Application to CO. Powell and Dawes have published sparse potential energy curves for carbon monoxide computed using a quantum Monte Carlo (QMC) method. To obtain spectroscopic constants, they fitted selected points from their data to a Morse function. The authors had difficulty with the curve-fitting, reporting that the Morse potential cannot fit the data over their full range. Note that a Morse potential unphysically truncates all anharmonicity at the first constant (exe). Here, we restrict our attention to the most expensive calculations, Table in ref. , and to the four QMC points that are relevant for the lower vibrational levels. These points lie at R/Å = 0.9, 1.0, 1.1 and 1.3. (The next point is at 2.6 Å.) 0.9 Å is close to the inner classical turning point for v = 24, for which the outer turning point is near 1.655 Å. Figure shows the sparse QMC data and their differences with the guiding potentials. The differences between QMC and the multireference methods are nearly linear. The uncertainties for the data points are not all the same, so the model function must be fitted using a weighted procedure. We choose each weight to be the inverse square of its stated uncertainty. Starting with fM() as a linear function, we obtain spectroscopic constants as shown in Table . (Potential energy curves are shown in the Supporting Information.) Experimental values are shown in the first row of Table for reference, but we can only expect to match them to the extent that the QMC data are accurate.