Electronic, magnetic and structural properties of neutral, cationic and anionic Fe2S2, Fe3S4 and Fe4S4 clusters

This work reports density functional calculations of geometric, electronic and magnetic properties of freestanding iron-sulfur Fe2S2, Fe3S4 and Fe4S4 clusters which are the ones most frequently contained in proteins. We investigate neutral, anionic and cationic clusters using a method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials and a generalized gradient approximation to exchange and correlation. The results are discussed in connection with available experimental data. We mainly show that the ground-state geometries of these free clusters are consistent with their structures in core proteins and they are the same in the neutral, anionic and cationic states, but with small distortions. In all cases, an antiferromagnetic order between Fe atoms is always preferred to ferromagnetic and paramagnetic ones. The geometric distortions induced by magnetism decrease with cluster size and the maximum deviation between Fe-Fe distances is 11% in Fe2S2, but only 4% in Fe3S4 and 3% in Fe4S4 clusters.