First principles calculation of the elastic constants and phonon modes of UO2 using GGA + U with orbital occupancy control

The actinide oxide UO2 is studied using the GGA + U method combined with monitoring of the orbital occupation matrices to avoid erroneous metastable states. The need to ensure strict convergence of the ground state energy with basis set energy cut-off is demonstrated, and a ground state is determined with a highly isotropic unit cell. Using this ground state the elastic constants and phonon modes of UO2 are calculated and found to be in excellent agreement with experimentally determined values. Peaks observed in the experimental far-infrared response spectra are reproduced, and related to the vibrational modes of the heavy atoms in a static oxygen sub-lattice.