High level ab initio binding energy distribution of molecules on interstellar ices: Hydrogen fluoride

TL;DR

This study develops a new ab initio molecular dynamics method to accurately determine the binding energy distribution of molecules, specifically hydrogen fluoride, on interstellar ices, aiding astrochemical modeling.

Contribution

The paper introduces a robust ab initio approach validated by high-level methods to systematically analyze molecule-ice interactions, providing detailed binding energy distributions.

Findings

13 unique binding structures of HF on water clusters identified.

Average binding energy of HF on amorphous water surface is 5313 K.

Electrostatic interactions significantly influence binding energies depending on site properties.

Abstract

The knowledge of the binding energy of molecules on astrophysically relevant ices can help to obtain an estimate of the desorption rate, i.e. the molecules residence time on the surface. This represents an important parameter for astrochemical models, crucial to determine the chemical fate of complex organic molecules formed on dust grains and observed in the densest regions of the interstellar medium. In this work, we propose a new robust procedure to study the interaction of atoms and molecules with interstellar ices, based on \textit{ab initio} molecular dynamics and density functional theory, validated by high-level \textit{ab initio} methods at a CCSD(T)/CBS level. We have applied this procedure to hydrogen fluoride (HF), a promising tracer of the molecular content of galaxies. In total we found 13 unique equilibrium structures of HF binding to small water clusters of up to 4 molecules, with binding energies ranging from 1208 to 7162 K. We computed a 22-molecules model of amorphous solid water (ASW) surface using \textit{ab initio} molecular dynamics simulations and carried out a systematic analysis of the binding sites of HF, in terms of binding modes and binding energies. Considering 10 different water clusters, we found a binding energy distribution with an average value of $5313\pm74$ K, and a dispersion of $921\pm115$ K. Finally, the effect of the electrostatic field of the 22 water molecules on the binding energies was investigated incrementally by symmetry adapted perturbation theory, in order to gauge the effect of the water environment. The results indicate that the extent of the electrostatic interaction of HF with ASW depends strongly on the properties of the binding site. We expect that this work will provide a solid foundation for a systematic development of a binding energy distribution database of molecules on interstellar surfaces.