Simulating the optical properties of plasmonic nanomaterials in COMSOL Multiphysics involves using either the RF or Wave Optics Module, depending on the frequency range. Key parameters include the nanoparticle's geometry, material properties (complex permittivity), excitation conditions, boundary conditions, and a fine mesh for accuracy. By calculating electric and magnetic field distributions, integration for cross-sections, and utilizing derived values and plots, you can analyze scattering, absorption, and extinction properties. For optimal results, consider mesh refinement, material dispersion, nonlinear effects, and potential coupling with other physics.