The electromagnetic response of metal contains the contribution of the free charges and the bound charges，which are usually theoretically descripted by the Drude model and the Lorentz model in collage physics respectively. However，only in a certain frequency range that the prediction from both models agrees with the experimental values. In practical scientific research，one should make use of the experimental values of the dielectric constant instead of the results from the Drude model or the Lorentz model when the physical quantity is related to the response over a wide frequency range in principal. In this work，the energy level shift of a ground two-level system around a metal nanosphere is numerically demonstrated with the electric permittivity of metal from the Drude model and the experimental values. Significant difference is observed，which implies the importance of the correct usage of the electric permittivity of metal.