Biased Spin-State Energetics of Fe(II) Molecular Complexes within Density-Functional Theory and the Linear-Response Hubbard U Correction

Lorenzo Antonio Mariano (1) , Bess Vlaisavljevich , Roberta Poloni (1, 2)

1 SIMaP - Science et Ingénierie des Matériaux et Procédés
2 CNRS - Centre National de la Recherche Scientifique

The spin-state energetics of six Fe(II) molecular complexes are computed using the linear-response Hubbard U approach within DFT. The adiabatic energy differences, ∆E H-L , between the high spin (S=2) and the low spin (S=0) states are computed and compared with accurate coupled clustercorrected CASPT2 results. We show that DFT+U fails in correctly capturing the ground state for strong field-ligands yielding ∆E H-L that are almost constant throughout the molecular series. This bias towards high spin together with the metal/ligand charge transfer upon U correction are here quantified and explained using molecular orbital diagrams involving both σ-and π-bonding interactions. With increasing ligandfield strengths this bias also increases owing to the stronger molecular character of the metal/ligand Kohn-Sham orbitals thus resulting in large deviations from the reference larger than 4 eV. Smaller values of U can be employed to mitigate this effect and recover the right energetics.

Domaines: Chimie

Lorenzo Antonio Mariano, Bess Vlaisavljevich, Roberta Poloni. Biased Spin-State Energetics of Fe(II) Molecular Complexes within Density-Functional Theory and the Linear-Response Hubbard U Correction. Journal of Chemical Theory and Computation, 2020, 16 (11), pp.6755-6762. ⟨10.1021/acs.jctc.0c00628⟩. ⟨hal-03046264⟩