Potential energy curves of 25 low-lying electronic states of IrO are calculated with the multi-state complete active space second order perturbation theory (MS-CASPT2) method. The lowest state calculated without spin-orbit interactions has the 2 P symmetry. However, when spin-orbit coupling effect is considered, the ground state is the X-4 Delta(7/2) and the (2)Pi(3/2) is located 633 cm(-1) above. The excited A(2)Delta state splits into A(2)Delta(3/2) and A(2)Delta(1/2), which are 8233 and 9741 cm(-1) higher than the X-4 Delta(2/7) state respectively. Electronic transitions from the two excited states H-2 Delta(Omega) and D-2 Pi(Omega) to the A(2)Delta(Omega) state produce the rotationally resolved bands observed experimentally. (C) 2012 Elsevier B. V. All rights reserved.