All DFT calculations were performed using VASP which can only do periodic calculations, so each cluster is in effect surrounded by translationally equivalent clusters. Hence it is essential to use a simulation cell that is sufficiently large to avoid interaction among periodic images. We found that this was best accomplished using two different criteria for different sets of elements. For elements in the Groups 1A and 2A on the periodic table, the minimum distance between neighboring clusters must be greater than 3.5 times the distance between nearest neighbor listed in Table in the SI. For all other elements the minimum distance between periodic images must be greater than 10 angstroms. If, after relaxation, the minimum distance between neighboring clusters was too small, then we increased the supercell size and ran the DFT calculation again. We found that these "box size" criteria are sufficient to reach energy convergence within 2 meV/atom in all 1135 tested cases (please see Figure and additional details in the SI) with a root mean squared error of 0.118 meV/atom.