From a high-resolution, high-level dataset we obtain reference, "correct" values for the vibrational energy levels (using the cubic spline). These "correct" values are E0 = 457.58 cm -1 and the successive vibrational intervals 𝐸 = 898.08 cm -1 , 𝐸 = 874.22 cm -1 , 𝐸 = 849.85 cm -1 , and 𝐸 = 824.93 cm -1 . The "correct" values do not represent experimental data except to the extent that the high-level method represents reality. However, they are the appropriate benchmarks for testing the fitting procedures. Note that the v = 4 level depends upon parts of the potential beyond the sparse high-level data (see Computational Details), and therefore reflects the accuracy of extrapolation. We expect the guiding potentials to improve the quality of extrapolation because the guiding potentials extend beyond the range of the nine high-level points.