The Hierarchy and Competition of Reaction Theories

One fundamental development of reaction theory is illustrated in Fig.3 when following the anti-clockwise direction. Beginning with the TST we have an increase in the level of sophistication as far as the area of dynamic quantum mechanics, and in the same direction a strong decrease in the applicability of the methods to polyatomic systems. In the clockwise direction (r.h.s. of Fig.3) the first methods require knowledge of the whole, analytically described PES of small chemical 

At the bottom of the cycle one observes a free field for new developments of the theory, coming from both the left as well as from the right side. 

On the l.h.s. of Fig.3, the theory with the most reduced input of PES information is the conventional which only requires the exact knowledge of minima (educts, intermediates) and saddle points of index 1 (SPl, transition structure) on a RP. In the conventional TST, 

This is of particular interest for reactions along a RP without a SP, i.e. reactions on attractive PES , e.g. potentials of unimolecular bond fission processes and the reverse bimolecular recombinations, ion molecule reactions or a large number of proton transfer reactions in 

The VTST approach seems to be the most basic level of an actual dynamic theory, which is capable of outlining the entropy barriers. 

---