TOWARDS A DOUBLE MANY-BODY EXPANSION METHOD FOR MULTIVALUED POTENTIAL-ENERGY SURFACES - THE H-3, FH2 AND NO2 SYSTEMS

Using the diatomics-in-molecules formalism to get a first-order potent ial matrix, an extension is proposed of the double many-body expansion (DMBE) method for multivalued potential energy surfaces. The method c onsists of dressing the diatomic states with many-body energy terms of the extended Hartree-Fock and dynamical correlation type, with spin-r ecoupling and orientational effects being treated as in the traditiona l diatomics-in-molecules approach. All states of the polyatomic dissoc iate properly. Examples are discussed for s(3), s(2)p, and sp(2) syste ms. Specifically, a DMBE potential energy surface is reported for NO2 which shows the correct energy and geometry at the equilibrium structu re and is in fair agreement with ab initio calculations at other regio ns of configuration space. Thus, this NO, potential energy surface may be useful for molecular dynamics studies.