How accurate is time-independent perturbation theory for calculating frequency shifts of diatomic molecules in rare gas fluids?

Time-independent perturbation theory is the standard method for calculating frequency modulations of molecules in liquids to obtain vibrational spectra and dephasing times. This approach is tested by comparison with a highly accurate method for calculating the vibrationally adiabatic energy levels in which the vibrational Schrödinger equation is solved explicitly. Several diatomic solutes in rare gas solvents are considered. Detailed information on the mechanism of the frequency shifts is also presented.