The molecular geometry and the normal modes properties of coronene are investigated by means of DFT͑B3LYP͒ and restricted/Hartree-Fock calculations utilizing basis sets of triple zeta + polarization quality. The interpretation of the infrared and Raman spectra of coronene, especially in solid state, is critically revised. The phantom bands in the solid state, previously not understood, are readily assigned after considering a minute out-of-plane molecular distortion from D 6h to C 2h .

An extension of the CEPA1 method to the multi-reference case is presented, the MR-CEPA1 approach. The method takes the variationally included terms into account as in MRDCEPA and corrects for the exclusion principle violating terms as in closed shell CEPA1. It is shown that this method yields potential energy curves that are close to those of the multi-reference coupled cluster method and parallel the full CI results quite well. The size consistency of the method is as good as the MRDCEPA method and much better than approaches that ignore the VI terms like MR-ACPF and MR-AQCC. A simpler method, where the EPV terms are not corrected on an individual basis for the doubly occupied orbitals, dubbed the multi-reference averaged CEPA1 method (MR-ACEPA), which is akin to methods previously suggested by Szalay et al., is comparable in performance.

The B3LYP/DZP++ adiabatic electron affinity (AEA) of nine (non)-alternant polycyclic aromatic hydrocarbons are reported and discussed. Calculations became feasible for molecules this size by projecting out the near-linearly dependent part of the one-electron basis. Non-alternant PAH consisting of an alternant PAH core modified by peri-annulation with unsaturated five-membered rings (CP-PAHs) possess markedly enhanced AEAs.