Calculations on the unstable CO-(X-2 Pi) anion

It is demonstrated that CO-(X(2)Pi) lies above CO(X(1)Sigma (+)) 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 aff inity of +1.4 eV, but in agreement with electron scattering results. It mig ht 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 i dentical calculations on N-2; however, analysis of the electronic wave func tion indicates that this may be fortuitous. On the other hand, CASSCF + mul tireference configuration interaction (MRCI) calculations on CO- seem to in dicate a viable way forward, and spectroscopic constants are derived. (C) 2 001 Elsevier Science B.V. All rights reserved.