Gibbs energy of activation (standard free energy of activation), Δ ‡ G o

https://dx.doi.org/10.1351/goldbook.G02631
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 constantkvia the conventional form of the absolute rate equation:
Δ ‡ G = R T ln ( k B h ) − ln ( k T )
where k B is the Boltzmann constant and h the Planck constant (k B h = 2.083 58 × 10 10 K −1 s −1). 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).
Source:
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1118
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 166