These calculations correlated the Th 5s5p5d electrons, since the correlation of these actinide semicore electrons has been shown to make important contributions to thermochemistry parameters . Furthermore, the unpaired electrons in the excited states occupy orbitals with higher angular momenta than the ground state. Therefore, the differential basis-set effects may be significant. To obtain a good treatment of basis-set effects, we have performed calculations of vertical excitation energies using uncontracted basis sets of triple-zeta (TZ) and quadruple-zeta (QZ) quality and then estimated the basis-set-limit (∞Z) values by means of an extrapolation using a two-point formula . These basis sets using primitive functions in the ANO-RCC set and correlation-consistent basis sets are documented in the Supporting Information. The TZ, QZ and ∞Z results of the computed vertical excitation energies are summarized in Table . We have further estimated the spin-orbit corrections as the difference between the X2CAMF-EOM-CCSD vertical excitation energies and the SFX2C-1e EOM-CCSD results. The X2CAMF and SFX2C-1e calculations have employed Dyall's triple-zeta basis set of Th recontracted for X2CAMF using a spin-orbit contraction scheme and for SFX2C-1e using a scalar-relativistic contraction scheme, respectively. The inclusion of core-correlation, basis-set, and spin-orbit corrections gives the current best estimate for vertical excitation energies listed in the "+SO" column of Table . The adiabatic excitation energies have been obtained by augmenting the vertical excitation energies with the geometry relaxation energies, the difference between the excited state energies computed in their equilibrium structures and those computed in the equilibrium structure of the ground state.