Revisiting fundamental properties of TiO$_2$ nanoclusters as condensation seeds in astrophysical environments

Abstract

The formation of inorganic cloud particles takes place in several atmospheric environments including those of warm, hot, rocky and gaseous exoplanets, brown dwarfs, and AGB stars. The cloud particle formation needs to be triggered by the in-situ formation of condensation seeds since it can not be assumed that such condensation seeds preexist in these chemically complex gas-phase environments. We aim to develop a methodology to calculate the thermochemical properties of clusters as key inputs to model the formation of condensation nuclei in gases of changing chemical composition. TiO$_2$ is used as benchmark species for cluster sizes N = 1 - 15. We create 90000 candidate geometries, for cluster sizes N = 3 - 15. We employ a hierarchical optimisation approach, consisting of a force field description, density functional based tight binding (DFTB) and all-electron density functional theory (DFT) to obtain accurate energies and thermochemical properties for the clusters. We find B3LYP/cc-pVTZ including Grimmes empirical dispersion to perform most accurately with respect to experimentally derived thermochemical properties of the TiO$_2$ molecule. We present a hitherto unreported global minimum candidate for size N = 13. The DFT derived thermochemical cluster data are used to evaluate the nucleation rates for a given temperature-pressure profile of a model hot Jupiter atmosphere. We find that with the updated and refined cluster data, nucleation becomes unfeasible at slightly lower temperatures, raising the lower boundary for seed formation in the atmosphere. The approach presented in this paper allows to find stable isomers for small (TiO$_2$)$_N$ clusters. The choice of functional and basis set for the all-electron DFT calculations have a measurable impact on the resulting surface tension and nucleation rate and the updated thermochemical data is recommended for future considerations.