Importance of correlation energy in collision dynamics: quasiclassical trajectory study of collinear He+H₂⁺(υ')→HeH⁺+H using HF and CI potential-energy surfaces

Abstract

We have investigated the need to include correlation energy in potential-energy surface calculations by studying the collinear He+H₂⁺(υ')→HeH+(υ)+H reaction dynamics as a test case. We have utilised the Hartree-Fock (HF) and configuration interaction (CI) potential-energy surfaces for this system recently published by McLaughlin and Thompson. The quasiclassical trajectory method has been used to follow the molecular dynamics. There are quantitative and even some qualitative differences in the reaction probability results on the two surfaces. The state (υ')-selected reaction probability (P_υ'^R) is generally larger on the CI than on the HF surface. Increase in υ' from 0 to 1 results in a marginal increase in P_υ'^R on the former and a substantial increase on the latter; further increase in υ' results in a dramatic decrease in P_υ'^R on the CI and not on the HF surface. The average product vibrational energy (V) and enhancment of V with increase in reagent vibrational energy (V') are also sensitive to the inclusion of correlation energy for υ'. Quasiclassical trajectory results for the reverse (exothermic) reaction show much larger V' on the HF than on the CI surface in agreement with the larger vibrational enhancement for the endothermic reaction noticed on the former than on the latter. Interestingly, experimental results on the effect of reagent vibration on the reaction cross section for the endothermic reaction are in accord, qualitatively, with the results on the HF surface and not on the more accurate CI surface demonstrating the inadequacy of the collinear model for this endothermic reaction. The state-selected and overall rate constants on the two surfaces are in good agreement with each other, when accounted for the difference in the barrier height for the endothermic reaction on the two surfaces, except for υ'=0. We have also examined the effect of changing the reagent mass combination on the importance of correlation energy on collision dynamics. For the H+LH and L+HH (H=80 amu, L=1 amu) mass combinations we find the reaction probabilities to be too small (for V' corresponding to υ'=0 and 3 for H₂⁺) to be distinguished for the two surfaces. For the H+HL mass combination, the results are qualitatively the same as for the He+H₂⁺ mass combination. In the case of nonreactive collisions, the energy dependence of the probability (P_0→1) for υ'=0→1 transition is distinctly different on the two surfaces. However, the rate constants (k_0→1) and P_0→−2 and P_0→3 are relatively insensitive to the inclusion of correlation energy.