Overview of reduced dimensionality quantum approaches to reactive scattering

In this overview I discuss recent advances as well as outstanding issues in reduced dimensionality quantum approaches to reactive scattering. "Reduced dimensionality" in the present context signifies treating a subset of all degrees of freedom (the most strongly coupled ones) by rigorous quantum methods and treating the remaining (weakly coupled) degrees of freedom by a variety of approximate methods, ranging from simple, so-called energy shifts to more elaborate adiabatic treatments. The most widely used example of this approach is termed "J-shifting", and this overview will concentrate on this method and discuss its application and generalization to both "direct" and "complex" reactions, exemplified by O(P-3) + HCl and O(D-1) + HCl, respectively. In addition, for O(P-3) + HCI, resonances in the tunneling region, due to van der Waals wells, are discussed and their challenge to reduced dimensionality methods is stressed. Another new aspect of the reduced dimensionality treatment of polyatomic reactions is the need to describe anharmonicity in a consistent fashion. This is exemplified by the H + CH4 reaction.