Requirements for very high temperature Kohn-Sham density functional 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 orbital, we derived a model based on the 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. [PoP 23 042707, 2016] allows to bypass these strong constraints on the number of orbitals at high temperature.