A model is proposed to analyze the interaction energy of CO molecules adsorbed on metallic nanostructured surfaces in an external electric field, including the one between effective dipoles and the local electric field as well as the one between effective dipoles . The local fields near the nanoparticles surfaces are also calculated and discussed. Using Monte-Carlo numerical computation and simulation, the configuration of CO molecules adsorbed on nanoparticle surfaces and the interaction energy are given in detail. The results show that the metallic nanostructures cause the CO molecules to agglomerate and the interaction energy increase may explain the abnormal IR effects (AIREs).