  As the dimensions of plasmonic structures or the field confinement length
approach the mean free path of electrons, mesoscopic optical response effects,
including nonlocality, electron density spill-in or spill-out, and Landau
damping, are expected to become observable. In this work, we present a
quantum-informed local analogue model (QILAM) that maps these nonclassical
optical responses onto a local dielectric film. The primary advantage of this
model lies in its compatibility with the highly efficient boundary element
method (BEM), which includes retardation effects and eliminates the need to
incorporate wavevector-dependent permittivity. Furthermore, our approach offers
a unified framework that connects two important semiclassical theories: the
generalized nonlocal optical response (GNOR) theory and the Feibelman
d-parameters formalism. We envision that QILAM could evolve into a multiscale
electrodynamic tool for exploring nonclassical optical responses in diverse
plasmonic structures in future. This could be achieved by directly translating
mesoscopic effects into observable phenomena, such as resonance energy shifts
and linewidth broadening in the scattering spectrum.
