  We report an experimental and theoretical investigation of the lattice
dynamics and thermal properties of the actinide dioxide NpO$_2$. The
energy-wavevector dispersion relation for normal modes of vibration propagating
along the $[001]$, $[110]$, and $[111]$ high-symmetry lines in NpO$_2$ at room
temperature has been determined by measuring the coherent one-phonon scattering
of X-rays from a $\sim$1.2 mg single-crystal specimen, the largest available
single crystal for this compound. The results are compared against ab initio
phonon dispersions computed within the first-principles density functional
theory in the generalized gradient approximation plus Hubbard $U$ correlation
(GGA+$U$) approach, taking into account third-order anharmonicity effects in
the quasiharmonic approximation. Good agreement with the experiment is obtained
for calculations with an on-site Coulomb parameter $U = 4$ eV and Hund's
exchange $J= 0.6$ eV in line with previous electronic structure calculations.
We further compute the thermal expansion, heat capacity, thermal conductivity,
phonon linewidth, and thermal phonon softening, and compare with available
experiments. The theoretical and measured heat capacities are in close
agreement with another. About 27% of the calculated thermal conductivity is due
to phonons with energy higher than 25 meV ($\sim$ 6 THz ), suggesting an
important role of high-energy optical phonons in the heat transport. The
simulated thermal expansion reproduces well the experimental data up to about
1000 K, indicating a failure of the quasiharmonic approximation above this
limit.
