First, we note that direct analysis of the high-level data from Boschen et al., using eq. ( ), gives results that depend upon the method of interpolation used to smooth the potential. Table lists the first six vibrational intervals and the ground (zero-point) energy relative to the (interpolated) minimum. The variability with splining method, an arbitrary choice, is shown in the last column. The high variability indicates that the data, of themselves, do not contain enough information to define vibrational levels to wavenumber precision. Alternatively, a physically derived guiding function may reduce the demands placed upon the model function, thus reducing the variability (i.e., uncertainty) from the arbitrary choice of splining method. By making this comparison, we are not suggesting that a linear spline is a good choice of model function. We are showing that the information carried by the model function has a significant impact on the spectroscopic results. Table . Vibrational intervals (cm -1 ) of C2 as computed from high-level data by Boschen et al. using different splining methods.