High Temperature Density Functional Theory Calculations

Kohn-Sham density functional theory calculations are widely used as an accurate ab-initio approach to solving the many-body problem in the warm dense matter regime, encompassing conditions approximately ranging from 0.1 to 50 times solid density and temperatures from 0.5 to 100 eV. Nonetheless computational costs can quickly become prohibitive with rising temperature due to the growing number of partially occupied states involved. This is particularly pertinent for accurate molecular dynamics simulation that require large box sizes and timescales to reach convergence. For these reasons, orbital-free methods are an attractive alternative for temperatures beyond approximately 10 eV, however the increased computational efficiency of simple kinetic energy functionals comes at the cost of physical accuracy. Here we present a hybrid approach in which the Kohn-Sham equation is solved for the low-lying states only, whilst an orbital-free method is applied to describe the remaining electrons. Our hybrid methods are used to perform equation of state calculations in the warm dense matter regime which we then benchmark against conventional Kohn-Sham calculations.