We have also performed SFX2C-1e equation-of-motion coupled-cluster singles doubles (EOM-CCSD) and EOM-CC singles doubles triples (EOM-CCSDT) calculations to determine the excitation energies of NiO2 in the anionic equilibrium structure. These are combined with the computed vertical detachment energy for the ground 1 g X + ∑  state of NiO2 to describe the vertical detachment energies from the ground state of NiO2¯ to these excited states of NiO2. Here, aug-cc-pVQZ basis sets were used for EOM-CCSD calculations. The triples corrections have been obtained as the differences between EOM-CCSDT and EOM-CCSD results using cc-pVDZ basis sets. All calculations have been performed using the CFOUR program package, except that the EOM-CCSDT calculations of triplet excited states have been carried out using the MRCC program package. IV.