DFT plus U calculations of the ground state and metastable states of uranium dioxide

We report a study of the ground state and metastable states of uranium dioxide using ab initio DFT+U calculations. We highlight that in order to avoid metastable states and systematically reach the ground state of uranium dioxide with DFT+U, the monitoring of occupation matrices is crucial, as well as allowing the 5f electrons to break the cubic symmetry. For this purpose, we use a method based on the monitoring of the occupation matrix of the correlated orbitals on which the Hubbard term is applied. We observe the presence of numerous metastable states in calculations both with and without taking into account the symmetries of the wave functions. We investigate the influence of metastable states on the total energy, as well as on the electronic and structural properties of uranium dioxide. We show that the presence of metastable states induces large differences in the total energy and explain the origin of the discrepancies observed in the results obtained by various authors on crystalline and defect-containing UO2. Finally, in order to check the consistency of the procedure, we determine the structural and electronic properties of the ground state of uranium dioxide and compare them with results obtained in previous studies using the DFT+U approximation and hybrid functionals, as well as experimental data.