In this talk, I will review the basic approximations used in crystal modeling and then discuss two examples that illustrate the effect of disorder in modeling real and reciprocal space data. The examples are chosen from fluorite structures, which actually constitute a rich playground despite their supposed simplicity. The first example concerns the problem of scattering from mosaic systems with correlated microdomains (static disorder). The second example concerns dynamical correlations in pair correlation functions emerging from optical phonons (dynamical disorder).