  Quantum aspects, such as electron tunneling between closely separated
metallic nanoparticles, are crucial for understanding the plasmonic response of
nanoscale systems. We explore quantum effects on the response of the
conductively coupled metallic nanoparticle dimer. This is realized by
stretching a nanorod, which leads to the formation of a narrowing atomic
contact between the two nanorod ends. Based on first-principles time-dependent
density-functional-theory calculations, we find a discontinuous evolution of
the plasmonic response as the nanorod is stretched. This is especially
pronounced for the intensity of the main charge-transfer plasmon mode. We show
the correlation between the observed discontinuities and the discrete nature of
the conduction channels supported by the formed atomic-sized junction.
