where H 11 and H 22 are the valence bond descriptions of the reactant and product state respectively, and H 12 is the coupling parameter. The H 11 and H 22 potentials are usually modeled using force field descriptions U reactants and U products. H 12 is a bit trickier as it needs to be parameterized using a reference reaction. This reference reaction can be experimental, typically from a reaction in water or other solvents. Alternatively quantum chemical calculations can be used for calibration.