Hamiltonian was employed, and the dyall.cv4z basis set was used for both uranium (U) and oxygen (O) atoms. In the UO₂²⁺ molecule, numerous quasi-degenerate orbitals are present around the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), necessitating a large active-space calculation. Consequently, the RASPT2 method was employed instead of CASPT2. The active orbital space was defined based on the energies of the molecular orbitals. Table details the molecular spinors near the frontier orbitals, including the assignment of the active space and the character of the orbitals used in the RASPT2 calculations. Preliminary CASCI calculations with a smaller active space indicated that the primary configurations for both the target ground and excited states consisted exclusively of singly and doubly excited configurations. In addition, the presence of numerous quasi-degenerate orbitals around the HOMO