Apart from this model-free analysis, we sought to quantitatively fit the PDFs. We focus again first on Zr12-acetate to develop the method because a single crystal XRD structure is available for this cluster. To demonstrate the importance of the organic ligands to the PDF, we theoretically calculated the PDF of Zr12-acetate, including either (i) no carbon atoms, (ii) only the carbonyl carbon atoms, or (iii) all carbon atoms, see Figure . The car-Figure : Experimental PDF of zirconium oxo clusters with different capping ligands. The C-C and Zr-Zr are assigned. We make a distinction between the Zr-Zr distances within one Zr6 cluster and the Zr-Zr distances that are characteristic for the dimer. bon atoms contribute significantly to the PDF, especially between 4 Å and 8 Å. This result stands in stark contrast to the regular practice of ignoring the ligands in the PDF refinement of nanocrystals and larger clusters. The ligand effect is further demonstrated by fitting the Zr12-acetate PDF with various models, see Figure . For each model, we refined thermal motion parameters while keeping the structure (i.e., the atomic positions) unchanged. In a first attempt, similar to traditional approaches, we only used the Zr6 cluster core as a model.