Thermal properties of UO2 with a non-local exchange-correlation pressure correction: a systematic first principles DFT plus U study

We present results of a comprehensive first principles density-functional-theory-based study of structural and thermal properties of the strongly correlated UO2 system. The Mott-insulating ground state of UO2 is reproduced within DFT + U formalism with a Hubbard parameter of U-eff = 4 eV. The thermal properties, such as thermal expansion coefficient, bulk modulus, heat capacity and thermal conductivity, are calculated using the density functional perturbation theory in conjunction with the quasi-harmonic approximation. Surprisingly, the agreement between most of the computed thermal properties and the corresponding available experimental data is found to be rather poor. Moreover, the discrepancy worsens as temperature increases. We find the main cause of this error to be overprediction of the equilibrium lattice parameter by the theory. We have incorporated a simple non-local exchange-correlation pressure correction in our calculations to get the computed equilibrium lattice parameter closer to the experimental value. This simple procedure resulted in significant reduction in the discrepancy between the computed and experimental values of thermal properties of UO2. Further, our calculated phonon dispersion incorporating the non-analytical term correction for the macroscopic electric field associated with the longitudinal optic modes is in very good agreement with the experimental results available in the literature.