We present an efficient method to obtain initial state-selective cross sections for bimolecular reactions that can account for certain nuclear quantum effects by employing the ring polymer molecular dynamics approach. The method combines the well known quasiclassical trajectory (QCT) approach with the description of the system in an extended ring polymer phase space. Employing the prototypical Mu/H/D + H 2 (v = 0, 1) reactions as a benchmark, we show that the presented approach does not violate zero-point energy constraints and that it can also capture the contributions of tunneling through the v = 1 vibrationally adiabatic barrier present for the Mu + H 2 (v = 1) reaction. This is a significant improvement over the QCT approach with only a small increase in numerical cost.
The inclusion of nuclear quantum effects (NQEs) in molecular dynamics simulations is one of the major obstacles for an accurate modeling of molecular scattering processes involving more than a couple of atoms. An efficient method to incorporate these effects is ring polymer molecular dynamics (RPMD). Here, we extend the scope of our recently developed method based on non-equilibrium RPMD (NE-RPMD) from triatomic chemical reactions to reactions involving more atoms. We test the robustness and accuracy of the method by computing the integral cross sections for the H/F + CH4/CHD3 reactions where the methane molecule is either initially in its vibrational ground or excited state (C–H stretch). Furthermore, we analyze the extent to which NQEs are described by NE-RPMD. The method shows significant improvement over the quasiclassical trajectory approach while remaining computationally efficient.
Understanding the influence of different forms of energy (eg, translational, vibrational, rotational) on chemical reactions is a key goal and great challenge in physical chemistry. Very recently, we proposed a new approach to obtain state-selective cross sections that approximately include quantum effects such as zero-point energy and tunneling. The method is a combination of the widely used quasiclassical trajectory approach (QCT) and the ring polymer molecular dynamics method and thus is numerically very efficient and easily employed. Here, we present a detailed description of the method and exhaustive tests of its accuracy and applicability. The robustness of the approach is tested, as well as the convergence with the number of beads. The approach is then applied to several prototypical X + H 2 (ν = 0, 1), X = Mu, H, D, F, Cl reactions over a wide range of collision energies. Good agreement with rigorous quantum dynamics simulations is found for most cases. Encouraging improvement over QCT results is found for particular cases, while only a small increase in numerical cost is required.
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