Thermal and state-selected rate coefficients for the O(P-3)+HCl reaction and new calculations of the barrier height and width

This paper compares several approximate methods for calculating rate coeffi cients for the O(P-3) + HCl reaction to presumably more accurate quantum me chanical calculations that an based on applying the J-shifting approximatio n (QM/JS) to an accurate cumulative reaction probability for J = O, All cal culations for this work employ the recent S4 potential energy surface, whic h presents a number of challenges for the approximate methods. The O + HCl reaction also poses a significant challenge to computational dynamics becau se of the heavy-light-heavy mass combination and the broad noncollinear rea ction path. The approximate methods for calculating the thermal rate coeffi cient that are examined in this article are quasiclassical trajectories (QC T), conventional transition state theory (TST), variational transition stat e theory employing the improved canonical variational theory (ICVT), ICVT w ith the microcanonical optimized multidimensional tunneling correction (ICV T/mu OMT), and reduced dimensionality quantum mechanical calculations based on adiabatic bend and J-shifting (QM/AB-JS) approximations. It is seen tha t QCT, TST, and ICVT rate coefficients agree with each other within a facto r of 2.7 at 250 K and 1.6 at 1000 K, whereas inclusion of tunneling by the ICVT/mu OMT, QM/AB-JS, or QM/JS methods increases the late coefficients con siderably. However, the ICVT/mu OMT and QM/AB-JS methods yield significantl y lower rate coefficients than the QM/JS calculations, especially at lower temperatures. We also report and discuss calculations for the state-selecte d reaction of O(P-3) with HCl in the first excited vibrational state. In ad dition to the dynamics calculations, we report new electronic structure cal culations by the Multi-Coefficient Gaussian-3 (MCG3) method that indicate t hat one possible source of disagreement between the QM/JS rate coefficients and experiment is that the barrier on the S4 surface may be too narrow.