We theoretically investigate gap plasmons for two silver nanocubes coupled through a molecular tunnel junction. In absence of tunneling, the red-shift of the bonding mode saturates with decreasing gap distance. Tunneling at small gap distances leads to a damping and slight blue-shift of the bonding mode, but no low-energy charge transfer plasmon mode appears in the spectra. This finding is in stark contrast to recent work of Tan et al. [Science 343, 1496[Science 343, (2014 [1,[3][4][5][6]. Molecular tunnel junctions enable tunneling over larger gap distances in the nanometer regime [7,8], and thus establish a novel platform for hybrid structures reconciling molecular electronics with plasmonics.Recent years have seen significant research efforts to understand the properties of gap plasmons, and have highlighted the importance of the tunneling strength as a trigger for the CTP appearance [9] and of the gap morphology which strongly influences the CTP spectral position [10]: for rounded gap terminations the bonding mode red-shifts with decreasing gap separation, until tunneling sets in, as evidenced by the appearance of a low-frequency CTP together with a broadening and blueshift of the bonding mode [1,3,6]. In contrast, for flat terminations the red-shift of the bonding mode saturates with decreasing gap distance, while at the same time the transversal cavity plasmon (TCP) modes shift to the red; here, the onset of tunneling has no significant impact on the bonding mode and no low-frequency CTP appears in the spectra.In this paper, we theoretically investigate the plasmonic properties of two coupled silver nanocubes, similarly to the electron energy loss spectroscopy (EELS) experiments of Tan et al. [7] for two nanocubes coupled through a molecular tunnel junction. We compute EEL and extinction spectra using the MNPBEM toolbox [11][12][13], supplemented with the quantum corrected model (QCM) [14] to account for quantum tunneling. We find that the red-shift of the bonding mode saturates with decreasing distance and an additional tunnel conductivity in the gap region leaves the spectral position unaffected. The TCP modes shift with decreasing gap distance to the red, and the tunnel conductivity damps these modes. All these findings are in perfect agreement with the observations of Esteban et al. [6] for flat gap terminations and would qualify our work as a systematic research paper, if it was not for this single point: despite serious efforts we were unable to confirm the emergence of the low-energy CTP observed by Tan et al. [7] and could not reproduce their simulation results. We will argue why we believe that our results are valid within the electrodynamic and QCM model under consideration, and why a re-interpretation of the experiments might be needed.In our simulations we model the cubes with rounded edges and corners as superellipsoides, whose boundaries are parameterized through u ∈ [0, π) and v ∈ [−π, π) according towhere a determines the cube size (we use side lengths of 35 nm throughout), r is a roundi...