For each frequency ω, the nanoparticle permittivity was set to ε p (ω) and the N particle induced dipoles p ij were computed for a particular field polarization E 0 = E 0 êj (i = 1, . . . , N, j = x, y, z) by solving eq 1 using GMRES with a relative error tolerance of 10 -3 . During each iteration, the right side of eq 1 is evaluated to an absolute error tolerance of 10 -3 using an Ewald summation procedure discussed in Supporting Information Sec. 1.2 and Ref. 39. The polarization-averaged extinction cross-section per nanoparticle volume is σ = j σ j /N E , where