Characterization and control of the shape of nanoparticles has a primary importance in nanoscience and nanotechnology since most of the physical and chemical properties are shape-dependent.In recent years,many in situ experimental observations have shown that metal nanoparticles can change their shapes and structures dramatically and reversibly under reactive gas conditions.However,despite the experimental achievements,the precise theoretical prediction of this kind of shape evolution is still a challenging and demanding task.In this work,using CO@Pt as a benchmark,we develop a multiscale structure reconstruction model to quantitatively illuminate the equilibrium geometries of metal nanoparticles at given temperature and gas pressure.This model perfectly reproduces the experimental results and explains some intriguing phenomena,including the CO-induced breakup of Pt surfaces.The shape evolution results of Pt,Pd,Cu,and Au nanoparticles under CO and NO gas environments are presented.Our study provides useful guidelines for improving and developing real catalysts.