Minimum Energy Path Diffusion Monte Carlo Approach for Investigating Anharmonic Quantum Effects: Applications to the CH₃⁺ + H₂ Reaction

Abstract

A method for evaluating anharmonic corrections to energies along a minimum energy path is developed and described. The approach is based upon the Diffusion Monte Carlo theory as initially developed by Anderson. Diffusion Monte Carlo has been shown to be effective for evaluating the ground-state properties of highly anharmonic systems. By using Jacobi coordinates, the evaluation of anharmonic corrections to the energies along a minimum energy path are straightforward to implement using Diffusion Monte Carlo. In this work, the CH₃⁺ + H₂ → CH₅⁺ reaction and its singly deuterated analogues are investigated. In addition to exploring how the energetics of this reaction change upon partial deuteration, projections of the probability amplitude onto various internal coordinates are evaluated and used to provide a quantum mechanical description of how deuteration affects the orientation of the two fragments as they combine to form the CH₄D⁺ molecular ion