Application to C 2 . Dicarbon is well-known for its challenging electronic structure. Here, we focus on the spectroscopic constants as determined by fitting the high-level CEEIS (correlation energy extrapolation by intrinsic scaling) data by Boschen et al. The authors reported difficulty fitting the data to a continuous form, which they ascribed to an avoided crossing near 1.7 Å. In the end they used a cubic spline. Here we re-analyze their 43 data points (up to R = 6 Å) using guided fitting. Abrams and Sherrill reported correctly-shaped curves at the fcFCI/6-31G* level, so we use it here as a guiding potential. We also use guiding potentials from CASSCF/cc-pVTZ and icMRCI+Q/aug-cc-pwCVTZ (active core), which are far less expensive than FCI despite the larger basis sets.