The value of f(Eeq) should depend on the curvature, f", which itself controls the transition between the initial horizontal asymptote and the limit at positive infinity. The curvature of f cannot be constant (changing between two linear limits), and so it reaches a maximum at Eeq due to the symmetry constraints (Equation ). Due the even symmetry of f", the general Taylor expansion of f around Eeq must only comprise even exponents aside from the linear term: