Green’s function technique has been applied to the calculation of force field changes when all the atoms in a linearXY₂ symmetric molecule undergo perturbation. The formulae for the valence force constant changes have been derived and verified in about 13 different perturbations.

Explicit and simple relations are derived for the Kivelson-Wilson parameters which can directly generate the vibrational spectrum of XY₂ bent type molecules. These relations are also shown to generate the above parameters for the different isotopes of a parent molecule. These predictions are verified in the case of some molecules of the above symmetry for which experimental microwave studies have been made.

Using Green’s function techniques, equations for the force field changes resulting from different chemical perturbations in XY₂ bent molecules have been derived and are shown to predict the changes reasonably well in a number of cases. In perturbations involving negligible change of the interbond angle, the equations generate the changes in a straightforward manner.