In the Cartesian model (model C) shown in Figure . 1B, the positions of the hydrogen atoms in the whole system [r H1 , r H2 ] were represented with their Cartesian coordinates. The variable x C shown in Fig. , describes all the relevant N (N -1) marginal DOFs of the ice crystal. Finally, the shift and scale layer (yellow in Fig. ) was initialised to fit the marginal variables of x C into the working interval of the 1D splines. The spline coupling layers in this work were implemented similarly to Ref. , with each marginal variable of model C assigned to a non-periodic [-1, 1] interval. The uniform log base distribution (ln p 0 ) in model C was therefore set to: