There also exist multiple modes of IFC fitting in terms of the order in which they are fitted and to what displacements. The most prevalent practice is fitting all IFC orders in a single shot, but one could fit them separately, e.g. harmonic-first then anharmonic, or sequentially by order, using the same or separate training sets for each. Fitting all IFCs in one shot using all training configurations sounds most straightforward, and ideally, forces of the correct order would perfectly distribute to IFC parameters of their corresponding orders. In practice, however, the inevitably imperfect training set and fitting errors cannot guarantee this to a desirable extent. Fitting harmonic IFCs using large displaced training structures is problematic and lacks physical justification. Harmonicity is predominantly present in the region of small displacements, while anharmonicity becomes significant at larger displacements. Numerically, large atomic displacements in fitting harmonic IFCs at 0 K could fictitiously attribute what in reality are anharmonic contributions to harmonic IFC description, as has been observed by a recent systematic study of this matter . As such, we decide to fit harmonic IFCs first to forces in the small-displacement set, and then fit anharmonic IFCs to the residual forces from the large-displacement set with the harmonic forces subtracted out. This decision of separate order, separatetraining-set fitting affects our subsequent decision of the supercell perturbing method.