H Atom Near the Amorphous H ₂ O Surface: The Revision of Binding Energy Estimations

Formation of molecular hydrogen is a fundamental process that controls the birth of stars and planetary systems. H2 is formed efficiently on the surfaces of interstellar dust grains, which are typically covered by amorphous water ice in cold clouds. The binding energy (Eb) of atomic hydrogen (H) on amorphous solid water (ASW) ice is a fundamental parameter for astrochemical models, governing key processes, such as adsorption, diffusion, and desorption. However, published estimates vary widely between 76 and 680 K, introducing significant uncertainty into simulations. In this work, we present a systematic and transparent hierarchical approach to determine Eb(ASW-H). Starting from ab initio calculations (MP2/aug-cc-pVTZ) of the H2O–H dimer, we progressively increase the complexity of the system by considering plane water clusters (H2O)n (n = 3–6) and planar ASW surface models comprising up to 16 water molecules. A novel method to generate realistic, nonoptimized amorphous monolayers is introduced, allowing us to sample the morphological heterogeneity of the ice surface. The calculated binding energies exhibit a clear saturating trend with the cluster size, which we fit with an exponential function to extrapolate to a macroscopic surface. Our final estimate, Eb(ASW-H) = 450 ± 90 K, is derived by averaging over multiple surface realizations and accounts for the inherent site heterogenity. This value resolves the discrepancy between earlier theoretical and experimental proxies, shows excellent agreement with the recommended value in the KIDA database, and provides a robust input for next-generation astrochemical models. The methodology outlined here is readily extensible to other astrophysically relevant atoms and molecules (C, N, O, and CO), paving the way for a more accurate and self-consistent description of grain surface chemistry.