The Fixed Displacement method, as implied by its name, generates displacements of a fixed, predetermined magnitude in random directions for each atom in a given supercell. This approach allows for a distinct separation for harmonic force constant extraction across different structure configurations. We designate displacements of ≤ 0.05 Å for harmonic IFC fitting and > 0.05 Å as anharmonic fitting. We benchmarked two sets of Fixed Displacements: (i) [0.01 Å, 0.03 Å, 0.1 Å, 0.3 Å] (labeled as in the unit of picometer), and (ii) [0.01 Å, 0.03 Å, 0.08 Å, 0.1 Å] (labeled as [1-3-8-10]) (see Figure ) by comparing their RMSE from fitting and calculated CTE. The set of [0.01 Å, 0.03 Å, 0.08 Å, 0.1 Å] demonstrates significantly lower RMSE and more accurate CTE compared with experimental data. Consequently, we choose this set as the default displacements for perturbing structures and multiply the configuration number based on the primitive cell size and structure symmetry as discussed in the next session. It is worth noting that the default setting we suggested here [0.01 Å, 0.03 Å, 0.08 Å, 0.1 Å] is not the universally optimal perturbing distances for all materials. For strongly anharmonic materials, such as cubic ZrO 2 , β-GeTe , BCC Zr (as shown in Figure ), larger displacements up to 0.5 Å are necessary to avoid large RMSE in IFC fitting and unreliable anharmonic properties caused by insufficient small-displaced training dataset.