The rate constant expression from transition state theory can be used to calculate the Δ"G"‡, Δ"H"‡, Δ"S"‡, and even Δ"V"‡ (the volume of activation) using experimental rate data. These so-called "activation parameters" give insight into the nature of a transition state, including energy content and degree of order, compared to the starting materials and has become a standard tool for elucidation of reaction mechanisms in physical organic chemistry. The free energy of activation, Δ"G"‡, is "defined" in transition state theory to be the energy such that formula_22 holds. The parameters Δ"H"‡ and Δ"S"‡ can then be inferred by determining Δ"G"‡ = Δ"H"‡ – "T"Δ"S"‡ at different temperatures.