Gravitational harmonics J₂, J₄ for Uranus (a) and Neptune (b) in case (ii) models (stars) and case (iv) models (dots). The colour of the dots indicates the parameter P_Z. The colour of the stars indicates the H₂ –H₂O phase diagram used. Dark pink crosses show observed values. Light pink crosses are wind-corrected static J₄ values (see text for details). In the case of Neptune, the V23 conv-1800 K and SFB-linear-4 GPa stars overlap. We note the change in units for J₂ and J₄ with respect to Table 1. Panels c and d show stars which are the same models as in (a) and (b), and circles: case (iii) models. This figure shows that a mixture of rocks (circles) in the deep interior below the 2000 K level, and water reduces the resulting |J₄| values.