Electron correlation and relativistic effects in atomic structure calculations of Th+, Th2+ ions

Relativistic two-component ab initio calculations through second-order Douglas-Kroll-Hess (DKH2) transformation are performed on Th+ and Th2+ ions. Spin–orbit-free calculations are done at SA-CASSCF and MS-CASPT2 levels. Spin–orbit coupled states are studied using effective mean-field operator. Spin–orbit states of Th+, below 23 000 cm−1 are compared with experimental values. Relative separations between various energy levels depend on the amount of electron correlation included in the calculation. For Th2+, spin–orbit energy levels below 20 000 cm−1 agree well with the experimental levels. Transition properties of several spin–orbit states in case of Th2+ ion are predicted.