The densities yield information about the compactification behavior of SOM. The five different compositions of SOM presented in Table were simulated. The system densities, respectively averaged over the last 3 µs of the production runs and calculated for the different levels of water contents in the system, denoted by x water , are shown in Figure . Please note that due to the utilization of Martini standard masses, the densities were rescaled, using the ratio of the mass of the respective atomistic to that of the coarse-grained system. The overall trends for the densities of the different SOM types are similar. As discussed previously , coarse-grained Leonardite (HA) systems tend to underestimate the density, especially for lower water contents, due to the calcium ions having fixed bead sizes. This results in the coarse-grained model being unable to form distinct calcium-carboxylate complexes. In particular, the so-called bidentate complexes, which contribute to a tight packing at lower water contents, cannot be reproduced by the coarse-grained model, which leads to an underestimation of the density for these cases. However, for Leonardite (HA), the deviations were still acceptable (maximum of around 10 %). The use of smaller-sized water beads (tiny water) facilitated a denser packing of the SOM molecules, thereby reducing the discrepancy between atomistic and coarse-grained densities. Nonetheless, at lower water concentrations, this discrepancy remained significant, emphasizing the critical influence of calciumcarboxylate complexes on the system's density.