where Dmig is the migration rate constant, D0 is the pre-exponential factor, Emig is the atomic migration energy barrier, and T is the absolute temperature. As mentioned above, the activation energy barrier is closely related to the metal-substrate interaction (Ebind) and the cohesive energy of bulk metal (Ec), which depends on the type of element, facet, defect, and reaction intermediate. External stimuli in working conditions, including temperature, electric field, and adsorbates, facilitate the atomic migration of single atoms. For example, high temperature induces a larger amplitude of atomic vibration near the equilibrium position, thereby increasing the possibility to cross the migration barrier. External electrical fields can drive the atomic diffusion or migration toward specific directions and even cause the uphill diffusion behavior. It should be pointed out that SACs can undergo dynamic structural transformations in both geometric (e.g., aggregation) and electronic structures in a catalytic cycle. The changes in local coordination environments (e.g., type and number of coordinated atoms) by bond dissociation and oxidation states by polarization or distortion of its electron clouds can interplay for a macroscopical behavior of structural evolution. Such evolution could be progressive, irreversible, and easily detected by ex situ techniques. By contrast, it may occur discreetly and transiently under reaction conditions, requiring the use of operando methods to understand the underlying mechanism. 1