  We investigate the role of Kekule and non-Kekule structures in the radical
character of alternant polycyclic aromatic hydrocarbons (PAHs) using
thermally-assisted-occupation density functional theory (TAO-DFT), an efficient
electronic structure method for the study of large ground-state systems with
strong static correlation effects. Our results reveal that the studies of
Kekule and non-Kekule structures qualitatively describe the radical character
of alternant PAHs, which could be useful when electronic structure calculations
are infeasible due to the expensive computational cost. In addition, our
results support previous findings on the increase in radical character with
increasing system size. For alternant PAHs with the same number of aromatic
rings, the geometrical arrangements of aromatic rings are responsible for their
radical character.
