The reaction pathway (RP) technique is a powerful tool in theoretical chemistry and chemical reaction theory. The present book discusses the background of the concept, both mathematical and physical, and outlines new developments. Different approaches to the RP are described, with particular emphasis on gradient extremals and the treatment of zero eigenvalues of rotation in the Cartesian Hessian matrix along the RP. There is also a review of geometric optimization methods, together with an outline of density functional theory. Some contributions reveal the progress made in interface dynamics calculations based on RP potentials and tunnelling with electronic structure theory ("direct dynamics"). The study also includes the latest results in excited-state RP calculations (H transfer) and a theoretical view of the RPs of photodissociation processes using time-resolved femtosecond spectroscopy. The passage from the RP to the reaction mechanism is described by means of fundamental groups and symmetry rules.
