USE OF SYMMETRY-ADAPTED BRILLOUIN THEOREM TO ANALYZE THE VARIATIONAL CONTENT OF MOLECULAR WAVE-FUNCTIONS ALONG POTENTIAL-ENERGY SURFACES - APPLICATION TO BH2 AND PO2

The analysis of the Brillouin conditions in a chain of groups is used to explain why self-consistent field (SCF), multiconfigurational SCF ( MCSCF), or configuration interaction (CI) methods can lead to an unbal anced description of the potential energy hypersurfaces along symmetry -broken pathways. A Wigner-Racah formulation of the generalized Brillo uin theorem is the appropriate theoretical tool for performing such an analysis. We apply this procedure to a Renner-Teller splitting in BH2 and to a pseudo-Jahn-Teller conical intersection in PO2. A systematic ab initio study of both systems leads to the definition of compact CA SSCF wave functions which ensure a balanced description of the ground Born-Oppenheimer potential energy surfaces. (C) 1997 John Wiley & Sons , Inc.