The relativistic all-electron ab initio calculations have been performed on 5f³ manifold of free U³⁺ ion to explore the electronic structure and spectra. The spin-orbit-free low lying electronic states have been calculated at the level of theories of State-Averaged Complete Active Space Self-Consistent Field (SA-CASSCF) and Multi-State Complete Active Space Second-Order Perturbation (MS-CASPT2) using different sets of active orbitals. The corresponding spin-orbit states and the energy levels have been computed using Douglas-Kroll type of atomic mean-field integral (AMFI) approach. The ground state is ⁴I_9/2 and the electron correlation has been found to be small in case of the excited ⁴I states. The low-lying energy levels obtained with the core-valence correlation effects in the treatment are in very good agreement with the experimental values.

在自由U ³⁺离子的5f ³流形上进行了相对论的全电子从头计算，以探索电子结构和光谱。在状态平均完整有源空间自洽场（SA-CASSCF）和多状态完整有源空间二阶扰动（MS-CASPT2）的理论水平上计算了无自旋轨道的低躺电子状态。使用不同组的活动轨道。相应的自旋轨道状态和能级已使用Douglas-Kroll类型的原子平均场积分（AMFI）方法进行了计算。基态为⁴ I _9/2，并且发现在激发^4的情况下电子相关性很小我说。在治疗中具有核心价相关效应的低能级水平与实验值非常吻合。