Fig. 7: Descriptor to evaluate HER performance.

a The average charge difference ΔQ_Cu-Cu between two adjacent Cu atoms on the edge site and vertical site for Cu_55-nM_n (M = Co, Ni, Ru, and Rh) alloy clusters, b linear relationship between the number of active sites (|ΔG_H| < 0.15 eV) and the average charge difference ΔQ_Cu-Cu.