*3.1. Intramolecular Level*

In order to initiate the study, *Mogul* was used to predict the geometric preferences of every bond length, angle, torsion angle and ring within the molecules present in both forms, by accessing a depository of CSD-based libraries [31]. The results for geometric characteristics of form I showed that there were no unusual parameters, as opposed to some properties of form II that were not within the normal CSD distributions. Although these outlying parameters might be a product of poor data quality, which could be inferred from the atomic displacement parameters of form II (Figure 2), some of these unusual geometric characteristics (Please refer to Supplementary Materials) were also observed in other systems involving the ganciclovir molecule, such as the HCl salt and the monohydrate [32].

The electrostatic map of form I (Figure 3) clearly displays the large variety of hydrogen bond participants, with maxima (dark blue) around the hydrogen atoms of the amine group, and minima (dark red) around the oxygen of the carbonyl group and N3 of the imidazole ring, due to the presence of lone pairs of electrons. In addition, the electrostatic map also proves the presence of other potential hydrogen bond contributors, such as the hydroxyl groups, the ether oxygen atom and also carbon atoms. The electrostatic map of form II was very similar to that of form I (Please refer to Supplementary Materials).

**Figure 3.** Electrostatic map of form I, mapped on the molecular vdW surface. Double bonds are omitted only for visualization purposes.
