The IllustrisTNG simulations are some of the most advanced hydrodynamical, large-volume cosmological simulations to date. While these simulations predict countless properties of the stars, black holes, and gas in galaxies, they do not currently compute the abundance of the different phases of neutral hydrogen: atomic and molecular (HI and H2). In Diemer et al. 2018, we post-processed the Illustris and IllustrisTNG simulations to compute their abundances in galaxies (see also the research page). The resulting data are publicly available as part of the IllustrisTNG data release.
In Diemer et al. 2019, we compare Illustris and IllustrisTNG to HI and H2 observations. To avoid overcrowding the figures in the paper, we have omitted the original Illustris-1 simulation from the plots. A complete set of figures that compares Illustris TNG100 with Illustris-1 can be downloaded below. Similarly, we add a set of figures that compare TNG100 with its lower-resolution cousin, TNG100-2, to highlight any issues with convergence.
From the first figure set, it is apparent that the HI and H2 abundances in Illustris-1 match the observations much more poorly than IllustrisTNG. Thus, the latter is the preferred simulation suite for any work related to HI and H2. If you have any questions about the data or these figures, please do not hesitate to contact me.