  We investigate the low energy acoustical and optical modes in
$\mathrm{HgBa_2CuO_{4+\delta}}$ using inelastic x-ray scattering (IXS). The
experimental phonon dispersion and the dynamical structure factor are compared
with an atomic shell model, and the set of the atomic potentials obtained are
discussed. Our results are also compared with those obtained by Raman
spectroscopy and with density-of-state data measured by inelastic neutron
scattering.
