Accurate a b i n i t i o potential curves for the X 2Πg, A 2Πu, a 4Σ−u, and 2Σ−u states of the 0−2 ion

A b initio potential energy curves and spectrocopic constants of the X 2Πg, A 2Πu, a 4Σ−u and 2Σ−u of the negative ion of molecular oxygen (0−2), of major interest in the upper atmosphere, are presented in a sequence of increasing computational sophistication. The unique features of the correlation problem for homonuclear ions are analyzed in detail. The most accurate 76-configuration MCSCF ground X 2Πg state results are examined and found (as expected) to be in excellent agreement with experiment. The most accurate A 2Πu excited state results (also involving a 76-configuration MCSCF wavefunction) are expected to be of comparable accuracy, while somewhat less accuracy is expected for the best calculations of the 4Σ−u and 2Σ−u states (64 and 65 configurations used, respectively).