Multiconfigurational character of the ground states of polycyclic aromatic hydrocarbons. A systematic study.

A systematic study of the electronic structure of polycyclic hydrocarbons from naphthalene to a system containing 80 fused benzene has been carried out. Geometries were optimized for closed shell singlet, open shell singlet, triplet and multiplet states at B3LYP/cc-pVDZ level of theory, D1 (second order Møller-Plesset) and D1 (second-order approximate coupled-cluster) diagnostics have been calculated for studied molecules. Complete active space self-consistent field (10,10)/6-31G(d) single point energy calculations have been carried out for all optimized structures. Multireference character of the ground state becomes important when the number of atoms in the polycyclic hydrocarbon exceeds 40-50. At this point, D1 diagnostics reaches 0.04-0.05 and the squared configuration interaction expansion coefficient for dominant configuration drops to about 0.6. However, only for the three largest systems predominantly polyradicalic ground states have been detected. All other polycyclic hydrocarbons showing significant multiconfigurational character of singlet ground state have only two dominant configurations which are closed shell singlet and doubly excited singlet, respectively. Thus, small polycyclic hydrocarbons have mostly single reference singlet ground state, the medium size systems have notably multireference ground state (singlet or triplet) with only moderate polyradicalic character. The ground state of largest systems is singlet polyradical.