Model systems were created in Avogadro 1.2.0. The models were equilibrated using the built-in energy minimization feature in Avogadro, using the MMFF94s force field. PDFs were generated from the models using the DebyePDFCalculator class from diffpy-cmi. Isotropic atomic displacement parameters, Biso = 8π 2 Uiso, were set to 2 Å 2 for each atom type. Qmin, Qmax, Qdamp, and Qbroad were set to 0, 25, 0, and 0 Å -1 , respectively.