We observe that a first-order relationship generally corresponds to a change of ±1 negative eigenvalues of the species' Hessian matrix where these values correspond to the associated bond stretches (e.g., LM ⇌ TS [6a ⇌ 7a], or TS ⇌ 2S [7a ⇌ 8a], see Fig. ). Similarly, a second-order relationship typically corresponds to ±2 negative bond-stretching Hessian eigenvalues (e.g., LM ⇌ 2S [6a ⇌ 8a], see Fig. ). This simple correspondence though is not followed for some pericyclic reactions, e.g., Diels-Alder reactions (Further details are given in the Discussion and Supplementary Information).