancies. Based on our new method, the Morokuma energy partition provides, in the framework of ab initio SCF-MO computation at the STO-3G level, the various π and σ-energies associated with the inter-and intrafragment interactions. The π-electronic delocalization is destabilization in cyclopentadiene, and its delocalized π-system is also destabilized. In the case of benzene, its QMREis stabilizing. However, it is the σ-framework of benzene, rather than the π-system itself, to be strongly stabilized, revealing the benzene is σ aromatic. It should be emphasized that the σ-π space interactions, expressed in terms of the Coulomb J_σπ and exchange K_sp integrals, have significant effects on the molecular behaviour.

Key words: Conjugation effect, Morokuma energy partition, Orbital interaction, Fragment molecular orbitals