An accurate description of the optical response of subwavelength metallic particles and nanogap structures is a key problem of plasmonics. Quantum hydrodynamic theory (QHT) has emerged as a powerful method to calculate the optical response of metallic nanoparticles since it takes into account nonlocality and spill-out effects. Nevertheless, the absorption spectra of metallic particles from QHT is affected, at energies higher than the main plasmon peak, by several additional peaks, which are instead largely damped in reference time-dependent density-functional theory calculations. Moreover, we show here that these peaks have a strong dependence on the simulation domain-size so that the numerical convergence of QHT calculations is problematic. In this article, we introduce a QHT method accounting for kinetic energy contributions depending on the Laplacian of the electronic density, thus beyond the gradient-only dependence of conventional QHT. In this way, only the main plasmon peak is obtained, with a numerically stable absorption spectrum and more accurate intensities of the plasmon peak. Thus, the Laplacian-level QHT represents a novel, efficient and accurate platform to study plasmonic systems.
1 Jan 2020