Perturbative and variational handling of the Renner–Teller effect in Δ electronic states of triatomic molecules

Second-order perturbative formulae for handling the Renner–Teller effect in Δ electronic states of triatomic molecules are derived. They are applied to calculate vibronic energy levels of the X 5Δg state of FeH2. The ab initio computed bending potential energy curves are employed. The results of perturbative calculations are compared with those obtained by means of a variational approach.