In contrast, coupling constants are highly stable entities and tend to be nearly unaffected by the above experimental conditions unless they induce a change in the structure's conformational space, which is rare. For example, the standard deviation of all(!) the coupling constants derived from the two independent HNMR data sets of α-santonin in CDCl3 was <0.01 Hz (0.02 ppb[!] at 500 MHz), and <0.3 Hz for parthenolide in DMSO-d6 vs. CDCl3. These variations are smaller than the natural line width in the corresponding spectra. Deriving NMR parameters from experimental data with this precision requires QM-based total line shape fitting, e.g. via HiFSA analysis, which takes into account the complete, continuous line shape of the resonances.