Entanglement and polyradical character of polycyclic aromatic hydrocarbons predicted by projected Hartree-Fock theory.

We study strong correlation effects in a series of fused benzene rings (acenes) of varying length and width using our recently developed projected Hartree-Fock (PHF) method. These molecules, commonly known as polycyclic aromatic hydrocarbons or nanographenes, are very challenging for electronic structure theory because of their strong multireference character. This challenge is here met by PHF at moderate computational cost optimizing a spin eigenfunction obtained by projection of an unrestricted Hartree-Fock (UHF) trial determinant. The resulting method, known as SUHF, predicts that polyradical behavior and orbital entanglement are enhanced with molecular size, especially in systems whose structural motifs are dominated by zigzag edges, like oligoacenes.