A double many-body expansion of molecular potential energy functions

Abstract

A general procedure recently proposed by one of the authors for constructing the potential energy surfaces of small polyatomic molecules and van der Waals molecules has been used to obtain explicit functions for HeH₂ ([Xtilde], ¹ A′), HeLi₂ ([Xtilde], ¹ A′) and HO₂ ([Xtilde], ² A″). The HeH₂ and HeLi₂ potentials compare well the best available correlated ab initio energies, and should be suitable for studying both vibration-rotation inelasticity in atom-diatom collisions and atom (H, Li) recombination in the presence of He atoms. For HO₂, the surface conforms with the ab initio data of Melius and Blint for the 3-body energy, and shows good agreement with available spectroscopic data for the energy and geometry of the minima referring to the equilibrium triatomic. Two secondary minima which have chemical interest are predicted. One refers to a T-shaped (C _2v ) H-O₂ weakly bound (℞ 25 kJ mol^-1 with respect to the H + O₂ dissociation limit) species, while the other is related to a linear (C _∞v ) weak hydrogen bonded OH … O structure (ε ℞ 26 kJ mol^-1, with respect to O + OH). The HO₂ surface shows no barrier for the H + O₂ two body addition reaction as implied from the best available ab initio calculations. There is a saddle point for isomerization between the two equivalent equilibrium HO₂ structures which lies ℞ 90 kJ mol^-1 below the dissociation limit into H + O₂. All potentials show the proper R^-n behaviour at the asymptotic channels. Hence, they are suitable for future dynamics studies.