properties, and various nanoparticle synthesis techniques can be used to finely control particle shape. Thus, there is the potential to use rod-and disk-like inclusions to introduce an anisotropic dielectric response. With this in mind, we considered five composites with rodand disk-like inclusions from Ref. 24 to investigate shape effects. All geometries were created by substitution of Ag at adjacent MgO lattice sites followed by optimization of the lattice and atomic coordinates. Hereafter, we denote these inclusions by the number of atoms substituted in each direction (2×2×N for rods, and N ×N ×2 for disks). These composites are described in more detail in Sec S1.2, and the smallest two anisotropic inclusions are shown in Fig. , alongside "spherical" Ag 8 .