I'm studying the dispersive properties of metals using Lorentz-Drude model, but I'm confused about the equation set-up. What is LD model different from the classical dielectric function using for other dielectrics? Namely: ε(w) = ε’ + iε’' where the imaginary part is the attenuation. In LD model, ε is composed of intraband (free-electron) effects and interband (bound-electron) effects. Physically what do they mean? Thanks for the help!