Computational work using density functional theory (DFT) has indicated that the two forms exhibit only a subtle energetic difference, and it was suggested that the two polymorphs were virtually degenerate. Later on, Reilly and Tkatchenko applied a many-body dispersion approach with DFT to investigate their energetic differences at room temperature, which inferred that the stability of form I was due to the coupling of low-frequency phonon modes and collective electronic fluctuations. A recent work of Vaksler et al. proposed a new method to study the mechanical properties using quantum chemical calculations constituted of two steps: analysis of the pairwise interactions between molecules and modelling displacement between strongly bound fragments. The results identified the crystallographic c axis, within the region between the common planes of the polytypes, as the most likely direction for shear deformation to occur. Even though crystals are defined as well-organised and ordered systems, defects and dislocations are commonly encountered in crystalline materials. The influence of defects, for example on the properties of metals, has been widely studied. It is well known that defects can also affect the performance of molecular materials such as their mechanical stability. The subtle differences in crystal structure between the two aspirin forms and the comparable energetic stabilities challenge the view of distinct polymorphs and make this an interesting model system for investigating the effect of disorder and the structural dynamics in organic molecular crystals beyond their perfect structures. Chan et al. have reported a detailed structural study of aspirin crystals, with disorder models fitted to diffuse X-ray scattering data. These results confirm that crystals of form I are well ordered, but that form II crystals contain stacking faults corresponding to the form I interlayer arrangement, which aggregate to form included regions of the form I structure. Hence, "pure" form II is generally not encountered, and may not be experimentally attainable.