The stability of bimetallic and multi-metallic SACs has received less attention. Besides changes in size, segregation phenomena can occur and be evaluated by DFT using the segregation and aggregation energies as descriptors of hetero-atom mobility. occur continuously toward alloying, dealloying, or segregation determined by the element type, atomic ratio, and external conditions in the phase diagram. With the development of SACs from monometallic to multi-metallic systems, the interaction between individual atoms has received increasing attention in regulating electronic structures and catalytic reactivity. Such metal-metal interaction becomes more prominent in ultrahigh loading SACs, whose inter-site distance is sufficiently close to enjoy a synergistic effect. In such an event, the bond length of the hetero-nuclear pair may experience dynamic changes.