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.