For comparison, Caldararu et al. performed QM-cluster calculations with TPSS/de2-TZVPD with a COSMO solvation model, using a dielectric constant of 80, and find that the dissociation of superoxide is unfavorable with an electronic energy of 126 kJ/mol and a free energy of 80 kJ/mol, suggesting zero-point vibration energy and entropy to contribute about -47 kJ/mol. The difficulties in obtaining an dissociated O • - 2 species was also observed then: An attempt to model the dissociation of superoxide with QM/MM also led to dissociation of O 2 from Cu(I), and releasing the restraint of 4.6 Å between Cu and O p resulted in superoxide binding again to Cu. In 2014, Kjaergaard et al. proposed that the dissociation of superoxide could be 43.1 kJ/mol uphill, but feasible, and report that superoxide abstracts a proton from a water molecule during the calculations. Such a proton transfer was not observed by Caldararu et al.. We obtain an electronic energy of 66.8 ± 6.7 kJ/mol and n = 6 with TPSS. However, taking into account the -47 kJ/mol contribution for the zero-point vibration energy and the entropy (taken from ref. 27), the dissociation of O • - 2 from Cu(II) is possible with both functionals, albeit uphill. The dissociation of OOH • from 3 in the triplet state is on average downhill by 14.0 ± 20.6 kJ/mol (n = 7), but it should be noted that the result is highly functional dependent (we obtain 42.7 ± 23.2 kJ/mol (n = 7) uphill with TPSS). For all structures, the triplet and open-shell singlet spin states are essentially degenerate. Independently of the spin-state or functional, most spin density is on Cu (0.6) and the oxygen atoms of the OOH • species (in sum 1.0), suggesting a dissociation of OOH • from Cu(II). Caldararu et al. tried to model this dissociation with linear transit calculations using TPSS, but without a restraint, OOH • binds back to Cu, even if the distance between Cu and O p is 4.6 Å.