While the neutron data supports modeling of a two and three atom dioxygen species at the active site, it does however not permit definitive discrimination between the hydroperoxyl or hydroperoxo species nor does it resolve the redox state of the copper ion. We performed DFT studies on active site models to further assess the intermediates modelled in the copper active site of Molecule A (Table ). The calculations indicate that the Nε-protonated His157 plus superoxo species and the imidazolate plus hydroperoxo species are both stable states in that they both remain bound when geometry optimized to a stationary point on the potential energy surface; however, they each have a different combination of charge and copper spin state (Table ) precluding a direct comparison of their DFT calculated energies.