Can range-separated functionals be optimally tuned to predict spectra and excited state dynamics in photoactive iron complexes?

Density functional theory is an efficient computational tool to investigate photophysical and photochemical processes in transition metal complexes, giving invaluable assistance in the interpretation of spectroscopic and catalytic experiments. Optimally-tuned range-separated functionals are particularly promising, as they were created to cure some of the fundamental deficiencies present in approximate exchange-correlation functionals. In this paper, we scrutinize the selection of optimally tuned parameters and its influence on the excited state dynamics, using the example of the iron complex [Fe(cpmp)_2 ]^{2+} with push-pull ligands. Various tuning strategies are contemplated, based on pure self-consistent DFT protocols, as well as on the comparison with experimental spectra and multireference CASPT2 results. The two most promising sets of optimal parameters are then employed to carry out nonadiabatic surface-hopping dynamical simulations. Intriguingly, we find that the two sets lead to very different relaxation pathways and timescales, showcasing the complexity of iron-complexes excited state landscapes and the difficulty of obtaining an unambiguous parametrization of long-range corrected functionals without experimental input.