text: Arrhenius argued that for reactants to transform into products, they must first acquire a minimum amount of energy, called the activation energy E. At an absolute temperature T, the fraction of molecules that have a kinetic energy greater than E can be calculated from statistical mechanics. The concept of activation energy explains the exponential nature of the relationship, and in one way or another, it is present in all kinetic theories.
  The calculations for reaction rate constants involve an energy averaging over a Maxwell–Boltzmann distribution with  as lower bound and so are often of the type of incomplete gamma functions, which turn out to be proportional to .