As depicted in Figure , various paths within the trapezoid region are conceivable, limited only by the available energy and curvature of the PES. For instance, in all HAT scenarios, the transition structure will lie along the ⃗ p and ⃗ d vectors. Should the direct path over the TS be prohibitively high in energy, an alternative lower-energy path such as the sketched Sshaped path in Figure might be taken. It is therefore likely that distinct kinetic and thermodynamic regimes for the CPET and HAT scenarios will exist. When comparing with experimental data, tunneling of the proton, and possibly of heavy atoms as well, should be considered.