Perspective on "Intermolecular orbital theory of the interactions between conjugated systems". I General theory; II Thermal and photochemical cycloadditions - Salem L (1968) J Am Chem Soc 90 : 543, 553

At the time where a reaction path between molecules could not be calculated easily, the interaction between conjugated molecules was calculated using perturbation theory. The theory of Lionel Salem gave a very interesting app roximation to a reaction path between rather large conjugated systems.