where E is potential energy, R is the internuclear distance, and fM(R) is a continuous model function. The model function is fitted to the ab initio data, which are a small set of pairs of distances and energies, (Ri, Ei). Polynomials remain a popular choice of model function. In the present strategy, we include a high-resolution potential energy function computed using a less expensive, presumably lower-level theoretical method. This provides the physics used to guide the interpolation of the high-level data. The low-level data are converted to continuous form, g(R), using spline interpolation. The low-level grid should be dense, so that interpolations are short and the method of interpolation does not matter. We replace eq. ( ) by