Construction of isotherms around the fluid-to-SI transition for temperatures between 2000 K and 6 · 10⁴ K. Isotherms for T > 2000 K cross from region 6 to region 7 through an interpolation region ranging from 0.6·P_6-to7 Pa to 38·P_6-to-7 Pa. Across the interpolation region, a cubic spline (shown in red) was used to connect the original AQUA isotherm (solid lines) to the new data from Mazevet et al. (2021) (dashed lines) with the +425 J/(kg K) shift applied. The 2041.74 K and 10000 K follow the Mazevet et al. (2021) data until the boundary between region 7 and region 5 is met. For the 2041.74 K case, this occurs at 6.08 · 10¹⁰ Pa. For the 10000 K isotherm, this is at 1.40 · 10¹² Pa. At these pressures, the isotherms stop following the Mazevet et al. (2021) data and begin following French et al. (2016) (dotted lines). This occurs up to 2.88 · 10¹² Pa, where a new spline until 4.47 · 10¹² Pa is applied to transition from French et al. (2016) back to Mazevet et al. (2021). Isotherms at 33884.42 K and 58884.37 K simply have the spline showing the transition from region 6 to 7 applied, and then continue following Mazevet et al. (2021). The grey curves show the final, revised isotherms.