  We present a comprehensive investigation of lattice dynamics in the
double-helix antiferromagnet FeP by means of high-resolution time-of-flight
neutron spectroscopy and ab-initio calculations. Phonons can hybridize with the
magnetic excitations in noncollinear magnets to significantly influence their
properties. We observed a rich spectrum of phonon excitations, which extends up
to $\sim$50 meV. We performed detailed analysis of the observed and calculated
spectra for all high-symmetry points and high-symmetry directions of the
Brillouin zone. We show that the DFT calculations quantitatively capture the
essential features of the observed phonons, including both dispersions and
scattering intensities. By making use of the detailed intensity comparison
between the theory and the data, we were able to identify displacement vectors
for the majority of the observed modes. The overall excellent agreement between
the DFT predictions and the experimental results breaks down for the lowest
mode at the $Y$-point, whose energy is lower than calculated by $\sim$13%. The
present study provides vital information on the lattice dynamics in FeP and
demonstrates applicability of the DFT to novel pressure-induced phenomena in
related materials, such as MnP and CrAs.
