A procedure for bypassing metastable states in local basis set DFT+U calculations and its application to uranium dioxide surfaces

We present a study of the bulk and the (1 0 0), (1 1 0) and (1 1 1) surfaces of uranium dioxide (UO2) using DFT+U in combination with a mixed Gaussian and plane waves basis set method, as implemented in the CP2K program package. The compromise of accuracy against computational efficiency by which this method enables calculations of relatively large UO2 systems is found to be acceptable in light of the almost 50-fold improvement in calculation times. A novel scheme is presented which reliably allows the system to escape the lower-lying metastable states that arise from the improved treatment of the strongly correlated 5f electrons of uranium. Based on the electronic configurations obtained by an f-occupation smearing combined with U ramping, this method relaxes the local energy minima by averaging the f-occupation matrices used to calculate the corrections in DFT+U. Various surface terminations are investigated and their calculated formation energies are found to be consistent with the experimentally observed morphologies. The direction of the antiferromagmentic ordering in relation to the surfaces exerts little influence on the results of the calculations while surface reconstructions can induce significant structural changes extending well into the bulk.