We study the bending motion in the tetratomic molecules C2H2 (˜X 1
+
g ), C2H2 ( ˜A 1Au) trans-S1,
C2H2 ( ˜A 1A2) cis-S1, and ˜X 1A1 H2CO. We show that the algebraic operator expansion method with
only linear terms comprised of the basic operators is able to describe the main features of the level
energies in these molecules in terms of two (linear) or three (trans-bent, cis-bent, and branched) parameters.
By including quadratic terms, the rms deviation in comparison with experiment goes down
to typically ∼10 cm−1 over the entire range of energy 0–6000 cm−1.We determine the parameters by
fitting the available data, and from these parameters we construct the algebraic potential functions.
Our results are of particular interest in high-energy regions where spectra are very congested and
conventional methods, force-field expansions or Dunham-expansions plus perturbations, are difficult
to apply