WHAT IS THE BEST SEMICLASSICAL METHOD FOR PHOTOCHEMICAL DYNAMICS OF SYSTEMS WITH CONICAL INTERSECTIONS

We present a systematic test of four general semiclassical procedures for the theoretical treatment of multistate molecular processes such a s electronically nonadiabatic photochemical reactions. The methods are tested by comparing their predictions to accurate quantal results for three two-state model reactions involving conical intersections. The four methods tested are Tully's fewest-switches version of trajectory surface hopping (1990), the Blais-Truhlar trajectory surface hopping m ethod (1983), the Ehrenfest scheme (1975-1979), and the Meyer-Miller m ethod (1979). We test the ability of the classical path methods to pre dict both electronic probabilities and product rovibrational distribut ions. For each of the four basic approaches we test six options for ex tracting final-state information from the calculated dynamics. We find that, although in most cases there is qualitative agreement between a verage quantum mechanical and trajectory results, the overall average error is about 50% for Tully's fewest-switches method, the Ehrenfest m ethod, and the Meyer-Miller method, and even higher, about 60%, for th e Blais-Truhlar method. These values do not include additional errors in the below-threshold regions, which are especially large for the Mey er-Miller method because of the electronic zero-point energy in the Me yer-Miller classical analog Hamiltonian. (C) 1998 American Institute o f Physics.