Construction of isotherms around the fluid-to-SI to ice X transition for temperatures between 1320 K and 2000 K. Isotherms 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 isotherms follow the Mazevet et al. (2021) data until the boundary between region 7 and region 5 is met. For the 1380.38 K isotherm, the Mazevet et al. (2021) data is followed until 5.09 · 10¹⁰ Pa. Then the isotherm will follow the French et al. (2016) data (dotted line), which will lead to an expected discontinuity, and until 6.04 · 10¹⁰ Pa where region 5 will finish. Then the isotherm will go from following French et al. (2016) to following the original AQUA data again, also leading to an expected but much larger discontinuity. After 3 · 10¹¹ Pa, the ice will be extrapolated linearly. This extrapolation is shown in brown and will be cut at 3.2 · 10¹² Pa in the final table. For the 1659.59 K, the transition from Mazevet et al. (2021) to French et al. (2016) occurs at 5.48 · 10¹⁰ Pa and the transition from French et al. (2016) back to the original AQUA at 8.47 · 10¹⁰ Pa. For the 1995.26 K, the isotherm follows Mazevet et al. (2021) after the spline until 6 · 10¹⁰ Pa and then French et al. (2016) until 1.25 · 10¹¹ Pa, after which it goes back to the original AQUA and then extrapolation (brown lines). The grey curves show the final, revised isotherms.