Theoretical study of the electronic structure of iridium monoxide

► It is first studied by high-level ab inito methods for the electronic structure for IrO molecule. ► Both the relativistic effect and electronic correlation are considered in the present Letter. ► The ground state of IrO is clarified and assigned to the X4Δ7/2. ► The calculated rotationally resolved spectra of IrO are in good agreement with experimental data.