Coupled–cluster calculations with large basis sets are used widely to make predictions of gas–phase thermochemistry. Wavefunction diagnostics are sometimes used to indicate whether or not there is problematic multireference character that may cause errors. Here, we investigate whether existing diagnostics, as well as diagnostics proposed by us, can be used to estimate these errors quantitatively. We calculate the atomization energy of 50 molecules, including known multireference molecules such as CN, C
-benzyne, formaldehyde oxide, and hydrogen trioxy radical. In addition to the
1, and %TAE[(T)] diagnostics, we assess the Hartree–Fock HOMO–LUMO gap, maximum occupation number defect, fi rst vertical excitation energy, a direct estimate of multireference effects, and combinations of diagnostics as indicators of errant thermochemistry.