ENERGIES, RESONANCE AND UHF INSTABILITIES IN POLYCYCLIC AROMATIC-HYDROCARBONS AND LINEAR POLYENES

We have performed ab initio restricted Hartree-Fock (RHF) and restrict ed second-order Moller-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 exh ibit large unrestricted Hartree-Fock (UHF) instabilities, the C14H10 P AHs were also studied using higher levels of electron correlation. Add itionally, the role of UHF instabilities was investigated for the the linear conjugated polyenes from C2H4 to C20H22. Our results suggest th at the RHF and RMP2 methods yield energies in relatively good agreemen t with highly electron-correlated methods, but the UHF instabilities i ndicate a fundamental shortcoming of the single determinant wavefuncti on description of these systems. (C) 1998 Elsevier Science B.V. All ri ghts reserved.