The likelihood of a reaction occurring can be determined using the activation energy. Regioselectivity can be determined by calculating the activation energy for the rate limiting step of the reaction at each possible site of reaction in the molecule. The ratio of each regioisomer is then found by inputting these activation energies into the Arrhenius equation. A large difference in activation energy between sites corresponds to a large proportion of the isomer with the lowest E_a and small amounts of product with substitution at higher E_a sites. A smaller difference in activation energies between sites leads to a mixture of regioisomers.

Currently activation energies are calculated at an AM1 level of theory. In future this will be updated to use HF/6-31G* derived energies which have been tested to more accurately reflect experimentally observed regioselectivity.