are coherent oscillations that exist as evanescent waves at both sides of the interface between any two materials where the real part of the dielectric function changes sign across the interface. The typical example is a metal-dielectric interface, such as a metal sheet in air [@SaridChallener]. Maxwell’s equations can be employed to model the macroscale electromagnetic waves and armed with classical electrodynamics there are numerous approaches ranging from classical electrodynamics to ab initio treatments [@Gallinet2015; @Varas2016]. Ab initio techniques can be used to simulate the microscopic dynamics on the atomic scale, but with ab initio methods one can only deal with systems with up to about ten thousand atoms [@Varas2016], thus calling for semiclassical treatments [@MortensenGNOR; @ToscanoEtal2015] or more effective inclusions of quantum phenomena into classical electrodynamics [@Luo2013; @Yan2015; @Christensen2016; @Zhu2016].

If one models the interaction of light with metallic nanostructures classically or semiclassically, then this calls for appropriate modelling of the material response as described for example by the Drude model [@Drude1900; @DresselScheffler2006], the Nonlocal Hydrodynamic Drude (NHD) model [@Bloch1933a; @RazaEtal2015JPCM], or the Generalized Nonlocal Optical Response (GNOR) theory [@MortensenGNOR], all in combination with and coupled to Maxwell’s equations. Except for some highly symmetric geometries, analytical solutions to the 