It is mainly to describe the overall experimental trend for comparison with O2  and with the more robust treatments to follow that we include two different 2p-3d mixing curves in Fig. (a), both calculated via eqn (2). The solid curve, representing the overall experimental trend, was generated with 𝛾 = 0.81, 𝐴 = 0.53 eV -1 , 𝐴 = 0.10 eV -1 , 𝐵 = 11.4 eV -1 , and cos2,0 = cos3,1 = 0.96. The phase-shifts were set equal to the O2  value, while the three Hanstorp coefficients are defined by effective charges 𝜁 = 1.05 and 𝜁 = 3.10 according to the formulae in eqn (6) in Ref.