Local chemical softness as a reactivity probe in mixed-valence compounds of class I and class II (Cr2F5 and Co3O4): The importance of site-dependent Hubbard U parameters

In this work, we employ local softness as a key reactivity descriptor to systematically analyze class I (Cr2F5) and class II (Co3O4) mixed-valence materials. We evaluated the performance of several exchange–correlation functionals, including PBEsol, NCAPR (Nearly Correct Asymptotic Potential Revised), SCAN (Strongly Constrained and Appropriately Normed), and PBE+U (Perdew–Burke–Ernezerhof + Hubbard U), to determine their ability to resolve electron donor and acceptor capacities. Our results demonstrated that local softness is a powerful tool for distinguishing non-equivalent metallic centers and coexisting oxidation states. Crucially, we found that the PBE+U functional, utilizing site-specific Hubbard parameters (U) calculated via density functional perturbation theory, is the most robust approach, consistently reproducing the expected one-electron transfer. This study highlights the necessity of integrating local reactivity descriptors with differentiated Hubbard corrections for the accurate design and characterization of redox-active functional materials.