The standard Gibbs energy difference between the transition state of a reaction (either an elementary reaction or a stepwise reaction) and the ground state of the reactants. It is calculated from the experimental rate constant k via the conventional form of the absolute rate equation: \[\Delta ^{\ddagger}G = R\ T\ \left[\ln (\frac{k_{\text{B}}}{h}) - \ln (\frac{k}{T})\right]\] where kB is the Boltzmann constant and h the Planck constant (kB/h = 2.08358 x 10E10 K-1 s ). The values of the rate constants, and hence Gibbs energies of activation, depend upon the choice of concentration units (or of the thermodynamic standard state).