Electron Spin Densities and Density Functional Approximations: Open-Shell Polycyclic Aromatic Hydrocarbons as Case Study.

The way different density functional approximations (DFAs) are able to predict, in open-shell systems, spin density, that is the difference between the densities of electrons with spin α and those of spin β, is investigated. Here, a large panel of functionals were tested on a set composed of seven π-radicals expected to amplify DFA errors in modeling electron delocalization and spin polarization effects due to their extended electronic conjugation coupled with their planar structures. Our results show that generally the DFA performances follow a systematic improvement in going from semilocal to hybrid functionals. More problematic is, instead, the case of double hybrid functionals, where the perturbative contribution to correlation damps the positive effect of the presence of a high percent of exact exchange. More interestingly, differences are observed in the spin delocalization and polarization patterns, thus restraining the possibility of applying some of current DFAs to study chemically relevant properties, like molecular magnetism or charge/electron transport.