Configuration mixing in the groundstate of Ce

4f electrons at the beginning of the rare earth series are weakly bound yet highly localized. Their virtual excitations create core change fluctuations which strongly enhance excitonic effects. The dynamic interaction lead to a new groundstate which may viewed as a boundstate of a 4f electron and an f-exciton. The 4f-hole Greens' function is also strongly modified and shows two poles instead of one pole in the one-electron picture. The photoemission experiments and the X-ray edge absorption experiments in cerium and its metallic compounds are explained in this model. The new groundstate also provides a basis for the understanding of the γ-α phase transition of cerium. (Auth.)