  In Yang et al. [Nature 576, 248 (2019)], the authors introduced a general
theoretical framework for nanoscale electromagnetism based on Feibelman
parameters. Here quantum effects of the optically excited electrons at the
interface between two materials are lumped into two complex-valued and
frequency-dependent parameters, which can be incorporated into modified
boundary conditions for Maxwell's equations, the so-called mesoscopic boundary
conditions. These modifications can in principle be implemeted in any Maxwell
solver, although the technicalities can be subtle and depend on the chosen
computational approach. In this paper we show how to implement the mesoscopic
boundary conditions in a boundary element method approach, based on a Galerkin
scheme with Raviart-Thomas shape elements for the representation of the
tangential electromagnetic fields at the boundary. We demonstrate that the
results of our simulations are in perfect agreement with Mie theory including
Feibelman parameters, and that for typical simulation scenarios the
computational overhead is usually small.
