Analytic potential energy surfaces and their couplings for the electronically nonadiabatic chemical processes Na(3p)+H-2 -> Na(3s)+H-2 and Na(3p)+H-2-> NaH+H

High-level ab initio electronic structure calculations are presented for th e ground and first excited state of the NaH2 system. These calculations inc lude full configuration interaction (FCI) theory, second-order Moller-Pless et perturbation (MP2) theory, and coupled cluster (CC) theory at a wide ran ge of geometries including those near the conical intersection between the ground and first excited state, points in the excited-state potential energ y well (exciplex), and the long-range van der Waals wells, and geometries i n regions where the interaction energy is dominated by long-range forces. A 2 x 2 diabatic potential energy matrix function is fit to the high-level a b initio points. Special attention is paid to the form that the diabatic co upling should take and to the method used to fit this coupling. Long-range multipole and dispersion forces, including the quadrupole-quadrupole intera ction, are included in the diagonal elements of the potential energy matrix . The matrix potential and its eigenvalues are compared to results that hav e appeared in the literature previously. (C) 1999 American Institute of Phy sics. [S0021-9606(99)30409-8].