Ionic Models and the Vibrational Spectra of Molecules

It is shown that an ionic model with polarization terms gives reasonable values for the vibrational frequencies of linear symmetric MX2 molecules. The potential function of such a model is indistinguishable from the valence force potential with interaction terms and this result is general for all molecules. Contrary to the literature, even a central force potential will give a nonzero bending frequency for a linear symmetric molecule.