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.

In the interstellar medium, the H 2 adsorption and desorption on the solid water ice are crucial for chemical and physical processes. We have recently investigated the probabilities of H 2 sticking on the (H 2 O) 8 ice, which has quadrilateral surfaces. We have extended the previous work using classical MD and ringpolymer molecular dynamics (RPMD) simulations to the larger ice clusters, (H 2 O) 10 and (H 2 O) 12 , which have pentagonal and hexagonal surfaces, respectively. The H 2 sticking probabilities decreased as the temperature increased for both cluster cases, whereas the cluster-size-independent profiles were observed. It is thought that the size independence of the probabilities is qualitatively understood from the similar binding energies for all the three cluster systems. Furthermore, the RPMD sticking probabilities are smaller than the classical ones because of the reduction in the binding energies owing to nuclear quantum effects, such as vibrational quantization.

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