Gaussian (14s13p10d8f6g)/[6s6p5d4f3g] atomic natural orbital valence basis sets have been generated for relativistic energy-consistent small-core actinide pseudopotentials of the Stuttgart-Bonn variety. Effective valence spin-orbit operators supplementing the scalar-relativistic pseudopotentials have been derived from multiconfiguration Dirac-Hartree-Fock reference data. Pseudopotentials, basis sets and spin-orbit operators have been used to determine the first and second ionization potentials of all actinide elements at the multiconfiguration self-consistent field and multireference averaged coupled-pair functional level. Comparison is made to results obtained from large-scale calculations using uncontracted basis sets up to i-type functions and extrapolation to the basis set limit as well as to experimental data. Molecular calibration studies using the coupled-cluster singles, doubles, and perturbative triples approach are reported for the ground states of AcH, AcO, AcF, and ThO. (C) 2003 American Institute of Physics.