Computer Generated Realistic Interstellar Icy Grain Models: Physicochemical Properties and Interaction with NH₃

Abstract

Interstellar grains are composed by a rocky core (usually amorphous silicates) covered by an icy mantle, the most abundant molecule being H 2 O followed by CO, CO 2 , NH 3 , and also radicals in minor quantities. In dense molecular clouds, gas-phase chemical species freeze onto the grain surface, making it an important reservoir of molecular diversity/complexity whose evolution leads to interstellar complex organic molecules (iCOMs). Many different models of water clusters have appeared in the literature, but without a systematic study on the properties of the grain (such as the H-bonds features, the oxygen radial distribution function, the dangling species present on the mantle surface, the surface electrostatic potential, etc.). In this work, we present a computer procedure (ACO-FROST) grounded on the newly developed semiempirical GFN2 tight-binding quantum mechanical method and the GFN-FF force field method to build-up structures of amorphous ice of large size. These methods show a very favorable accuracy/cost ratio as they are ideally designed to take noncovalent interactions into account. ACO-FROST program can be tuned to build grains of different composition mimicking dirty icy grains. These icy grain models allow studying the adsorption features (structure, binding energy, vibrational frequencies, etc.) of relevant species on a large variety of adsorption sites so to obtain a statistically meaningful distribution of the physicochemical properties of interest to be transferred in numerical models. As a test case, we computed the binding energy of ammonia adsorbed at the different sites of the icy grain surface, showing a broad distribution not easily accounted for by other more size limited icy grain models. Our method is also the base for further refinements, adopting the present grain in a more rigorous QM:MM treatment, capable of giving binding energies within the chemical accuracy.

Theoretical distribution of the ammonia binding energy at interstellar icy grains: a new computational framework

“…The water-ice grain model used throughout this work, the binding energy sampling procedure and the preliminary BE geometric optimization were taken from a previous work by our group, which is summarized in this section. 24 Water-ice grain model. In order to build-up the grain model a bottom-up approach was followed, i.e.…”

“…Data taken from Ref. 24 Preliminary geometry optimization. After the NH 3 sampling, a preliminary geometry optimization via xTB-GFN2 34,48 was performed.…”

“…16,[20][21][22][23] Therefore, the icy grains should be large and varied enough to allow to reconstruct the BE distribution of a species and not just a value. To overcome the above mentioned problems, we have recently proposed 24 an automatic and unbiased approach to construct water ice clusters and obtain the binding energy distribution of any species (see the Methodology section for further details).…”

“…Here, we propose a new method which optimises the computation accuracy on very large icy grain models. Specifically, we implemented an automatic procedure which is based on the ACO-FROST code, recently developed by our group, 24 to construct a large (≥ 1000 water molecules) icy grain. Briefly, only a selected portion of the icy grain, where the adsorption takes place, is treated at a very high level of theory, while the whole cluster is treated at a lower level.…”

“…Figure 1: (Left) Icy grain model, (Central) with the overlapped 162 vertices grid points in blue color and (Right) with the same vertices projected closer (2.5-3 Å) to the grain surface.Atom color legend: oxygen in red, hydrogen in white. Data taken from Ref 24. …”