  Metallic nanostructures exhibit a multitude of optical resonances associated
with localized surface plasmon excitations. Recent observations of plasmonic
phenomena at the sub-nanometer to atomic scale have stimulated the development
of various sophisticated theoretical approaches for their description. Here
instead we present a comparatively simple semiclassical generalized nonlocal
optical response (GNOR) theory that unifies quantum-pressure convection effects
and induced-charge diffusion kinetics, with a concomitant complex-valued GNOR
parameter. Our theory explains surprisingly well both the frequency shifts and
size-dependent damping in individual metallic nanoparticles (MNPs) as well as
the observed broadening of the cross-over regime from bonding-dipole plasmons
to charge-transfer plasmons in MNP dimers, thus unraveling a classical
broadening mechanism that even dominates the widely anticipated
short-circuiting by quantum tunneling. We anticipate that the GNOR theory can
be successfully applied in plasmonics to a wide class of conducting media,
including doped semiconductors and low-dimensional materials such as graphene.
