ON THE DIRECT CALCULATION OF THERMAL RATE CONSTANTS .2. THE FLUX-FLUXAUTOCORRELATION FUNCTION WITH ABSORBING POTENTIALS, WITH APPLICATION TO THE O-]OH+CL REACTION(HCL)

We present a method for obtaining the thermal rate constant directly ( i.e., without first solving the state-to-state reactive scattering pro blem) from the time integral of the Aux-flux autocorrelation function, C-ff(t). The quantum mechanical trace involved in calculating C-ff(t) is efficiently evaluated by taking advantage of the low rank of the B oltzmannized flux operator. The time propagation is carried out with a Hamiltonian which includes imaginary absorbing potentials in the reac tant and product exit channels. These potentials eliminate reflection from the edge of the finite basis and ensure that C-ff(t) goes to zero at long times. In addition, the basis can then be contracted to repre sent a smaller area around the interaction region. We present results of this method applied to the O + HCl reaction using the J-shifting an d helicity conserving approximations to include nonzero total angular momentum. The calculated rate constants are compared to experimental a nd previous theoretical results. Finally, the effect of deuteration (t he O + DCl reaction) on the rate constant is examined. (C) 1997 Americ an Institute of Physics.