The alloy segregation is a basic and important issue in studying the surface property of nanoalloy.Most of the previous works focus on the segregation under vacuum condition due to the complex of the environment effect.However resent experiments show the surface reconstruction under the reactive condition[1-3].One of our previous works also proves the CO induced Pd migration to the low-coordinated surface sites[4].How to simulate the segregation in complicated environment is a challenging and interesting task in todays bimetallic nanoalloy research.In our previous study,on the basis of DFT calculations of adsorption energies of CO on different AuPd(100)surfaces,we already established a rigid lattice Ising model to study the isotherms of Pd surface concentration at given CO partial pressure.In the present work we try to fit an empirical potential to describe the interaction between CO and AuPd surface,which will allow us to extend our model to relaxed AuPd alloy cluster.