In this thesis relativistic energy-consistent pseudopotentials (PPs) for f-elements have been adjusted. The PP approach restricts the explicit calculations to the chemically relevant valence electron system and implicitly includes relativistic effects by means of a simple parameterization. Thus, it is a commonly used approximation to study molecules containing f-elements, where the large number of electrons and the significant relativistic effects are the main obstacles. Even difficulties due to open shells can be avoided, if these are included in the core, as it is the case for f-in-core PPs. However, if the f shell is not treated explicitly, one PP for each oxidation state has to be adjusted. This thesis completes the already existing quasirelativistic f-in-core PPs, i.e. 5f-in-core PPs for di- (Pu-No), tetra- (Th-Cf), penta- (Pa-Am), and hexavalent (U-Am) actinides and 4f-in-core PPs for tetravalent (Ce-Nd, Tb, Dy) lanthanides are presented. Corresponding molecular basis sets of polarized valence double- to quadruple-zeta quality have been derived. Smaller basis sets suitable for crystal calculations form subsets of these basis sets. Furthermore, core-polarization potentials for di-, tri-, and tetravalent actinides have been adjusted to account for the neglect of static and dynamic core-polarization. Atomic test calculations on actinide ionization potentials as well as molecular test calculations on actinide and lanthanide fluorides using the Hartree-Fock and coupled cluster method show satisfactory agreement with calculations using f-in-valence PPs and experimental data, respectively, except for plutonium difluoride and neptunium, plutonium, and americium hexafluoride. While for plutonium difluoride the large deviations are due to the fact that for plutonium the divalent oxidation state is not stable, in the hexavalent case the 5f-in-core approximation seems to reach its limitations except for uranium. Moreover, the 5f-in-core PPs are successfully applied to actinocenes, actinyl ions, and uranyl(VI) complexes. Thus, the f-in-core PPs should be an efficient computational tool for those compounds, where the f orbitals do not participate significantly in chemical bonding. In addition to the quasirelativistic f-in-core PPs, the recently adjusted 5f-in-valence uranium PP including scalar-relativistic effects as well as spin-orbit coupling have been tested by calculating the fine-structure splittings of U5+ and U4+. These test calculations gave reliable results and thus confirm earlier benchmark calculations on uranium monohydride
