In order to reach a model dependent on physically meaningful parameters, the question must be approached in the most general way possible by considering the properties common to any kind of chemical reaction. For a given set A of similar reactions, the function should map every reaction energy in the set to its activation energy (Equation ), where ΔEr is the reaction energy and ΔE ‡ is the activation energy: This non-linear function would have to satisfy a number of physically meaningful requirements that define its properties, particularly those concerning the symmetry of the function and its derivatives. The origin of these constraints would arise from the fundamental relationship between the energies of a given reaction and its reverse (Figure ). By considering how the energy barriers and reaction energies invert between a general forward and reverse reaction, the following constraints on the nature of the non-linear relationship are obtained: