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

TL;DR

This paper models the orbital requirements for accurate high-temperature Kohn-Sham DFT simulations and demonstrates how an extended FPMD method can bypass these computational constraints.

Contribution

It introduces a model linking orbital count to occupation levels at finite temperature and shows how to circumvent high orbital requirements using an extended FPMD approach.

Findings

The model accurately predicts the number of orbitals needed for desired precision.

Occupation levels below 10^-4 are necessary for 1% accuracy at high temperature.

Extended FPMD method effectively bypasses the orbital count constraints.

Abstract

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.