When computing the potential-energy curve of a diatomic molecule for
predictive spectroscopy, high-level calculations are usually desired.
The best calculations are expensive, so few points are usually
available. The points are fitted to a continuous function, such as a
polynomial. Ro-vibrational energy levels are then computed using the
fitted function, and spectroscopic constants extracted. However, there
may be problems with overfitting, with inadequate flexibility of the
fitting function, or with dependence of results upon the choice of
fitting function. More fundamentally, the fitting function is selected
using aesthetics or convenience, instead of physics. Here we suggest
using a lower-level, high-resolution ab initio potential as a guide.
Instead of fitting the sparse, high-level data directly, the energy
differences between the high-level points and the guiding potential are
fitted. The results are improved even with an inexpensive guiding
potential. This simple strategy involves little additional effort and
can be recommended for routine use. It is similar to some interpolation
strategies in the literature of polyatomic molecules. When the guiding
potential extends beyond the high-level data, extrapolations are also
improved.