Comment on 'Polaron formation and symmetry breaking' by L. Zuppiroli et al. [Chem. Phys. Lett. 374 (2003) 7]

L. Zuppiroli et al. [Chem. Phys. Lett. 374 (2003) 7] have theoretically studied polaron formation in oligo(phenylene vinylene) radical ions. In particular, they obtained with the AM1/UHF method a stepwise increase of the relaxation energy with increasing chain length. In this Comment, we suggest that this result is likely to be an artifact. We argue that UHF is particularly inappropriate for studying energies of open-shell pi-conjugated systems because of inherent spin contamination leading to wrong molecular structures. We show that, within the AM1 methodology, relaxation energies are rather insensitive to chain length and discuss the origin of this behavior.