2. Computational details

All calculations were performed at UB3LYP/6-311G++(d,p) level of theory with ultrafine integration grid within the framework of the Gaussian09 package.[11] For initial Fe4O4(OH)4 cluster with parallel spins on all iron centers the solution for the spin projection of 10 appears to have minimal energy among all possible iron spins configurations. Ferromagnetic coupling of spins on metal centers seems to be a consequence of cubic geometry of tetramer Fe4O4(OH)4 having right angles Fe-O-Fe in a perfect agreement with the prediction of negligible superexchange for the Fe3+-O2--Fe3+ linkage at the Fe-O-Fe angle of 90°.[12,13] For selected most important complexes the presence of water solvent was accounted within Polarizable Continuum Model (PCM) using the integral equation formalism (IEFPCM)[14] which is the default SCRF method in Gaussian09. In order to get some insight into the electron state of the oxygen and iron centers participating in the interaction with water molecules, the core 1s energies were estimated by the lowest Kohn-Sham molecular orbitals. These energies allow to indirectly interpret the change of electron density on particular center via the shift of the 1s energy of this center: the lower core level, the less electron density.