Initial conditions and paths in semiclassical tunneling

The choice of initial conditions and tunneling paths for semiclassical tunneling are investigated for a two-dimensional double-well system which is often used for modeling hydrogen tunneling in unimolecular isomerization processes. Tunneling effects are incorporated in a standard trajectory simulation. The exact quantum mechanical calculations are performed for comparison. We find that tunneling splittings are in close agreement for initial conditions obtained by using adiabatic switching and normal-mode sampling methods. These results suggest that the approximate normal-mode sampling can be used to compute accurate level splittings. We also find that simple straight-line tunneling paths work well from the sudden to intermediate regimes. However, the method is less accurate for the intermediate cases. (C) 1997 American Institute of Physics.