Density functional theory (DFT) has been used to calculate geometies and magnetic properties of iron-sulfur clusters [Fe~2S~2(SCH~3)~4]^2^- , [Fe~3S~4(SCH~3)~3]^2^- and [Fe~4S~4(SCH~3)~4]^2^- in the active sites of FeS proteins. The Fe－Fe spin coupling is described by a broken symmetry (BS) state. the calculations show that the Fe－Fe distances obtained from high spin state (HS) are deviated from the experimental values significantly, whereas the Fe－Fe distances obtained by BS method are in good agreement with crystal structure data. Based on the energies of HS and BS states, Heisenberg coupling constan J of these clusters can be estimated and the calulated J values compare fairly well with the experimental data.

Key words: IRON COMPOUNDS, SULFUR COMPOUNDS, COUPLING REACTION, METALLOPROTEIN