As shown in Table , basis-set effects expectedly reduce the excitation energies. This is consistent with the expectation that the excited states with open-shell electrons in Th 6d, 5f, and 6p orbitals are stabilized more than the ground state with the increase of the size of basis sets. The basis-set effects beyond the triple-zeta basis sets range from 400 cm -1 to 2000 cm -1 for these excitation energies. Spin-orbit effects also play important roles in accurate calculations of the excited-state energies. For example, spin-orbit coupling increases the splitting between the two lowest-lying excited states, the 1 2 A" and 2 2 A' states, from around 1000 cm -1 to more than 3000 cm -1 . The geometry relaxation makes substantial contributions to the 4 2 A' state, for which the adiabatic and vertical excitation energies differ by more than 3000 cm -1 .