Assessment of the Accuracy of Shape-Consistent Relativistic Effective Core Potentials Using Multireference Spin−Orbit Configuration Interaction Singles and Doubles Calculations of the Ground and Low-Lying Excited States of U⁴⁺ and U⁵⁺

Multireference spin−orbit configuration interaction calculations were used to determine the accuracy of 60-, 68-, and 78-electron shape-consistent relativistic effective core potentials (RECPs) for uranium V and VI ground and low-lying excited states. Both 5fn and (5f6d)n, (n = 1, 2) reference spaces were investigated using correlation-consistent double-ζ quality basis sets. Accuracy was assessed against gas-phase experimental spectra. The 68-electron RECP calculations yielded low relative and rms errors and predicted the empirical ordering of states most consistently.