Electron correlation and relativistic effects in atomic structure calculations of the thorium atom

Relativistic two-component ab initio calculations have been performed for the Th atom. The spin free low lying states have been calculated at state-averaged complete active space self-consistent field (SA-CASSCF) and multi-state complete active space second-order perturbation (MS-CASPT2) level of theories using different sets of active orbitals. The spin-orbit states have been computed using Douglas-Kroll type of atomic mean-field integral approach. The effects of dynamic electron correlation have been studied at the MS-CASPT2 level. The energy levels of spin-orbit states below 30,000 cm(-1) obtained by the inclusion of dynamic electron correlation are in very good agreement with the experimental values. The radiative properties such as weighted transition probabilities (gA) and oscillator strengths (gf) among several spin-orbit states have been calculated at the SA-CASSCF and MS-CASPT2 levels and are expected to be very helpful for future experiments.