All density functional calculations (DFT) have been carried out with Gaussian 09 (revision D) except those with the SCAN functional, which have been performed using the Q-Chem 5.0 electronic structure suite. All calculations have been converged to 10 -8 for the density matrix elements, and the corresponding vibrational analysis was done to ensure that they were minima along the potential energy surface. The fully optimized basis set from Ahlrichs and co-workers, including polarization functions, was employed for all atoms. In particular, five different basis set schemes were tested during the benchmarking process: TZV for all elements (BS1), TZVP for manganese and TZV for the rest (BS2), TZVP for all elements (BS3), QZVP for manganese and TZVP for the rest (BS4) and QZVP for all elements (BS5) (see results section and Supporting Information). To compute the transition temperatures, several post pro-cessing scripts were used (see Supporting Information). The n-electron valence perturbation theory (NEVPT2) calculations were performed with the Orca 4.0 code. In these calculations, we employed the def2TZVPP basis set, including the corresponding auxiliary basis set for the correlation and Coulomb fitting. The active space contains the 5 d-orbitals of the metal and 4 electrons, and the ab initio ligand-field theory (AILFT) approach was employed to extract the related orbitals.