THE EFFECT OF NONADIABATIC COUPLING ON THE CALCULATION OF N(E,J) FOR THE METHANE ASSOCIATION REACTION

A classic example of a barrierless reaction, CH3+H-->CH4 is used as a model to test the extent of nonadiabatic coupling on the reaction rate constant. This coupling has two contributions. The first arises from the anisotropy of the CH3+H potential and the second is Coriolis coupl ing. A method is presented which calculates adiabats formally equivale nt to those calculated by statistical adiabatic channel model (SACM) w hile still permitting the determination and inclusion of nonadiabatic coupling. Using the discrete variable representation (DVR) for the int erfragment distance R, the re-vibrational Hamiltonian is solved at par ticular R values. The eigenvalues and eigenfunctions are calculated at each of these R values to create the surface and the coupling element s used in a wave packet propagation. The dynamics of the reaction are investigated through a study of the cumulative reaction probability N( E,J) using energy resolved flux methods. We find that for J=0, 1, and 2, neglecting the coupling due to the changing anisotropy as a functio n of R results in a 20% error in N(E,J). Neglecting the Coriolis coupl ing results in average errors of 2% lending support to the helicity-co nserving approximation. Finally, within the adiabatic approximation, t he calculated adiabats provide a more realistic view of the barriers t han the analytic functions of SACM, require no fitting parameter, and are obtained at reasonable computational cost. (C) 1998 American Insti tute of Physics. [S0021-9606(98)01344-0].