It was based on statistical sampling, which required very large unit cells to be effective. (pnnl.gov)
«Low - Energy Networks of the T - Cage (H2O) 24 Cluster and Their Use in Constructing Periodic Unit Cells of the Structure I (sI) Hydrate Lattice.» (pnnl.gov)
The ground state of the solid is assumed to be an incommensurate crystal, with quantum zero - point vacancies and interstitials and thus a non-integer number of atoms per unit cell. (science.sciencemag.org)