  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.
