Perturbative handling of the Renner-Teller effect and spin-orbit coupling in Δ electronic states of triatomic and tetra-atomic molecules

Second-order perturbative formulae for handling the Renner-Teller effect combined with the spin-orbit coupling in Delta electronic states of triatomic and symmetric (ABBA-type) tetra-atomic molecules with linear equilibrium geometry are derived via two schemes for partition of the model Hamiltonian. The formulae for triatontic molecules are employed to compute the spectrum of the X-5 Delta(g) state of FeH2. The parameters entering the model Hamiltonian are generated by means of ab initio calculations. (C) 2002 Elsevier Science (USA).