Spin Manifolds in the [4Fe-4S] Cluster from Localized Active Space State Interaction Singles and Pair-Density Functional Theory

Iron--sulfur clusters play central roles in biological photosynthesis, respiration and catalysis. The [Fe$_4$S$_4$(SCH$_3$)$_4$]$^{2-}$ cluster possesses a dense manifold of low-lying electronic states arising from strong magnetic interactions among the Fe centers, making its electronic structure challenging to describe theoretically. Previous multireference studies have generally optimized excited states individually, while neglecting the interactions among low-lying states. In this work, we investigate the low-lying electronic states of [Fe$_4$S$_4$(SCH$_3$)$_4$]$^{2-}$ using localized active space state interaction singles (LASSIS) combined with pair-density functional theory (PDFT). By treating all states within a common model-space Hamiltonian, LASSIS generates a single set of orthogonal low-lying states. The resulting spin-state energies organize into distinct manifolds. The same overall pattern is obtained for active spaces ranging from 22 electrons in 20 orbitals to 54 electrons in 36 orbitals. To enable these calculations, we also implement GPU acceleration for the most computationally demanding components of the LASSIS workflow.