This chapter addresses high‐accuracy calculations of actinide systems. Lanthanide systems, where the broader availability of experimental data provides more extensive checks of computed properties, are discussed. The chapter presents predictions for the eka‐actinide atoms E121 (eka‐Ac) and E122 (eka‐Th). Actinides (as well as their lanthanide homologues) present severe problems to theory and computations, caused by large, non‐additive relativistic and correlation effects, further complicated by the multireference character of many electronic states, involving the 5f, 6d, 6p, and 7s orbitals. The chapter gives an overview of a state‐of‐the‐art ab initio approach used in computational actinide chemistry and atomic physics: the relativistic Fock‐space coupled cluster (RFSCC) method. The Fock‐space and intermediate Hamiltonian coupled cluster methods were applied to the ground and excited levels of the second actinide element, thorium, and its heavy homologue eka‐thorium.