Zigzag graphene nanoribbons feature remarkable spin-polarization and half-metallic properties. Quantum chemical investigations play a major role in clarifying the outstanding electronic properties of carbon nanoflakes displaying bi- or even multi-radical character. Multireference (MR) methods are especially well suited since they are not affected by spin contamination or energy instability problems. In this work extensive MR averaged quadratic coupled cluster calculations have been performed on polyaromatic hydrocarbons (PAH) used as model systems for graphene nanoflakes. These calculations illustrate (see figure below) the strong polyradical character which is established at the zig zag edges of the nanoribbons. The comparison of the properties of the different PAHs investigated in this work illustrates the remarkable variety in open shell character when following different pathways in extending the molecular size of the PAHs.

Unpaired electron density for 5-armchair, 6-zig zag periacene.