Common force field for graphite and polycyclic aromatic hydrocarbons

A valence force field based on H\"uckel's theory has been developed, which allows us to establish a close correlation between phonons of graphite and the normal modes of small polycyclic aromatic hydrocarbons (such as coronene and hexabenzocoronene). The results show that in these systems two kinds of motions dominate the Raman spectrum: the {} mode and the ``breathing'' A mode. These modes are the equivalent, in a finite domain, of the E2g phonon of graphite at the {Γ} point and the A^' phonon at the K point of the first Brillouin zone. This study provides a useful basis for the understanding of the Raman spectra of any material containing sp² carbon domains.