A combination of ( ), Δ ( ), and data points. This extension of the "mixture only" training set is included to explore whether including the density of pure systems helps to constrain the optimization, or whether ( ) alone is sufficient. 4. (Δ ( ), ( ), , Δ ) ("pure and mixture"): A combination of the "pure only" and the "mixture only" training sets. This data set tests whether including pure Δ alongside Δ ( ) improves the parameterization of the cohesive energies between molecules, or whether Δ ( ) alone is sufficient.