For the dissociation of an oxygen species from 3l red , the different micro-solvated dissociated states are a mixture of Cu(I)•OOH • and Cu(II) • OOH -, based on the Mulliken spin populations. However, none of the dissociated states clearly contains the Cu(I) and OOH • dissociation channel, whereas a few of the micro-solvated dissociation products clearly contain Cu(II) and OOH -(since they have zero spin density on OOH -). The dissociation of OOH - is generally uphill with 67.2±16.2 kJ/mol (n = 3) with B3LYP (and not seen with TPSS). If we also include states with higher character of Cu(I) and OOH • , the dissociation energies are even higher, i.e., on average 91.5 ± 32.3 kJ/mol (n = 6) with B3LYP. Interestingly, we obtain two structures where the OOH • or OOH -species spontaneously abstracts a proton from water during the geometry optimization, leading to the formation of H 2 O 2 and OH -, where the latter coordinates to Cu. These structures are significantly lower in energy than the others (-6.2 and 37.0 kJ/mol with B3LYP and -1.6 and 34.7 kJ/mol with TPSS). For both structures, the spin density is highest on Cu (0.6) while the remaining spin density is distributed among the nitrogen atoms of the histidine brace and the oxygen of OH -(all coordinating Cu). A similar distribution is observed for the three structures corresponding to Cu(II) and OOH -. On the other hand, in those cases with higher character of Cu(I) and OOH • , the total spin density on the dissociated OOH • species is 0.32 (0.14 for TPSS) or larger. Taking into account that the previous calculations by some of us gave zero-point vibration energy and entropy contributions of about -47 kJ/mol, we conclude that dissociation of OOH • from Cu(I) is unfavorable compared to dissociation of OOH -from Cu(II) which could be possible, although the formation of H 2 O 2 via proton abstraction from water seems the more plausible pathway. The previous investigation by Caldararu et al. finds that dissociation of OOH • from Cu(I) is slightly uphill but much more favorable (barrier of 60 kJ/mol and a reaction energy of 28 kJ/mol).