The structures and energies of the electronic ground states of the FeS0/−, FeS20/−, Fe2S20/−, Fe3S40/−, and Fe4S40/− neutral and anionic clusters have been computed systematically with nine computational methods in combination with seven basis sets. The computed adiabatic electronic affinities (AEA) have been compared with available experimental data. Most reasonable agreements between theory and experiment have been found for both hybrid B3LYP and B3PW91 functionals in conjugation with 6-311+G* and QZVP basis sets. Detailed comparisons between the available experimental and computed AEA data at the B3LYP/6-311+G* level identified the electronic ground state of 5Δ for FeS, 4Δ for FeS−, 5B2 for FeS2, 6A1 for FeS2−, 1A1 for Fe2S2, 8A′ for Fe2S2−, 5A′′ for Fe3S4, 6A′′ for Fe3S4−, 1A1 for Fe4S4, and 1A2 for Fe4S4−. In addition, Fe2S2, Fe3S4, Fe3S4−, Fe4S4, and Fe4S4− are antiferromagnetic at the B3LYP/6-311+G* level. The magnetic properties are discussed on the basis of natural bond orbital analysis.