Ab initio calculations of harmonic vibrational frequencies deviate from observed vibrational spectra because of errors in the ab initio potential and anharmonicity. Based upon the Morse model potential, an effective mass can be used to mimic anharmonic effects. Exploratory calculations show that this strategy is effective for diatomic molecules. Parametric expressions are provided for predicting fundamental frequencies and zero-point energies from calculated harmonic frequencies. (C) 2005 Published by Elsevier B.V.