Robust binding energy distribution sampling on amorphous solid water models

Method testing and validation with NH₃, CO, and CH₄

¹ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, University of Liège, Quartier Agora, 19c, Allée du 6 Aôut, B5c, 4000 Sart Tilman, Belgium
² Theoretical Chemistry Lab, Unit of Theoretical and Structural Physical Chemistry, Namur Institute of Structured Matter, University of Namur, Rue de Bruxelles, 61, 5000 Namur, Belgium

^★ Corresponding author.

Received: 9 April 2025
Accepted: 24 April 2025

Abstract

Context. The astrochemically efficient icy mantles surrounding dust grains in molecular clouds have been shown to be of an amorphous water-rich nature. This therefore implies a distribution of binding energies (BEs) per species instead of a single value. Methods proposed so far for inferring BEs and their distributions on amorphous ices rely on different approaches and approximations, leading to disparate results or BE dispersions with partially overlapping ranges.

Aims. This work aims to develop a method based on a structurally reliable ice model and a statistically and physicochemically robust approach to BE distribution inference, with the aim of being applicable to various relevant interstellar species.

Methods. A multiscale computational approach is presented, with a molecular dynamics heat and quench protocol for the amorphous water ice model, and an ONIOM(B3LYP-D3(BJ)/6-311+G(d,p):GFN2-xtb) scheme for the BE inference, with a prime emphasis onto the convergence of the computed BEs with the real system size. The sampling of the binding configurations is twofold, exploring both regularly spaced binding sites as well as various adsorbate-to-substrate orientations on each locally distinct site. This second source of BE diversity accounts for the local roughness of the potential energy landscape of the substrate. Three different adsorbate test cases are considered, NH₃, CO, and CH₄, owing to their significance in icy dust mantles, and their distinct binding behavior with water ices.

Results. The BE distributions for NH₃, CO, and CH₄ have been inferred with converged statistics. The distribution for NH₃ is better represented by a double Gaussian component profile. Three starting adsorbate orientations per site are required to reach convergence for both Gaussian components of NH₃, while two orientations are sufficient for CO , and one unique one for CH₄ (symmetric). Further geometrical and molecular surrounding insights have been provided. These results encompass previously reported results.