  We present full quantum mechanical calculations of the hybridized plasmon
modes of two nanowires at small separation, providing real space visualization
of the modes in the transition from the classical to the quantum tunneling
regime. The plasmon modes are obtained as certain eigenfunctions of the
dynamical dielectric function which is computed using time dependent density
functional theory (TDDFT). For freestanding wires, the energy of both surface
and bulk plasmon modes deviate from the classical result for low wire radii and
high momentum transfer due to effects of electron spill-out, non-local
response, and coupling to single-particle transitions. For the wire dimer the
shape of the hybridized plasmon modes are continuously altered with decreasing
separation, and below 6 {\AA} the energy dispersion of the modes deviate from
classical results due to the onset of weak tunneling. Below 2-3 {\AA}
separation this mode is replaced by a charge-transfer plasmon which blue shifts
with decreasing separation in agreement with experiment, and marks the onset of
the strong tunneling regime.
