To observe ST inversion, spin polarization is needed to bring the energy of the lowest excited singlet below the energy of the lowest triplet in systems with small HOMO-LUMO exchange energy. Spin polarization is driven by electron correlations, and cannot be modeled in single-CI approaches. Accordingly, TD-DFT, arguably the most widely applied quantum chemical approach to excited states in molecular systems, fails to describe the ST inversion that requires at least double excitations. By exploiting the PPP model, we investigated the role of higher order CIs, showing that reliable results on inverted ST systems can only be obtained if at least triple excitations are accounted for (see Fig. ).