Labile electronic and spin states of the CaMn4O5 cluster in the PSII system refined to the 1.9 Ã X-ray resolution. UB3LYP computational results

► A first ab initio study of the manganese cluster with the 1.9 Å X-ray resolution. ► We examined all possible axial spin structures using B3LYP method of Mn(IV)4 and Mn(III)4 cases. ► We analyzed the energy spectra of spin states using Heisenberg 6 J model. ► Energy gaps are quite small, which is related to the labile character of the cluster.