Calculations on the unstable CO−(X2Π) anion

It is demonstrated that CO−(X2Π) lies above CO(X1Σ+) and hence is unstable with respect to autodetachment. This is in disagreement with an oft-cited experimental result, which concluded that CO has an electron affinity of +1.4 eV, but in agreement with electron scattering results. It might be concluded that the RCCSD(T) approach with aug-cc-pVQZ and aug-cc-pV5Z basis sets gives reliable electron affinities based upon comparison with identical calculations on N2; however, analysis of the electronic wave function indicates that this may be fortuitous. On the other hand, CASSCF + multireference configuration interaction (MRCI) calculations on CO− seem to indicate a viable way forward, and spectroscopic constants are derived.