Spin-state energy differences of octahedral Fe(II) complexes using self-interaction-corrected methods.

Accurate prediction of spin-state energy difference is crucial for understanding the spin crossover (SCO) phenomena and is very challenging for the density functional approximations, especially for the local and semi-local approximations, due to delocalization errors. We use recent locally scaled self-interaction-correction (LSIC) [Zope, R. R. et al., J. Chem. Phys. 151, 214108 (2019)] and Perdew-Zunger self-interaction-correction (PZSIC) method to study the spin-state gaps (SSG) of Fe(II) complexes with four different ligands of various strengths and compare them with reference diffusion Monte Carlo (DMC) results. Results show the surprising failure of the PZSIC method, which favors low spin states for all systems. The perturbative LSIC-LSDA using PZSIC densities significantly improves the gaps with a mean absolute error of 0.51 eV but slightly overcorrects PZSIC for the stronger ligand. The quasi self-consistent LSIC method with the simplest LSDA functional gives the correct sign of SSG for all ligands with a mean absolute error (MAE) of 0.56 eV, comparable to that of CCSD(T) (MAE = 0.49 eV).