Double exchange in polynuclear mixed-valence clusters .2. Iron-sulfur proteins [Fe4S4](+) and [Fe4S4](3+)

The energy states of tetrameric iron clusters with d(6)-d(6)-d(6)-d(5) and d(5)-d(5)-d(5)-d(6) electronic configurations, which form the metal frameworks of the [Fe4S4](+) and [Fe4S4](3+) iron-sulfur proteins, are calculated using a general approach. It is revealed that the structural distortions of these systems significantly affect the composition of the magnetic states. The effect of the isotropic Heisenberg exchange is considered.