Use of the simple perturbation molecular orbital (PMO) method allows the first-order estimation of energies of formation (I$ of polycyclic aromatic hydrocarbons (PAH) as well as their localization (EL) and bislocalization (E,,) energies. These calculations give theoretical support to experimental evidence in the literature which suggests that massive systems (more than six condensed aromatic rings) are not likely to be found in significant quantities in petroleum samples. From EL and Ef it is predicted that ortho- and perifused systems such as pyrene are more likely than ortho-fused systems such as chrysene. The PM0 method is also used for the estimation of first-order interaction energies responsible for the stabilization of free radicals. Copyright 0 1996 Elsevier Science Ltd.