There are multiple options in choosing a shape for the nanoparticle inclusions in the FEM model: nanoparticles can be described by meshing the full Bader volume, or a geometric shape such as prisms, cylinders or ellipsoids may be used. We discuss the use of rounded prism shapes in Sec. S9. However, in the following pages we chose to implement the theory to ellipsoidal shapes for several reasons: 1) There are simple analytical expressions for their polarizability in vacuum that facilitate analysis of isolated particles. 2) They are commonly used as models for anisotropic inclusions in effective medium theory. 3) There are available procedures for fitting ellipsoids to arbitrary shapes, such as Bader surfaces. 4) It is simple to scale and extrapolate ellipsoidal shapes for larger inclusions that are not simulated quantum mechanically. Using Bader surfaces themselves as inclusion shapes, in contrast, would require a quantum simulation of each inclusion to be simulated prior to performing the FEM.