Ring-Polymer Molecular Dynamics Calculations of Thermal Rate Coefficients and Branching Ratios for the Interstellar H₃⁺ + CO → H₂ + HCO⁺/HOC⁺ Reaction and Its Deuterated Analogue

Abstract

The reaction between H3 + and CO is important in understanding the H3 + destruction mechanism in the interstellar medium. In this work, thermal rate coefficients for the H3 + + CO and D3 + + CO reactions are calculated using ring-polymer molecular dynamics (RPMD) on a high-level machine-learning potential energy surface. The RPMD results agree well with the classical molecular dynamics results, where nuclear quantum effects are completely ignored, whereas the agreement between the RPMD results and the previous quasi-classical trajectory is good only at low temperatures. The calculated [HCO+]/[HOC+] product branching ratios decrease as the temperature increases, and the product branching is exclusively determined by the initial collisional orientation, which governs the formation of an ion–dipole complex, H3 +···CO or H3 +···OC, that dissociates into H2 + HCO+ or H2 + HOC+, respectively, via a direct mechanism. However, the contribution of the indirect mechanism via the rearrangement between H3 +···CO and H3 +···OC increases as the temperature increases, although its absolute fraction is small.

Ring-Polymer Molecular Dynamics and Kinetics for the H– + C2H2 → H2 + C2H– Reaction Using the Full-Dimensional Potential Energy Surface

“…The RPMD method is a powerful tool for expressing reaction dynamics to represent nuclear quantum effects, such as zero-point energy and tunneling . The accuracy of the RPMD calculations has been examined extensively for gas-phase bimolecular chemical reactions and condensed-phase diffusion processes. − The RPMD method used in this study is the direct trajectory approach, which is a similar approach on the dRPMD program, a direct version of the RPMD method based on the RPMDrate code . Our RPMD procedure has two steps, thermalization and real-time dynamics, similar to the dRPMD program.…”

Ring‐polymer Molecular Dynamics Simulation for the Adsorption of H2 on Ice Clusters (H2O)n (n=8, 10, and 12)

“…The method has been widely utilized and has successfully described chemical reactions such as collision processes in the gas phase and diffusion processes in the condensed phase. [12,[25][26][27][28][29][30][31][32][33][34][35] Our RPMD method employs the direct trajectory technique, which is similar to the dRPMD program [36] implemented in RPMDrate code. [37] For our RPMD approach, we performed thermalization and implemented real-time dynamics on the open-source PIMD Ver.…”

“…Further details of the collision procedure have been presented in our previous H 2 + (H 2 O) 8 , H 3 + + CO, and H À + C 2 H 2 studies. [12,34,35] Classical MD simulations were also performed in essentially the same way as the PIMD/RPMD calculations mentioned above. The classical Hamiltonian H p;r ð Þ has the conventional form for the n-atom system:…”

“…Nuclear quantum effects, such as zero‐point energy and tunneling [23,24] are represented by the RPMD method. The method has been widely utilized and has successfully described chemical reactions such as collision processes in the gas phase and diffusion processes in the condensed phase [12,25–35] . Our RPMD method employs the direct trajectory technique, which is similar to the dRPMD program [36] implemented in RPMDrate code [37] .…”

“…After the first impact between H 2 and (H 2 O) n , the EOM was integrated up to 3 ps. Further details of the collision procedure have been presented in our previous H 2 +(H 2 O) 8 , H 3 + +CO, and H − +C 2 H 2 studies [12,34,35] …”