We have performed a systematic first-principles investigation to calculate the electronic structures, mechanical properties, and phonon dispersion curves of NpO$_{ 2}$. The local density approximation$+U$ and the generalized gradient approximation$+U$ formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Np $5f$ electrons. By choosing the Hubbard \emph{ U} parameter around 4 eV, the orbital occupancy characters of Np 5\emph{ f} and O 2\emph{ p} are in good agreement with recent experiments [J. Nucl. Mater. \textbf{ 389}, 470 (2009)]. Comparing with our previous study of ThO$_{ 2}$, we note that stronger covalency exists in NpO$_{ 2}$ due to the more localization behavior of 5\emph{ f} electrons of Np in line with the localization-delocalization trend exhibited by the actinides series.
Categories: cond-mat.mtrl-sci cond-mat.str-el