Assembly and disassembly of protein-bound iron-sulfur clusters are involved in a wide variety of vital biological processes, ranging from biosynthesis to regulation of biological function. The study of the fission of analogue clusters can provide valuable insight into the various reaction mechanisms that can occur in proteins as well as tests of the ability of theoretical studies to interpret experiment. Previously, we observed symmetric fission of doubly charged Fe-S cluster anions, [Fe4S4X4](2-) --> 2[Fe2S2X2](-) (X = SC2H5, Cl, Br) in the gas phase, which is surprising because four strong Fe-S bonds are being broken. Here, we report a study of the detailed fission mechanism using density functional theory in conjunction with photoelectron spectroscopic results for X = Cl. Both the experimental and theoretical results suggest that the fission daughter products are low-spin [Fe2S2Cl2](-) (S = 1/2) species. However, the layered structure of the cubane would seem to indicate a mechanism that would give high-spin daughter products. Thus, we investigate the symmetric fission of [Fe4S4Cl4](2-) along two possible reaction pathways, involving high-spin and low-spin [Fe2S2Cl2](-)fragments, respectively. Though the high-spin channel is endothermic by 1.34 eV with a high barrier of 2.65 eV, the reaction along the low-spin fission channel is more favorable with an exothermicity of 0.53 eV and a lower barrier of 1.51 eV. Two intermediates are observed along the low-spin fission channel, a spin-localized cubane [Fe4S4Cl4](2-) cluster, which contains two valence-localized Fe3+ centers and two valence-localized Fe2+ centers, and a half-opened [Fe4S4Cl4](2-) Cluster. The spin-localized cluster is crucial to breaking the four strong Fe-S bonds in the symmetric fission.