We study high-frequency spectra of classical gases interacting via Lennard-Jones and similar potentials with applications to the problem of vibrational energy relaxation. We derive expressions for the spectra which are asymptotically exact at high frequencies and compare these with classical trajectory and molecular dynamics simulations. Trajectory calculations are in quantitative agreement with the theory, but conventional NVE molecular dynamics simulations are qualitatively in error at high frequencies due to insufficient simulation time.