![Binding energies of CO, CO2 and NH3 on pristine and cavity-containing ice clusters consisting of 100 and 97 H2O molecules, respectively. The error bars represent the spread in adsorption energies across the three clusters. Dispersion energy as fraction of the total binding energy per species, site and functional. -- [astro-ph.GA]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/09/How-Small.png?fit=1200%2C676&ssl=1)
[astro-ph.GA] Binding energies of molecules on ice mantles are important to understand the evolution of molecular complexity in molecular clouds.
Binding energies are often computed using density functional theory (DFT) calculations, typically on either small amorphous ice clusters or crystalline slabs. Since these calculations require an accurate description of the electronic structure, hybrid functionals with dispersion corrections, basis set superposition error corrections and zero point energy corrections are typically employed.
This, however, comes at a high computational cost, so most often small ice clusters are considered, frequently containing no more than twenty water molecules or so. While several recent studies have explored binding energy distributions, few have addressed the importance of substrate size.
We here perform DFT calculations, using six different functionals, on ice clusters containing 10 to 100 H2O molecules to quantify the ice cluster size effect and to separate the electronic from the geometrical contribution. As probe molecules, we use CO, CO2 and NH3. These calculations demonstrate that, irrespective of the molecule and functional used, interaction energies only start to converge from thirty to forty water molecules onwards.
The dispersion energy flattens out earlier, whereas induction and polarization effects require larger clusters to stabilize, particularly at structurally confined (cavity) sites. We conclude that ice cluster sizes of at least 30-40 water molecules are needed to obtain reliable binding energies, and is as important as the choice of the functional.
Erik C. Neyts, Christopher King, Irina Grubova, Tobe Vorsselmans
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2609.17036 [astro-ph.GA] (or arXiv:2609.17036v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2609.17036
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Submission history
From: Erik Neyts
[v1] Tue, 15 Sep 2026 11:41:38 UTC (2,564 KB)
https://arxiv.org/abs/2609.17036
Astrobiology,
