AbstractThe highly accurate valence internally contracted multireference configuration interaction (MRCI) approach has been employed to investigate the potential energy curves (PECs) for the X2Π, b4Σ−, C2Σ− states of PO and the X1Σ+ state of PO+. For these electronic states, the spectroscopic parameters of the isotopes (P16O, P18O, P16O+, and P18O+) have been determined and compared with those of the investigations reported in the literature. The comparison shows that excellent agreement exists between the present results and the available experiments. With the PECs determined here, the first 30 vibrational states for P16O(X2Π, b4Σ−), P18O(X2Π, b4Σ−), P16O+(X1Σ+), and P18O+(X1Σ+) are computed when the rotational quantum number J equals zero (J = 0). The vibrational level G(υ), inertial rotation constant Bυ and centrifugal distortion constant Dυ are determined when J = 0. All the results of vibrational states except for P16O (X2Π) are reported for the first time. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011