MnO+:: a challenge for density functional theory methods

The (5)Sigma(+) and (5)Pi states of MnO+ are studied using density functional theory (DFT) and the complete-active-space self-consistent-field (CASSCF)/multireference configuration interaction (MRCI) and CASSCF/average coupled pair functional (ACPF) levels of theory. All the DFT methods give a reasonable description of the (5)Sigma(+) state, but only the hybrid functionals give (5)Pi spectroscopic constants that agree with those obtained using the MRCI and ACPF approaches. The origin of the difference between the pure and hybrid functionals is discussed.