Balancing accuracy and efficiency in density functional theory studies of SiO₂ polymorphs

The ability of dispersion-corrected density functional theory (DFT) calculations to reproduce framework densities and relative stabilities of silica polymorphs has been the subject of a number of prior investigations. Most of these studies either considered only a limited number of DFT approaches or included relatively few structures in the validation against experimental data. Using the Gaussian and plane wave DFT code CP2K, this work aims at a more comprehensive assessment, comparing 27 semilocal approaches that include dispersion interactions either by means of a pairwise correction ("Grimme-type" D3) or in the framework of a nonlocal density functional. The set of silica polymorphs encompasses three minerals and 16 all-silica zeolites. For those approaches that perform well with a moderately sized (triple-zeta) basis set, the effect of adding additional basis functions is evaluated. This (slightly) improves the performance in the majority of cases, especially for relative energies. All in all, those functionals that deliver best agreement with experiment achieve overall errors (as expressed by the mean unsigned error) on the order of 0.2 T atoms per 1000 ų for framework densities and of 1.0 kJ mol^-1 (per SiO₂ formula unit) for relative energies. Due to the favourable scaling behaviour of CP2K, structure optimisations of complex zeolites structures are routinely feasible. This is demonstrated through additional calculations for three recently reported all-silica zeolites with extra-large pores.