Artificial thermalization in ring-polymer molecular dynamics: The breakdown of RPMD for gas-phase reactions with pre-reactive complexes and how to fix it

Ring-polymer molecular dynamics (RPMD) has become a popular method for describing chemical reactions due to its ability to simultaneously capture tunneling, zero-point energy, anharmonicity, and recrossing. Here, we highlight that despite its many successes, great care must be taken when applying RPMD to study gas-phase reactions at low pressure. We show that, for bimolecular reactions that proceed via pre-reactive complexes, RPMD predicts spuriously large rates at low temperatures and pressures. Using the rigorous connection between RPMD and semiclassical instanton theory, we demonstrate that this breakdown can be understood in terms of an intrinsic problem with RPMD that we call "artificial thermalization." In the present context, this opens up reactive channels below the reactant asymptote that should be energetically inaccessible, resulting in erroneously large rates. We discuss practical strategies to overcome this problem by combining the steepest-descent inverse Laplace transform with Bleistein's uniform approximation to calculate the thermal rate given an appropriate lower energy bound.