Where does the barrier come from

The barrier always results from the intersection of diabatic potential energy hypersurfaces. We may think of diabatic states as preserving the electronic state (e.g., the qfstem of chemical bonds) I and II, respectively. 

Sometimes it is said that the barrier results from an avoided crossing (cf. Chapter 6) of two diabatic hypersurfaces that belong to the same irreducible representation of the symmetry group of the Hamiltonian (in short of the same q mmetry ). This, however, caimot be taken literally, because, as we know from Cliapter 6, the non-crossing rule is valid for diatomics only. The solution to this dilemma is the conical intersection described in Chapter 6 (cf. Fig. 6.15). Instead of diabatic we have two adiabatic hypersurfaces ( upper and lower ), each consisting of the diabatic part I and the diabatic part II. A thermic reaction takes place as a rule on the lower hypersurface and corresponds to crossing the border between I and n. 

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