Finite-Temperature Orbital-free Density Functional Calculations for Warm Dense Lithium

Warm dense matter (WDM) defines the region between condensed matter and plasmas. This regime is characterized by high pressure and elevated temperature. The standard theoretical and computational approach, which is a combination of finite-temperature Kohn-Sham density functional theory (KS-DFT) and classical molecular dynamics, becomes computationally very expensive at elevated temperature. The orbital-free (OF) version of DFT is a less expensive alternative to the orbital-based methods. We have implemented finite-temperature Thomas-Fermi, second-order gradient expansion, and new generalized gradient approximation free energy functionals in an OF-DFT code. These non-interacting free-energy functionals are used in combination with zero-temperature exchange-correlation in local density approximation. Self-consistent OF-DFT calculations with these functionals are performed for lithium for the range of densities $\rho_{\rm Li}=0.5-10$ g/cm$^3$ and temperatures between 100 K and 100 kK. OF-DFT results are compared to the standard Kohn-Sham data. Local pseudopotentials used on OFDFT calculations are validated by comparison between Kohn-Sham results obtained with standard non-local pseudopotentials and with the same local pseudopotentials.