The generalized Heitler-London theory for the H-3 potential energy surface

A systematic perturbation theory has been developed to analyze the terms co ntributing to the H-3 potential energy surface. Group theory is used to fin d the irreducible representations of the Hamiltonian matrix elements which then are decoupled to their simplest level. In the case of diatomic molecul es, this approach leads naturally to the Generalized Heitler-London (GHL) t heory derived previously [K. T. Tang and J. P. Toennies, J. Chem. Phys. 95, 5918 (1991)]. Many previous semi-empirical potential surfaces for the H-3 triatomic system including the well known LEPS surface are examined in the light of the present results. In particular, the Cashion-Herschbach (or dia tomics-in-molecules without overlap) surface is shown to encompass far more information than previously recognized. The new theory now contains all th e terms needed for an exact perturbation calculation of the potential energ y surface. (C) 1999 American Institute of Physics. [S0021-9606(99)30832-1].