Testing on F 2 . Difluorine has only a formal single bond, but is an anomalous molecule with strong dynamical electron correlation. To determine the distance resolution needed to exceed wavenumber precision, a uniform grid of 2201 points (from R = 1.1 to 2.1 Å; resolution = 0.0005 Å) is computed at the fcCCSD/cc-pVDZ level. The ground vibrational level, E0, is computed along with the first four intervals: 𝐸 , 𝐸 , 𝐸 and 𝐸 . The numerical values are affected by the choice of spline interpolation method. The difference between cubic and linear splines indicates how much information originates in the fitting procedure, instead of the quantum theory. This difference is computed for each of the energy quantities as a function of grid resolution. The value of E0 is the most sensitive to the grid resolution, by far. A grid resolution of 0.0075 Å is needed to converge E0 to a precision of 1 cm -1 . A resolution of 0.035 Å is adequate for the four energy intervals. Throughout the present report, a resolution of 0.005 Å is used for "high-resolution" grids, except as noted in the Computational Details.