RASPT2/RASSCF vs Range-Separated/Hybrid DFT Methods: Assessing the Excited States of a Ru(II)bipyridyl Complex

The excited states of the trans(Cl)-Ru(bpy)Cl2(CO)2 (bpy = bypyridyl) transition-metal (TM) complex are assessed using the newly developed second-order perturbation theory restricted active space (RASPT2/RASSCF) method. The delicate problem of partitioning the RAS subspaces (RAS1, RAS2, and RAS3) is addressed, being the choice of the RAS2 the bottleneck to obtain a balanced description of the excited states of different nature when TMs are present. We find that the RAS2 should be composed by the correlation orbitals involved in covalent metal-ligand bonds. The level of excitations within the RAS1 and RAS3 subspaces is also examined. The performance of different flavors of time-dependent density functional theory including pure, hybrid, meta-hybrid, and range-separated functionals in the presence of solvent effects is also evaluated. It is found that none of the functionals can optimally describe all the excited states simultaneously. However, the hybrid M06, B3LYP, and PBE0 functionals seem to be the best compromise to obtain a balanced description of the excited states of trans(Cl)-Ru(bpy)Cl2(CO)2, when comparing with the experimental spectrum. The conclusions obtained in this molecule should pave the road to properly treat excited states of larger Ru-polypyridyl complexes, which are of particular interest in supramolecular chemistry.