Sommerfeld expansion of electronic entropy in an INFERNO-like average atom model

In the average atom (AA) model, the entropy provides a route to compute thermal electronic contributions to the equation of state (EOS). The complete EOS comprises in many modelings an additional 0 K isotherm and a thermal ionic part. Even at low temperature, the AA model is believed to be the best practical approach. However, when it comes to determining the thermal electronic EOS at low temperatures, the numerical implementation of AA models faces convergence issues related to the pressure ionization of bound states. At contrast, the Sommerfeld expansion tells us that the variations with temperature of thermodynamic variables should express in simple terms at these low temperatures. This led us to tackle the AA predictions with respect to the Sommerfeld expansion of the electronic entropy. We performed a comprehensive investigation for various chemical elements belonging to s, p, d, and f blocks of the periodic table, at varying densities. This was realized using an INFERNO-like model since this approach provides the best theoretical framework to address these issues. We found that the Sommerfeld expansion is valid for a ratio of the temperature to the Fermi energy of <0.05. This allows us to extend the study to other atomic numbers at a low enough unique temperature. Comparing the AA results with the free electron gas model, the discrepancies are <10% at very high densities but can reach an order of magnitude at the metal-insulator transition.