Quite interestingly, despite the lack of complementarity between the HOMO and LUMO, the exchange energy is fairly small when the electron-withdrawing nature of the central N atom is large. As a result, a region with ST inversion is obtained for both molecules at large negative ϵ py N . Indeed, the best descriptor for ST inversion is not the complementarity of the frontier orbitals, but rather the value of the HOMO-LUMO exchange energy.