QUANTUM-MECHANICAL PRESSURE-DEPENDENT REACTION AND RECOMBINATION RATES FOR O-]H+O-2, HO2(OH)

We extend recent flux-flux autocorrelation function methods for the di rect computation of thermal reaction rate constants and unimolecular r ecombination rates to the case where both reaction and recombination a re possible. Rather than a single transition state dividing surface, d ividing surfaces are placed on both the reactant (r) and product (p) s ides of the intermediate collision complex region. The thermal recombi nation rate expression then involves a flux cross-correlation function C-rp(t) in addition to the usual autocorrelation function C-rr(t), bo th of which are computed during a single quantum time propagation. Thi s method is applied to the three-dimensional O + OH reversible arrow H + O-2 (J = 0) reactions, employing parallel computation because of th e necessary large basis (2(18) grid points) and long propagation times (2-3 ps). Thermal rate constants (in the absence of recombination eff ects) are presented for T = 500-2000 K, using the J-shifting approxima tion to account for nonzero total angular momentum; good agreement is found with experimental measurements of bath forward and reverse rate constants. Collisional recombination by a bath gas is included via the strong collision assumption, and rate constants for the competing O OH reaction (H + O-2) and recombination (HO2) channels are calculated as a function of collision frequency, i.e., pressure of the bath gas.