Reverse Engineering of Spin Crossover to Scrutinize Density Functional Approximations

Electronic structure calculations for crossover molecules can produce differing trends based on methodology, especially in small metal complexes. The spin gap energy in these complexes, typically ≤ 10 kJ/mol, complicates accurate quantification due to the limitations of wave function theories. Most large-scale calculations rely on density functional approximations, significantly influencing outcome quality. Unfortunately, there is no systematic guide for selecting an appropriate density functional or single-determinant exchange fraction, which hampers the balance between accuracy and computational efficiency.

 

This work presents a tuning strategy using the LC-ωPBE functional, which offers a single tuning parameter aligned with experimental results. We optimized the ω parameter for various metal complexes to match calculated transition temperatures with experimental data. This calibrated functional enabled us to extract crossover energies for each molecule, serving as a reference to evaluate results from several common density functionals. Our findings detail the impact of different methodological choices on the precision of the spin energy gap, offering valuable insights for researchers in the field.