The geometric structures of FeS3 and FeS3- with spin multiplicities ranging from singlet to octet were optimized at the B3LYP level, allowing two low-lying conformations for these clusters to be identified. The planar D-3h conformation contains three S2- atomic ligands (S3Fe0/-), whereas the C-2v structure contains, in addition to an atomic S2- ligand, also a S-2(2-) ligand that is side-on-bound to the iron cation: an eta(2)-S2FeS conformation. Subsequently, energy differences between the various states of these conformations were estimated by carrying out geometry optimizations at the multi-reference CASPT2 level. Several competing structures for the ground state of the anionic cluster were recognized at this level. Relative stabilities were also estimated by performing single-point RCSSD(T) calculations on the B3LYP geometries. The ground state of the neutral complex was unambiguously found to be B-5(2). The ground state of the anion is considerably less certain. The 1(4)B(2), 2(4)B(2), B-4(1), and (6)A(1) states were all found as low-lying eta(2)-S2FeS- states. Also, B-4(2) of S3Fe- has a comparable CASPT2 energy. In contrast, B3LYP and RCCSD(T) mutually agree that the S3Fe- state is at a much higher energy. Energetically, the bands of the photoelectron spectra of FeS3- are reproduced at the CASPT2 level as ionizations from either the B-4(2) or (6)A(1) state of eta(2)-S2FeS. However, the Franck-Condon factors obtained from a harmonic vibrational analysis at the B3LYP level show that only the B-4(2)-to-B-5(2) ionization, which preserves the eta(2)-S2Fe-S conformation, provides the best vibrational progression match with the X band of the experimental photoelectron spectra.