  The optical response of a coupled nanowire dimer is studied using a fully
quantum mechanical approach. The translational invariance of the system allows
to apply the time--dependent density functional theory for the plasmonic dimer
with the largest size considered so far in quantum calculations. Detailed
comparisons with results from classical electromagnetic calculations based on
local and non local hydrodynamic response, as well as with results of the
recently developed quantum corrected model is performed. We show that electron
tunneling and dynamical screening are the major nonlocal quantum effects
determining the plasmonic modes and field enhancement in the system. Account
for the electron tunneling at small junction sizes allows semi-quantitative
description of quantum results within classical framework. We also discuss the
shortcomings of classical treatments using non-local dielectric permittivities
based on hydrodynamic models. Finally, the implications of the actual position
of the screening charge density for the plasmon ruler applications are
demonstrated.
