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Due to the dependence of the chemical and physical properties of the bimetallic nanoparticles (NPs) on their structures,a fundamental understanding of their structural characteristics is crucial for their syntheses and wide applications.In this article,a systematical atomic-level investigation of Au-Pd bimetallic NPs is conducted by using the improved particle swarm optimization (IPSO) with quantum correction Sutton-Chen potentials (Q-SC) at different Au/Pd ratios and different sizes.In the IPSO,the simulated annealing is introduced into the classical particle swarm optimization (PSO) to improve the effectiveness and reliability.In addition,the influences of initial structure,particle size and composition on structural stability and structural features are also studied.The simulation results reveal that the initial structures have little effects on the stable structures,but influence the converging rate greatly,and the convergence rate of the mixing initial structure is clearly faster than those of the core-shell and phase structures.We find that the Au-Pd NPs prefer the structures with Au-rich in the outer layers while Pd-rich in the inner ones.Especially,when the Au/Pd ratio is 6:4,the structure of the nanoparticle (NP) presents a standardized PdcoreAushell structure.