Opacity calculations for aluminum, hydrogen, and gold using FLYCHK with improved free-free opacity formalism

Opacity is an essential property for understanding the fundamental properties of high-energy-density (HED) and astrophysical plasmas. FLYCHK, a collisional-radiative code, has been used to calculate the opacities of HED plasmas under various conditions because of its simplicity and availability. However, the opacity calculations of FLYCHK have limitations in a strongly coupled region, partly because of the problem of a free-free opacity formalism. In this study, we improved the free-free opacity calculation model of FLYCHK and generated opacities of various materials, including high-Z materials (Al, H, and Au). The FLYCHK opacities were in good agreement over a wide range of conditions with those obtained using the Los Alamos opacity code ATOMIC and the detailed level accounting model.