Construction of isotherms around the fluid-to-ice X transition for temperatures between 1200 K and 1320 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-to7 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. These isotherms follow Mazevet et al. (2021) after the spline until the boundary between region 7 and region 3 is met, which occurs at 4.49 · 10¹⁰ Pa for 1202.26 K and at 5.12 · 10¹⁰ Pa for 1318.26 K. After that boundary, the isotherms follow the original AQUA again up to 3 · 10¹¹ Pa and then the linear extrapolation (brown lines). The grey curves show the final, revised isotherms.