ANALYTICAL POTENTIAL-ENERGY SURFACES FOR ALKALI DIHALIDE MOLECULES BASED ON THE DIATOMICS-IN-MOLECULES FORMALISM - APPLICATION TO LIF2

By judicious neglect of some of the off-diagonal elements in a diatomi cs-in-molecules Hamiltonian matrix for the ionic states of MX2 systems (M = alkali metal atom, X2=halogen diatomic), we are able to reduce t he matrix to three 2 x 2 blocks, yielding analytical expressions for s ix diabatic potential energy surfaces. The resulting expressions are e xact for C2v and linear geometries. All diatomic potentials include th e appropriate two-body long-range electrostatic, induction, and disper sion forces, while their short-range part has been modelled using spec troscopic data or ab initio calculations. Specific results are present ed for the LiF2 system, and compared with existing experimental and ab initio data. The agreement is good. The potentials are well suited fo r modelling accurate ab initio energies for these triatomic systems.