Secondly, as the energetic component of the interfacial free energy in eqn ( ) is calculated assuming an idealised packing structure for a fully face-on and side-on interfacial layer, variations in the packing structure at the interface reduce the accuracy of the estimated free energy. The positions of fluid particles are less strongly constrained by the vapour interface than the solid substrate, and the side-on packing that predominates at the vapour interface appears to be less robust to disorder than the face-on packing that predominates at the solid interface, due to the tendency of nearest neighbours to be displaced parallel or angled slightly with respect to another. Weaker ordering at the vapour interface results in molecular packings that deviate from the idealised structures, as shown in the ESI in Fig. . Similarly, the calculation of the energetic component of the free energy based solely on the fully face-on and fully side-on configurations means that the predictability of the model is likely to be poorer for systems with interfacial orientation that is closer to isotropic.