It is also encouraging that the method has been effective de- spite differences in the types of basis set used for the GGA (plane wave basis sets) and hybrid functional (Gaussian type basis sets) calculations. That these differences have apparently not harmed the performance of the models allows us to take advantage of different solid state DFT implementations to gain maximum efficiency in the calculations. More generally, the savings that we see in high level calculations open up the use of even more accurate and demanding calculations as the highest level of the model, because of the small number of structures on which these calculations are necessary. An option here is the implementation of the multi-fidelity GP modelling approach with fragment-based lattice energy models, enabling the use of wavefunction-based higher levels.