Do large polycyclic aromatic hydrocarbons and graphene bend? : How popular theoretical methods complicate finding the answer to this question

Theoretical studies of the vibrational frequencies of C6n 2H6n (n =2-12) coronenes, show that, despite full conjugation, delocalization and aromaticity, the stability of the planar geometry rapidly decreases with size. A switch to a nonplanar geometry can be expected at around n =9-12; any larger gas-phase coronene, including graphene, should be nonplanar. When applied to coronenes, popular quantum chemical methods, including Hartree-Fock and density functional theory, are shown to produce anomalous imaginary frequencies suggesting unrealistic geometry distortions; this reveals a serious methodological problem in calculations on extended systems which needs to be resolved through further basis set development.