In the subsection about guided point-selection, the high-level method for F2 is the same as described above, to allow comparison with "correct" results. The high-level method for C2 is CCSDT(Q) with all electrons correlated. Correlation energies are extrapolated to the basis-set limit, assuming n -3 dependence, from calculations using the aug-cc-pwCVTZ and aug-cc-pwCVQZ basis sets. Hartree-Fock energies are obtained using the aug-cc-pwCV6Z basis sets, without extrapolation. These calculations on C2 are done using CFOUR.