To further ensure that our reference data set is sufficiently broad, we consider the prevalence of pairwise data between the various environments, as shown in Figure . Across the entire matrix, the majority of pairwise interactions are represented by at least 10 structures. Whilst this might seem like a relatively small number of structures, because each structure contains information regarding both the crystal energy and crystal geometry (cell geometry and atomic positions), each structure contributes multiple data points into the parameter fitting objective function. Below, we attempt to rationalise why the remaining interactions do not meet this threshold and the significance of this relative data paucity.