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'Site values' are used to obtain a set of spin projection coefficients for different iron sulfur clusters in different oxidation states. The best spin state in a pairwise model having two ions dimer combined to a tetramer is (7/2,3,1/2), for the (Fe4S4)3+ cluster. For 4Fe ferrodoxins, the spin projection coefficients occur between the (7/2,3,1/2) and (5/2,2,1/2) states, and for the aconitase cluster, the coefficients intermediate between the states (9/2,4,1/2) and (7/2,3,1/2).

An analysis of the magnitude and signs of Fe internal magnetic fields and other Mosssbauer/EPR data for the redox state P(OX) of the P-cluster of nitrogenase helps arrive at a consistent picture of oxidation states and spin coupling patterns in all four observed P-cluster redox states. The 8Fe cluster is made of two (Fe4S4) units, but the two halves exhibit different spin properties due to the presence of an additional serine ligand at one iron site. One cluster has a spin of 1/2 and other 7/2 and both spins are coupled parallel to give a total spin of 4.