Ensemble Hubbard-U Correction for Improved Transition Temperatures for Spin-Crossover Materials

Thermally induced transitions in spin-crossover condensed phases are attractive for use in data display technologies, actuators [Science 279, 44 (1998)] or quantum gates [Dalton Trans. 44, 17819 (2015)]. The problem at hand is that calculation of transition temperatures and crossover profiles is challenging, even with state-of-the-art density functional approximations. The main difficulty is in computing accurate energetic differences between the meta-stable spin states for these types of metal-organic complexes [Comput. Mat. Sci. 206, 111161 (2022)]. We present a novel ensemble-average determination of a Hubbard-U correction on a reference spin-state and demonstrate that the method improves adiabatic total energy differences and thermodynamic properties of crossover aggregates. We find that the U values using this correction alleviate the overestimation of transition temperatures by as much as 90% with respect to the uncorrected results for a commonly used generalized gradient approximation.