Energies, resonance and UHF instabilities in polycyclic aromatic hydrocarbons and linear polyenes

We have performed ab initio restricted Hartree–Fock (RHF) and restricted second-order Møller–Plesset perturbation theory (RMP2) calculations of the relative energies of all C20H12, C18H12 and C14H10 polycyclic aromatic hydrocarbons (PAHs). Since all of these PAH wavefunctions exhibit large unrestricted Hartree–Fock (UHF) instabilities, the C14H10 PAHs were also studied using higher levels of electron correlation. Additionally, the role of UHF instabilities was investigated for the the linear conjugated polyenes from C2H4 to C20H22. Our results suggest that the RHF and RMP2 methods yield energies in relatively good agreement with highly electron-correlated methods, but the UHF instabilities indicate a fundamental shortcoming of the single determinant wavefunction description of these systems.