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.