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.
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