The parameters of dielectric function are shown in Table . The cubic simulation box with a side length of 40.00 nm and a grid size of 0.20 nm is adopted in all FDTD calculations. We use the geodesic Levenberg-Marquardt (LM) algorithm to fitting the FDTD field into a polynomial function which can be read in every TDDFT time step.We note that the current FDTD method is a classic electromagnetic model, the quantum size effect in Au NPs is ignored, which can be accounted for by the nonlocal hydrodynamic model. The neglecting of the quantum effect may bring some errors to the field enhancement. For a metal tetrahedron with the side length longer than 3 nm, however, its quantum size effect seems not so obvious. The excited-state population is computed by using orbital projections. The occu-