where ξ is a unitless reaction coordinate. The spiro cation is at an equilibrium geometry when ξ = ±0.5. For ξ = 0, an average of the two equilibrium structures is obtained, and this can be understood as the midpoint structure for intramolecular charge transfer. It has previously been shown that multireference perturbation theory can reasonable describe the PES only when going to the third order, and this shows that the spiro cation a very difficult system to treat. CMS-PDFT has been shown to predict the correct PES topology, 28 whereas XMS-PDFT and MC-PDFT fail in that they produce unphysical dips at ξ = 0 27 (Figure ). The L-PDFT and HL-PDFT potential curves are shown in Figure . Like CMS-PDFT, these methods do not have a dip at the high symmetry point ξ = 0, as is present in the MC-PDFT, HMC-PDFT, and XMS-PDFT potential curves 27 (Figure ). In fact, L-PDFT and HL-PDFT hardly differ at all from CMS-PDFT, even at the high-symmetry point, giving good quantitative agreement between the methods. Furthermore, the coupling term between these two states (H L-PDFT