On the limitations of the DFT+U approach to energetics of actinides

Scalar-relativistic density functional theory calculations are performed on three phases of Np within the framework of LSDA+U approach. Material properties examined include lattice parameters, atomic volumes, bulk modulus, magnetic moments, and cohesive energies. Dependence of these properties as a function of the Hubbard-type Coulomb U parameter is investigated, and it is observed that beyond the optimized value at which resulting properties compare well with experimental data, calculations suggest anomalously large changes in volumes supplemented by magnetic transitions. Several reasons that may have caused these instabilities in our calculations are discussed, and which should be considered by those resorting to DFT+U techniques for predicting phase stability in actinides and their alloys.