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 vibrationaliy adiabatic energy levels in which the vibrational Schrodinger 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. (c) 2005 Elsevier B.V. All rights reserved.