3 ∆𝐸 𝑟 2 + 𝑂(∆𝐸 𝑟 4 ) and in particular, the first term is reminiscent of the outcome of the quadratic approximation proposed by an approximation which however only holds true for a isodromic reaction around ΔEr = 0, conditions that require equal unbiased response on the TS from both the reactants and products.