The smallest molecular cluster we consider is a simple dimer consisting of a water molecule hydrogenbonded to the nitrogen of DIPA. This is shown as the left-most structure in Figure . The resulting total PDF of this molecular cluster structure, GMCS, is shown as the black curve in Figure . The first two peaks in the GMCS come from the covalent bonds in the DIPA molecule. Strikingly, the model displays peaks at r = 5.1 and 7.5 Å (Figure , brown), mirroring the peaks present in ddG (reproduced in red in Figure for reference) at the same location. Evidently, ddG is capturing water-DIPA dimer interactions at low r. The computed GMCS is for a single arrangement of the atoms. However, the highest-r peak in this dimer model (brown) is at ~5 Å, meaning that GMCS of the dimer cannot explain the intensity that we see in the ddG above 5 Å. In other words, larger molecular cluster structures are needed to explain the intensity in the intermediate-r range (5 Å < r < 20 Å).