Optically active materials, and particularly metals can be described fairly well with Classical Electrodynamics. For example, interfaces can be modeled using the Drude model, and for spherical particles we have Mie theory. This is valid for large systems (for example microns in depth or diameter). However, as systems become smaller, microscopic quantum effects can play an important role in the description of the electrodynamics. These effects are commonly known as "Quantum Confinement Effects".
A classic and illustrative example of change in optical activity as a function of size is in silver and gold nanoparticles.
In the completely classical regime, Finite Difference Time Domain (FDTD) simulations are used for electrodynamic simulations. Is it possible to use FDTD for simulating mesoscopic structures (nanostructures) where quantum size effects matter? What is the limit in size that can accurately be described by classical electrodynamics for a gold nanoparticle?
What methods are there available to model quantum-confinement effects in nanostructures of different sizes?