Importance of Electron Correlation on the Geometry\nand Electronic Structure of [2Fe–2S] Systems: A Benchmark Study\nof the [Fe<sub>2</sub>S<sub>2</sub>(SCH<sub>3</sub>)<sub>4</sub>]<sup>2–,3–,4–&

Iron–sulfur\nclusters are crucial for biological electron\ntransport and catalysis. Obtaining accurate geometries, energetics,\nmanifolds of their excited electronic states, and reduction energies\nis important to understand their role in these processes. Using a\n[2Fe–2S] model complex with Fe<sup>II</sup> and Fe<sup>III</sup> oxidation states, which leads to different charges, i.e., [Fe<sub>2</sub>S<sub>2</sub>(SMe)<sub>4</sub>]<sup>2–,3–,4–</sup>, we benchmarked a variety of computational methodologies ranging\nfrom density functional theory (DFT) to post-Hartree–Fock methods,\nincluding complete active space self-consistent field (CASSCF), multireference\nconfiguration interaction, the second-order N-electron valence state\nperturbation theory (NEVPT2), and the linearized integrand approximation\nof adiabatic connection (AC0) approaches. Additionally, we studied\nthree experimentally well-characterized complexes, [Fe<sub>2</sub>S<sub>2</sub>(SCys)<sub>4</sub>]<sup>2–</sup>, [Fe<sub>2</sub>S<sub>2</sub>(S-<i>o</i>-tol)<sub>4</sub>]<sup>2–</sup>, and [Fe<sub>2</sub>S<sub>2</sub>(S-<i>o</i>-xyl)<sub>4</sub>]<sup>2–</sup>, via DFT methods. We conclude that the\ndynamic electron correlation is important for accurately predicting\nthe geometry of these complexes. Broken symmetry (BS) DFT correctly\npredicts experimental geometries of low-spin multiplicity, while CASSCF\ndoes not. However, BS-DFT significantly overestimates the difference\nbetween the low- and high-spin electronic states for a given oxidation\nstate. At the same time, CASSCF underestimates it but provides relative\nenergies closer to the reference NEVPT2 results. Finally, AC0 provides\nenergetics of NEVPT2 quality with the additional advantage of being\nable to use large CASSCF sizes. NEVPT2 gives the best estimates of\nthe Fe<sup>III</sup>/Fe<sup>III</sup> → Fe<sup>II</sup>/Fe<sup>III</sup> (4.27 eV) and Fe<sup>II</sup>/F<sup>III</sup> →\nFe<sup>II</sup>/F<sup>II</sup> (7.72 eV) reduction energies. The results\nprovide insight into the electronic structure of these complexes and\nassist in the understanding of their physical properties.