As indicated in the introduction, to gain qualitative insights into the main factors driving HAT reactivity, we took inspiration from qualitative Valence Bond (VB) theory. In a VB reactivity analysis, reactivity diagrams, reflecting the shape of the potential energy surface (PES) associated with a given chemical system along a chosen reaction coordinate, are constructed. Usually, one starts by examining the evolution of the so-called "diabatic curves", which represent the wave function associated with a limited number of VB structures; i.e., they correspond to a localized VB configuration (e.g., a specific Lewis structure of the reactants or products). These individual diabatic curves interact along the reaction coordinate, and collectively they give rise to the "adiabatic" curve, i.e., the full ground-state PES associated with the chemical system under consideration. The transition state then corresponds to the top of the hill of the adiabatic state, which usually lies in the same region as the crossing point between the reactant and product diabatic states. An in-depth discussion of the theory behind the construction VB reactivity diagrams is provided in Ref. . In Figure , a simplified sketch of such a diagram is presented for the model reaction between H 2 and H • . Along the reaction coordinate, the reactant diabatic state (blue) rises in energy (since the hydrogen atoms contributing electrons to the formal covalent bond on the left become increasingly separated as the reaction proceeds), the product state (red) drops in energy, since the product covalent bond becomes increasingly stabilized as the associated hydrogens approach. The adiabatic state (black) is the full ground-(or adiabatic) state resulting from the mixing of the localized diabatic states.