Using CI-NEB, the activation energies for Cu migration between sites X, M, and H were calculated on all three monolayers. The activation barrier to transition from M to H is essentially identical on all three monolayers, with an activation barrier of 0.16 eV on the sulphide and selenide monolayers, and 0.15 eV on the telluride monolayer. However, the activation energy to transition from H to M increases, albeit with small absolute values, as the size of the chalcogen atom increases. On WS 2 , WSe 2 , and WTe 2 , the barrier becomes 0.04, 0.08, and 0.12 eV respectively. This is due to the relative difference in adsorption stability between M and H decreasing as the size of the chalcogen atom increases, which can be seen from the binding energies. These low barriers suggest facile diffusion of a single Cu atom over the TMD monolayers, most likely moving between these two adsorption sites.