Note that the result of eq. ( ) is not a "dual-level" potential energy curve because the low-level method does not contribute directly to the energy. E(R) is the high-level potential with interpolation and possibly extrapolation. The low-level guiding potential, g(R), although lacking adjustable parameters, contributes in the same way as the model function, fM(R). Unlike the model function, g(R) is derived from the electronic structure of the molecule under study. This strategy is described as "physics-guided" because the guiding potential is from actual calculations on the molecule of interest. No feasible calculation contains "all" the physics, but quite a bit can be captured using a low-cost guiding potential. In contrast, mathematical forms such as splines, polynomials, and Morse functions are not based upon any physical laws. Such functions are still required for fitting the energy differences, but their numerical influence is diminished, compared with direct fitting of raw data. The examples presented here are simple: covalently-bonded molecules composed of light elements and closed-shell ( 1   ), single-well electronic ground states. Such simple potentials are the easiest to fit using conventional, direct fitting, eq. ( ).