The key to this relationship involves using a directed graph encoding of the PES, for examples, as shown in Fig. where each directed graph edge has a vibration associated with it (e.g., 6a transforms from the displacements of vibrational normal modes ν6 and ν13). Assuming an ensemble, appropriate statistics ("weights") can be assigned to each graph vertex (species) and their contribution to the thermodynamic entropy calculated. Given the reaction graph and its PES describe a whole system of interconvertible species, the corresponding formulation of the entropy contribution is also reflective of the full system and not just a single species. It is important to note that these expressions do not reflect the full entropy as other vibrational modes describing, for example, stereoisomeric configurations of each species, will contribute to a degree reflective of their relative energy regimes.