Analytic potential energy surfaces and their couplings for the electronically nonadiabatic chemical processes Na(3p)+H2→Na(3s)+H2 and Na(3p)+H2→NaH+H

High-level ab initio electronic structure calculations are presented for the ground and first excited state of the NaH2 system. These calculations include full configuration interaction (FCI) theory, second-order Moller-Plesset perturbation (MP2) theory, and coupled cluster (CC) theory at a wide range of geometries including those near the conical intersection between the ground and first excited state, points in the excited-state potential energy well (exciplex), and the long-range van der Waals wells, and geometries in 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 ab initio points. Special attention is paid to the form that the diabatic coupling should take and to the method used to fit this coupling. Long-range multipole and dispersion forces, including the quadrupole-quadrupole interaction, are included in the diagonal elements of the potential energy matrix. The matrix potential and its eigenvalues are compared to results that have appeared in the literature previously. (C) 1999 American Institute of Physics. [S0021-9606(99)30409-8].