Zeolite Structure Prediction and the Identification of Useful Synthetic Targets We have found that almost all real zeolite frameworks exhibit a flexibility window, where the rigid SiO4 tetrahedra can rotate about the corner-shared linkages without causing any stress. This flexibility may be an important topological property allowing us to tell which hypothetical zeolite frameworks are plausible. The packing of shapes, such as molecules, into arbitrary volumes is a difficult problem. We have written a simulated annealing code that attempts to pack spheres densely into zeolite frameworks. The goal is to predict the adsorption properties quickly.