S-Neighbour-M forms the second structure seen in the Neighbouring adsorption. This structure is unstable and relaxes to the non-equivalent configuration S-Non-Equivalent-XM. For the S-Neighbour-M relaxation, one of the Cu atoms remains adsorbed at M while the second Cu atom migrates to X. This migration likely occurs to reduce the distance between both adsorbed Cu atoms, which then increases the stability of the nanocluster due to the formation of a Cu-Cu bond. The formation of this bond of course also increases the metal-metal interaction energy. The distance between both Cu atoms is ∼ 0.1 Å larger than the other Neighbouring structure. This small difference means the structures have identical energies.