Assessment of density functionals for the investigation of iridium(III) complexes

The ground state geometries of 12 phosphorescent iridium(III) complexes have been optimized using four widely used hybrid density functionals B3LYP, PBE1PBE, mPW1PW91 and MPWB1K in combination with the standard basis sets (LANL2DZ for iridium and 6-31G(d) for the other atoms). PBE1PBE, mPW1PW91 and MPWB1K yield better results than B3LYP when comparing with crystal structures. The incorporation of exact exchange in the hybrid density functionals significantly reduces the errors. By comparing the geometries obtained with different combinations of exchange-correlation functionals, we find that the choice of the correlation functional is much more important than the exchange functional. The functionals which satisfy the uniform electron gas limit in the correlation functional perform much better than those do not. More systematic tests suggest that the performances of density functionals are mainly affected by the admixture of exact exchange and the choice of correlation functionals. The basis set effect was also tested. It is suggested that the standard basis set used in this study, LANL2DZ for iridium and 6-31G(d) for main group atoms, could provide comparable accuracy as large basis sets.