Relation between electron number density, n_e, and the DM density for the hydrodynamical simulations (blue crosses) and the LDL surrogate (red crosses) for the fiducial parameters and at z = 0.21. The blue and red lines denote the mean locus (respectively for the hydrodynamical and the LDL-predicted voxels). The voxel deviation from the mean for both methods is quite correlated (ρ_corr = 0.56), but the LDL predictions do not reproduce the same scatter as in the originalsimulations.