Having shown that the fragmented training procedure produces accurate DNA only and solvent only electron density models, we now combine all of the training sets in Table to construct a model for a solvated DNA system. The test structure for this model is a DNA base-pair step surrounded by 100 water molecules (Fig. ) which is slightly more than a single water shell. We include ten test structures for each combination of base-pair step (ten combinations total), as well as ten structures with an Mg 2+ bound to the phosphate for a total of 110 test structures (S8 Table ). Each of the test structures contains around 480 atoms. Due to the size of the test structures, we omit calculating ϵ ρ M L , which requires an additional projection to the auxiliary density basis, and only calculate ϵ ρtrue , noting that ϵ ρ M L , which is the error from fitting the model, will always be lower.