Modeling Polymer Degradation by Atomic Oxygen in Low Mars and Earth Orbits: Are Diffuse Functions Necessary?

Materials used in satellites, often composed of organic polymers, are subject to degradation by highly reactive atomic oxygen in low Earth and Mars orbits. The local structures of these polymers often feature fused aromatic rings, resembling small polycyclic aromatic hydrocarbons (PAHs). Computational chemistry calculations, such as density functional theory (DFT), provide a method for estimating the chemical resistance of these moieties. The choice of basis sets in DFT calculations is a critical factor in accurately describing the molecular electronic density in the fragmentation pathways of PAH degradation. In particular, the inclusion of diffuse functions in the basis set is necessary for capturing long-range interactions and charge transfer effects. However, when modeling PAHs, computational limitations become a challenge. Here, we assess the accuracy of Pople basis sets, polarization-consistent contracted basis sets (pcseg-n), and correlation-consistent basis sets (cc-pVnZ) using the established ωB97X-D3BJ long-range DFT functional. Additionally, we evaluate the impact of including diffuse functions on the performance of the calculations in terms of energy accuracy and computational cost. These basis sets were tested on two PAH samples, naphthalene and phenanthrene. We found that diffuse functions do not significantly affect the performance of the ωB97X-D3BJ functional, and that cc-pVTZ provides the best balance of accuracy and computational efficiency.