Electronic structure calculations for the 2 g X ∏  state of NiO2¯ and the 1 g X + ∑  state of NiO2 were carried out at the coupled-cluster singles doubles augmented with a noniterative triples [CCSD(T)] level of theory to determine the equilibrium structures and the harmonic vibrational frequencies of these states as well as to enable a Franck-Condon (FC) simulation for the electron detachment transitions from the electronic ground state of NiO2¯ to that of NiO2. Scalar-relativistic effects have been taken into account using spin-free exact two-component theory in its one-electron variant (SFX2C-1e) together with the aug-cc-pVTZ basis set recontracted for SFX2C-1e calculations. Calculations using a hierarchy of CC methods including CCSD, CC singles doubles triples (CCSDT), and CC singles doubles triples quadruples (CCSDTQ) have also been performed for the