Comparison of the combined phase-space/trajectory and quasiclassical trajectory methods in the study of reaction dynamics: H + I2 and H + Br2

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Abstract
Results for the combined phase−space/classical trajectory (CPST) and standard quasiclassical trajectory (SQCT) procedures for investigating the dynamics of homogeneous gas−phase reactions are compared in the case of H + X2 → HX + X (X = Br or I). The relative absence of dynamical effects for these systems permits SQCT calculations with low statistical error and would seem to enhance the validity of the basic assumptions of CPST. The high reduced mass of X2 also reduces differences between CPST and SQCT that arise from quantization of internal X2 states in the latter method. When the critical phase−space surface is chosen so that its configuration−space projection traverses only regions of low three−body interaction (< 2 kcal/mole), the CPST computed rate coefficients and angular distributions are in qualitative accord with SQCT results. When phase−space points on the critical surface are integrated back into the reactant phase space, nonequilibrium distributions are obtained for the distribution of relative translational energies and for the initial H−I2 angular orientation. If the critical surface is altered so that its configuration−space projection traverses regions of larger three−body interaction, differences between the CPST and SQCT results are found to increase if the probability density is not weighted by the square of the BC distance. Finally, it is demonstrated that the combined method may require more computation time than for a standard trajectory calculation of the same accuracy.