Iron-sulfur clusters are crucial for biological electron transport and catalysis. Obtaining accurate geometries, energetics, manifolds of their excited electronic states, and reduction energies is important to understand their role in these processes. Using a [2Fe-2S] model complex with FeII and FeIII oxidation states, which leads to different charges, i.e., [Fe2S2(SMe)4]2-,3-,4-, we benchmarked a variety of computational methodologies ranging from density functional theory (DFT) to post-Hartree-Fock methods, including complete active space self-consistent field (CASSCF), multireference configuration interaction, the second-order N-electron valence state perturbation theory (NEVPT2), and the linearized integrand approximation of adiabatic connection (AC0) approaches. Additionally, we studied three experimentally well-characterized complexes, [Fe2S2(SCys)4]2-, [Fe2S2(S-o-tol)4]2-, and [Fe2S2(S-o-xyl)4]2-, via DFT methods. We conclude that the dynamic electron correlation is important for accurately predicting the geometry of these complexes. Broken symmetry (BS) DFT correctly predicts experimental geometries of low-spin multiplicity, while CASSCF does not. However, BS-DFT significantly overestimates the difference between the low- and high-spin electronic states for a given oxidation state. At the same time, CASSCF underestimates it but provides relative energies closer to the reference NEVPT2 results. Finally, AC0 provides energetics of NEVPT2 quality with the additional advantage of being able to use large CASSCF sizes. NEVPT2 gives the best estimates of the FeIII/FeIII → FeII/FeIII (4.27 eV) and FeII/FIII → FeII/FII (7.72 eV) reduction energies. The results provide insight into the electronic structure of these complexes and assist in the understanding of their physical properties.

中文翻译：

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电子相关性对 [2Fe-2S] 系统几何形状和电子结构的重要性：[Fe2S2（SCH3）4]2-，3-，4-、[Fe2S2（SCys）4]2-、[Fe2S2（S-p-tol）4]2-和 [Fe2S2（S-o-xyl）4]2- 配合物的基准研究。


铁硫簇对于生物电子传递和催化至关重要。获得准确的几何形状、能量学、激发电子态的流形和还原能对于了解它们在这些过程中的作用非常重要。使用具有 FeII 和 FeIII 氧化态的 [2Fe-2S] 模型复合物，导致不同的电荷，即 [Fe2S2（SMe）4]2-，3-，4-，我们对从密度泛函理论 （DFT） 到后 Hartree-Fock 方法的各种计算方法进行了基准测试，包括完全有源空间自洽场 （CASSCF）、多参考构型相互作用、二阶 N 电子价态扰动理论 （NEVPT2）， 以及绝热连接的线性被积函数近似 （AC0） 方法。此外，我们通过 DFT 方法研究了三种实验表征良好的复合物，[Fe2S2（SCys）4]2-、[Fe2S2（S-o-tol）4]2- 和 [Fe2S2（S-o-xyl）4]2-。我们得出结论，动态电子相关性对于准确预测这些复合物的几何形状很重要。打破对称性 （BS） DFT 正确预测低自旋多重性的实验几何形状，而 CASSCF 则不能。然而，BS-DFT 明显高估了给定氧化态的低自旋和高自旋电子态之间的差异。同时，CASSCF 低估了它，但提供了更接近参考 NEVPT2 结果的相对能量。最后，AC0 提供了 NEVPT2 质量的能量学，并具有能够使用大尺寸 CASSCF 的额外优势。NEVPT2 给出了 FeII/FeIII → FeII/FeIII （4.27 eV） 和 FeII/FIII → FeII/FII （7.72 eV） 还原能的最佳估计。 结果有助于深入了解这些复合物的电子结构，并有助于了解它们的物理性质。