The approximate coupled-cluster singles and doubles model CC2 is widely used for calculations of excited states in large molecules. We demonstrate a surprising failure of the CC2 geometry optimization of ozone, whereby the CC2 method predicts a barrierless, exothermic, and symmetric dissociation to three oxygen atoms. This is particularly astonishing since both second-order Moller-Plesset perturbation theory and coupled-cluster singles and doubles give reasonable equilibrium structures. We find that [[H, T-1], T-1] leads to an unbalanced treatment of T-1 and T-2 in the CC2 equations. Results presented here suggest that considerable caution should be exercised when applying the CC2 model to systems with singlet biradical character. (C) 2010 Elsevier B.V. All rights reserved.