A Ring Polymer Molecular Dynamics Approach to Study the Transition between Statistical and Direct Mechanisms in the H2 + H3 + → H3 + + H2 Reaction

Because of its fundamental importance in astrochemistry, the H + H → H + H reaction has been studied experimentally in a wide temperature range. Theoretical studies of the title reaction significantly lag primarily because of the challenges associated with the proper treatment of the zero-point energy (ZPE). As a result, all previous theoretical estimates for the ratio between a direct proton-hop and indirect exchange (via the H complex) channels deviate from the experiment, in particular, at lower temperatures where the quantum effects dominate. In this work, the ring polymer molecular dynamics (RPMD) method is applied to study this reaction, providing very good agreement with the experiment. RPMD is immune to the shortcomings associated with the ZPE leakage and is able to describe the transition from direct to indirect mechanisms below room temperature. We argue that RPMD represents a useful tool for further studies of numerous ZPE-sensitive chemical reactions that are of high interest in astrochemistry.Y.V.S. thanks the European Regional Development Fund and the Republic of Cyprus for support through the Research Promotion Foundation (Project Cy-Tera NEA YΠOΔOMH/ ΣTPATH/0308/31). S.G.-C., A.A., and O.R. acknowledge the support of the Ministerio de Economıá y Competitividad (SPAIN) under Grants FIS2014-52172-C2 and FIS2017- 83473-C2 and the support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 610256 (NANOCOSMOS). We also acknowledge the financial support of the European COST program, CM1401, “Our Astrochemical History”. The calculations have been performed at the CyI (Cy-Tera), CESGA (finisterrae2), UAM (CCC), and CSIC (trueno) supercomputer centers.Peer Reviewe