Requirements for very high temperature Kohn-Sham DFT simulations and how to bypass them

In high-temperature density functional theory simulations (from tens of eV to keV), the total number of Kohn-Sham orbitals is a critical quantity to get accurate results. To establish the relationship between the number of orbitals and the level of occupation of the highest energy orbital, we derived a model based on the homogeneous electron gas properties at finite temperature. This model predicts the total number of orbitals required to reach a given level of occupation and, thus, a stipulated precision. Levels of occupation as low as 10 - 4, and below, must be considered to get converged results better than 1%, making high-temperature simulations very time consuming beyond a few tens of eV. After assessing the predictions of the model against previous results and Abinit minimizations, we show how the extended FPMD method of Zhang et al. [Phys. Plasmas 23, 042707 (2016)] allows us to bypass these strong constraints on the number of orbitals at high temperature.