Towards a double many-body expansion method for multivalued potential energy surfaces

Abstract

Using the diatomics-in-molecules formalism to get a first-order potential matrix, an extension is proposed of the double many-body expansion (DMBE) method for multivalued potential energy surfaces. The method consists of dressing the diatomic states with many-body energy terms of the extended Hartree-Fock and dynamical correlation type, with spin-recoupling and orientational effects being treated as in the traditional diatomics-in-molecules approach. All states of the polyatomic dissociate properly. Examples are discussed for s³, s²p, and sp² systems. Specifically, a DMBE potential energy surface is reported for NO₂ which shows the correct energy and geometry at the equilibrium structure and is in fair agreement with ab initio calculations at other regions of configuration space. Thus, this NO₂ potential energy surface may be useful for molecular dynamics studies.