This substitution helps to understand the nature of θ, which defines not only the curvature but the additional barrier associated to the thermoneutral process (or at Eeq for an anisodromic process). The barrier at ΔEr = Eeq, 𝐸 𝑒𝑞 ‡ , is itself a consequence of θ, which mainly defines the curvature of the energy relationship and as a result determines the activation energy at Eeq. The specific characteristics of a process that lead it to respond more or less sharply to changes in the reaction energy are the origin of this barrier. Nonetheless, the re-expression of the normalised transition probabilities as a function not of θ but of 𝐸 𝑒𝑞 ‡ (equation 17, from equation 7) -for an idealised isodromic system with no ΔEr-independent behaviour-shows that the energy response can also be thought of as an 𝐸 𝑒𝑞 ‡ -normalised binary partitioning between the forward and backward reaction. This new expression highlights how the implicit role of θ modulates energy barriers with respect to that at Eeq