By comparing the different compositions, the densities can potentially be related to the abundance of certain functional groups, such as carboxylate groups. As mentioned previously, the interaction between these groups and calcium ions contributes to the SOM packing, which is especially important at lower water contents. In the case of the SOM systems with a similar aromatic content that were investigated, the densities are higher for the compositions with a higher acidity. From high to low acidity, the respective SOM types are: SOM I (blue), SOM IV (red) and SOM II (orange). Therefore, this could again be explained by the higher number of available carboxylate-calcium complexes that might contribute to structures that are denser in overall. This behavior has also been observed in studies investigating humic substances using atomistic simulations . Consequently, this is a feature that can be reproduced with the coarsegrained model as well.