Quantum reaction dynamics of heavy-light-heavy systems: Reduction of the number of potential curves and transitions at avoided crossings

Two decoupling procedures are proposed within the framework of the hypersph erical coordinate approach in order to reduce the number of states in the c lose-coupling calculations and to clarify the reaction mechanisms. Sharply avoided adiabatic states are diabatically connected and relabeled without a ny diabatic coupling there. This is named diabatic decoupling and is useful for decoupling two manifolds of states belonging to different categories. Furthermore, the number of states is reduced in such a way that only a limi ted number of adjacent adiabatic states are taken into account in the close -coupling calculations for each relevant state. This is called adiabatic de coupling scheme. These reductions of the number of states enable us to anal yze reactions in terms of nonadiabatic transitions at avoided crossings amo ng the small number of adiabatic potential curves. The method is applied to O(P-3) + HCl --> OH + Cl and Cl + HCl --> HCl + Cl. The idea of vibrationa lly nonadiabatic transitions at avoided crossings together with the concept of potential ridge introduced in our previous paper can untangle the conge sted potential curves and clarify reaction mechanisms.