Driving and retarding forces in a chemical reaction

The reaction force F(ξ) is the negative gradient of the potential energy of a chemical process along the intrinsic reaction coordinate ξ. We extend the rigorous concept of F(ξ) to the "activation strain model" of Bickelhaupt et al., to formulate the "strain" force F _str(ξ) that retards a reaction and the "interaction" force F_int(ξ) that drives it. These are investigated for a group of Diels-Alder cycloadditions. The results fully support the interpretation of the minimum of F(ξ) as defining the beginning of the transition from deformed reactants to eventual products.