Nuclear Quantum Effects in H₂ Adsorption Dynamics on a Small Water Cluster Studied with Ring-Polymer Molecular Dynamics Simulations

Abstract

Molecular hydrogen H2 is the most abundant molecule in dense interstellar clouds. To understand the role of H2 in the chemical and physical processes in astrochemical modeling, understanding the sticking probabilities of H2 to the water ice surface is important. In this work, we calculate H2 sticking probabilities for a small cluster consisting of eight water molecules using both the quantum ring-polymer molecular dynamics and classical molecular dynamics simulation methods to understand nuclear quantum effects in the H2 adsorption dynamics. The calculated sticking probabilities decrease with the increase in temperature for both the quantum and classical results. The sticking probabilities calculated using the present small cluster model are comparable with those obtained from the classical simulations using a much larger water ice model. This suggests that the H2 sticking probability is largely determined by the local nature of the H2–water interaction. Furthermore, nuclear quantum effects slightly decrease the H2 sticking probabilities because of a weaker binding energy due to vibrational quantization.

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

“…[22] The function can represent a proper PES including the terms up to three-body interaction and a suitable description of weak van der Waals interactions. Previously, the H 2 -(H 2 O) 8 problem was investigated by our group [12] using the PES. The collision dynamics showed reliable results using the RPMD method, which is introduced in detail below.…”

“…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.…”

“…Note that the RPMD simulation for H 2 -(H 2 O) 8 has already been performed. [12] The initial coordinates and momenta have been sampled by PIMD thermal sampling for each temperature. During the PIMD simulation corresponding to the NVT ensemble, the distance between the center of mass (COM) of H 2 and the COM of the water cluster was maintained at 15 Å using the harmonic constrained potential.…”

“…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:…”

“…Recently, H 2 adsorption processes on small (H 2 O) 8 clusters have been investigated by our group. [12] We have shown that the nuclear quantum effect including the zero-point energy and temperature dependence of the adsorption plays an important role by carrying out ring-polymer molecular dynamics (RPMD) simulations, [13][14][15][16][17] which are a semiclassical description for realtime dynamics. However, this study was performed using only the (H 2 O) 8 cluster with D 2d cubic-like structure.…”