Activation energies for atomic transitions were calculated using the climbing image nudged elastic band method (CI-NEB) with 5 images, including starting and ending structures. The convergence criteria in the CI-NEB are 0.02 eV/ Å Vibrational analysis and ab initio molecular dynamics were used to determine the stability of single atom adsorption at an adsorption site (vide infra). Vibrational analysis was performed by computing second-order force constants using finite differences, with no symmetry imposed. The convergence criteria for energy minimisation was set to 10 -7 eV to ensure accuracy. Only sulphur atoms directly below the adsorbed Cu atom, the three nearest W atoms to the Cu, and the Cu atom itself were not frozen in this analysis. The molecular dynamics simulations were performed in the NVT ensemble with an energy cutoff of 250 eV, and temperature of 100 K. A timestep of 2.5 fs for 2000 steps was used resulting in an overall simulation time of 5 ps.