Floating in Space: How to Treat the Weak Interaction between CO Molecules in Interstellar Ices

Abstract

In the interstellar medium, six molecules have been conclusively detected in the solid state in interstellar ices, and a few dozen have been hypothesized and modeled to be present in the solid state as well. The icy mantles covering micrometer-sized dust grains are, in fact, thought to be at the core of complex molecule formation as a consequence of the local high density of molecules that are simultaneously adsorbed. From a structural perspective, the icy mantle is considered to be layered, with an amorphous water-rich inner layer surrounding the dust grain, covered by an amorphous CO-rich outer layer. Moreover, recent studies have suggested that the CO-rich layer might be crystalline and possibly even be segregated as a single crystal atop the ice mantle. If so, there are far-reaching consequences for the formation of more complex organic molecules, such as methanol and sugars, that use CO as a backbone. Validation of these claims requires further investigation, in particular on acquiring atomistic insight into surface processes, such as adsorption, diffusion, and reactivity on CO ices. Here, we present the first detailed computational study toward treating the weak interaction of (pure) CO ices. We provide a benchmark of the performance of various density functional theory methods in treating the binding of pure CO ices. Furthermore, we perform an atomistic and in-depth study of the binding energy of CO on amorphous and crystalline CO ices using a pair-potential-based force field. We find that CO adsorption is represented by a large distribution of binding energies (200-1600 K) on amorphous CO, including a significant amount of weak binding sites (<350 K). Increasing both the cluster size and the number of neighbors increases the mean of the observed binding energy distribution. Finally, we find that CO binding energies are dominated by dispersion and, as such, exchange-correlation functionals need to include a treatment of dispersion to accurately simulate surface processes on CO ices. In particular, we find the ωB97M-V functional to be a strong candidate for such simulations.

Figure 1

CO dimer configurations used to determine the interaction energies utilized in our benchmark study (Table 2). Full geometry coordinates are given in the Supporting Information.

Figure 2

Distributions of binding energies calculated by DFT (blue) and FF (pink) methods for cluster sizes of (a) 8, (b) 10, (c) 12, and (d) 350 CO molecules. The mean (μ) and standard deviation (σ), both in K, for each distribution are shown in the plot legends. Dotted lines are probability density functions fitted to the binding energy distributions. For all distributions 10 equally spaced bins were used for plotting. Distributions consist of (a) 60 DFT samples and 200 FF samples, (b) 50 DFT samples and 250 FF samples, (c) 40 DFT samples and 300 FF samples, and (d) 550 FF samples. Note that overlapping blue and pink bars result in a third color within the plots.

Figure 3

Average contribution to the binding energy of each part of the force field employed. Points are the mean of the contribution distributions, error bars are the standard deviations, and dashed/dotted lines are guides for the eye.

Figure 4

Black and purple dots are binding energy distributions, and the gray and fuchsia dots are the median values of the distributions. Dashed lines do not indicate predicted trends.

Figure 5

All FF calculated binding energies with ZPE corrections plotted as a function of number of nearest neighbors. Results for cluster sizes less than 200 molecules are shown in pink and all larger clusters are shown in purple.

Figure 6

Distribution of binding energies calculated for a CO molecule adsorbed on the (100) surface of an α-CO crystal. Colors depict categories (see Figure 7) found by a clustering algorithm with reference to the orientation of the adsorbed CO. The inset is an enlargement of the low energy sites.

Figure 7

Representative image of each category for binding sites on the (100) surface of an α-CO crystal. Colored boxes indicate the category color in the histogram (see Figure 6), and text in the upper left corner of each box indicates which atom is closest to the crystal face. Text in the lower right corner of each box is the median binding energy of the categories distribution. “Noise” points (shown in red in Figure 6) are not shown here, since all orientations in the distribution differ significantly from each other. Note that all molecules shown here are adsorbed on a hollow site (over a CO molecule in the second layer).