The four configurations studied are: four metal atoms separated as far as the computational supercell allows (denoted as apart), a straight line of metal atoms at adjacent adsorption sites (denoted as line), a flat 2D-rhombus of metal atoms (rhombus), and a 3D-tetrahedron (tetrahedron). For the apart and line configurations, the atoms were initially adsorbed at the A site as this was the most favourable single atom adsorption site. energy as Ru forms a cluster. A strong metal-to-substrate interaction energy is therefore the most important factor in having a strong binding energy for Ru.