A method and codes for two-step correlation calculations of heavy-atom molecules in ²⁰⁵TlF were presented using the generalized relativistic effective core potential (GRECP) and relativistic coupled cluster (RCC) methods. A two-component electronic (pseudo)wave function was obtained with the 21-electron GRECP for Tl which provides electron density in the valence and outer core regions. The result shows that electron density obtained from the two-component GRECP/RCC (pseudo)wave function in the valence and outer core region is close to that of the corresponding all-electron four-component function.