  Previously reported inelastic x-ray scattering spectra of a typical van der
Waals molecular liquid CCl4 were reanalyzed by using a generalized Langevin
formalism with a memory function including a thermal and two viscoelastic
relaxation processes together with a simple sparse modeling. The obtained
excitations of longitudinal acoustic phonons show a largely positive deviation
from the hydrodynamic value by about 57%, much larger than about 37% by the
previously reported damped harmonic oscillator result. Such large values of
fast sounds in molecular liquids being larger than about 15-20% of typical
liquid metals are interpreted as extra energy-losses for terahertz phonons by
vibrational and rotational motions of molecules. The rates of the fast and slow
viscoelastic relaxations in the memory function at low Q indicate large values,
about 0.5 and 2 ps, which correspond to the vibrational and rotational motions
of CCl4 molecules, respectively. These values are larger than those of the
typical polar molecular liquid acetone, which may reflect the heavier atomic
mass of CCl4. The Q dependences of the viscoelastic relaxation rates are
discussed in terms of lifetime and propagating length of the terahertz phonon
oscillations. The microscopic kinematic longitudinal viscosity rapidly
decreases with Q from a reasonable macroscopic value.
