Within the Drude dielectric function, ε p /ε f can be expressed in terms of dimensionless parameters describing electron damping γ = γ/ω * p and high-frequency permittivity ε ∞ = ε ∞ /ε f , each of which affect the dipole polarizability and the structure-dependent optical response. These effects are evident when considering the far-field spectral features of Drude hard-sphere nanoparticles at different volume fractions (Figure ). The lineshapes of the spectra are qualitatively similar to those in Figure , so only the resonant frequencies ω res and linewidths W are presented for quantitative comparison. Also shown as solid lines are the mean-field predictions for ω res and W from Table . As γ decreases (Figure ), |α| increases with little change in the location of maximum |α|. This enhanced dipole polarizability, and therefore increased structural sensitivity, leads to larger deviations of the MPM optical response from the "structure-free" mean-field predictions. Relative to the ω res and W for a single particle (i.e., η = 0 limit), smaller values of γ result in larger redshifts for ω res and more pronounced lineshape broadening (especially at intermediate η). As ε ∞ decreases (Figure ), |α| increases, so the trends in ω res and W look similar to those for decreasing γ; the magnitude of the ω res redshift relative to η = 0, the linewidth W , and the deviation from mean-field behavior all increase. Varying ε ∞ also changes the frequency of maximum |α| and therefore shifts the frequency range over which the particles polarize appreciably. Therefore, by estimating the maximum |α|, we may qualitatively predict how sensitive the optical properties of an assembly with a particular type of nanoparticle will be to structural changes. A similar observation has recently been reported for plasmonic nanoparticle monolayers. For results presented from this point forward (Figures ), we fix the dielectric parameters to γ = 0.05 and ε ∞ = 1.