  The ferropnictide superconductors exhibit a sensitive interplay between the
lattice and magnetic degrees of freedom, including a number of phonon modes
that are much softer than predicted by nonmagnetic calculations using density
functional theory (DFT). However, it is not known what effect, if any, the
long-range magnetic order has on phonon frequencies above 23 meV, where several
phonon branches are very closely spaced in energy and it is challenging to
isolate them from each other. We measured these phonons using inelastic
time-of-flight neutron scattering in ~40 Brillouin zones, and developed a
technique to determine their frequencies. We find this method capable of
determining phonon energies to ~0.1 meV accuracy, and that the DFT calculations
using the experimental structure yield qualitatively correct energies and
eigenvectors. We do not find any effect of the magnetic transition on these
phonons.
