Time-dependent density functional theory applied to ligand-field excitations and their circular dichroism in some transition metal complexes

Ligand-field transitions in [Co(en)₃]³⁺ and [Rh(en)₃]³⁺ as well as the low-energy part of the electronic spectrum of [Fe(phen)₃]²⁺ are investigated with time-dependent density functional theory (TDDFT). There is a strong functional dependence for [Co(en)₃]³⁺ and [Fe(phen)₃] ²⁺. ΔSCF methods reproduce the ligand-field singlet excitation energies of [Co(en)₃]³⁺ and [Rh(en)₃] ³⁺ very well. The case of [Co(en)₃]³⁺ is analyzed in some detail, in particular regarding the possibility of applying a charge-transfer (CT) correction [M.E. Casida, F. Gutierrez, J. Guan, F.-X. Gadea, D.R. Salahub, J.-P. Daudey, J. Chem. Phys. 113 (2000) 7062]. A simple CT correction would not be sufficient, but the magnitude of the charge transfer correction term in comparison with the calculated excitation energy appears to be indicative of self-interaction problems in the ground state electronic structure and in the calculated excitation energies. For the ligand-field transition of [Co(en)₃]³⁺ a hybrid functional with about 25% exact exchange performs well. Range separation/long range correction/Coulomb attenuation offers little improvement for the ligand-field transitions in [Co(en)₃]³⁺ because the occupied and unoccupied orbitals involved are in close spatial proximity.