Deviations from an ideal crystal lead to diffuse scattering (DS) intensity, both between and beneath the Bragg peaks in diffraction patterns (Guinier, 1963). First characterized using simple ionic crystals in early studies of X-ray diffraction (Lonsdale, 1942), DS has a rich history (Welberry & Weber, 2016) and is a well established technique in smallmolecule crystallography (Welberry, 2004). DS studies in macromolecular crystallography began more recently (Phillips et al., 1980) and now the potential for obtaining information about protein motions is fueling the growing interest in DS (Meisburger et al., 2017).As noted in a previous IUCrJ commentary ( Keen, 2016), accurate modeling of smallmolecule DS requires not only information about the variations of individual molecules or unit cells, but also information about the correlated variations in a more extended environment. Similarly, macromolecular DS studies indicate the importance of modeling interactions across unit-cell boundaries in normal-modes models (Riccardi et al., 2010), as well as the molecular dynamics models (Wall, 2018) of macromolecular diffuse scattering that are shown in this issue. The liquid-like motions (LLM) model (Caspar et al., 1988), in which the correlated variations are modeled as if the crystal were a soft homogeneous material, explains the overall DS pattern in several protein crystals (Caspar et al., 1988;Clarage et al., 1992;Van Benschoten et al., 2016;Wall, Clarage & Phillips, 1997;Wall, Ealick & Gruner, 1997). However, the consequences of including intermolecular interactions for the accuracy of the LLM model were not clear until now.In this issue, Peck and co-workers (Peck et al., 2018) investigate the importance of intermolecular interactions by assessing the accuracy of two alternative versions of an LLM model (Caspar et al., 1988) (Fig. 1). In the original version of the model, the correlations extend across molecular boundaries (Fig. 1a). In this case, the diffuse intensity is derived from the squared crystal transform, which is sharply peaked. In a modified version of the model, correlations terminate at the molecular boundary (Fig. 1b). In this case, the diffuse intensity is derived from the squared molecular transform of the asymmetric unit (in the cases considered, a single molecule), which is continuous in reciprocal space. In both cases, the transform is blurred; shorter correlation lengths correspond to a larger scale blurring of the transform. Both models are optimized to maximize the agreement with the data, enabling a well controlled comparison.To be consistent with the state of the art (Meisburger et al., 2017), three-dimensional diffuse datasets were used for the comparison, obtained from crystalline cyclophilin A (CypA) (PDB entry 4yuo; Fraser, 2015), WrpA (PDB entry 5f51; Herrou & Crosson, 2015) and alkaline phosphatase (PDB entry 5c66; Peck et al., 2017). The CypA data were the subject of a prior DS study (Van Benschoten et al., 2016) and the others were newly analyzed for this study, providing valuable addi...