DDSCAT

You can use the DDSCAT (Discrete Dipole Scattering) package for nanoparticles >5 nm. Many of the optical properties of nanoparticles (absorption, scattering, extinction coefficient) can be solved via classic electrodynamics, i.e. solving the Maxwell Equations when light interacts with a nano-sized object with no quantum packages necessary. To determine the optical properties of nanomaterials classically, the most rigorous approach is solving Maxwell's Equations analytically via Mie Theory (see section 2.2 of Bertens). However, exact analytical solutions to the scattering problem only exist for a limited number of geometries (sphere, cylinder, etc.). The DDSCAT package approximates a continuous object utilizing a series of point dipoles on a 3D lattice, of which a direct solution to Maxwell's Equations can readily be solved.

The image above shows an example of point dipoles constructed for input into DDSCAT (Draine, 1988). In addition to the point geometry, to run DDSCAT, the complex polarizability of the material and an 'effective' radius must be provided. The complex polarizability of materials like gold, silver, etc. can be found readily online. The effective radius of an object is solved by determining the total volume of your object, then solving for radius as if the object were spherical (V_obj = 4/3𝜋r_eff³).