Experimental single-crystal X-ray diffraction revealed that at 173вЂ…K norleucine exists in the ОІ-phase with stacking disorder. Their simulated diffraction patterns show sharp reflections for h + k = 2 n and diffuse streaks parallel to c* through all reflections with h + k = 2 n + 1. According to the calculated stacking probabilities large models containing 100 double layers were constructed. Stacking probabilities were calculated from the DFT-D energies. The calculated energies depend not only on the arrangement of neighbouring double layers, but also of next-neighbouring double layers. Lattice-energy minimizations by force fields and by dispersion-corrected density functional theory (DFT-D) were performed on a set of ordered model structures with Z = 4, 8 and 16 with different stacking sequences. A symmetry analysis was carried out to derive possible stacking sequences. Diffraction patterns of the О±- and ОІ-phases frequently show diffuse streaks parallel to c*, which indicates a stacking disorder of the layers. The crystal structure of the ОІ-phase was redetermined at 173вЂ…K. Dl-Norleucine (2-aminoВhexanoic acid, C 6H 13NO 2) forms a double-layer structure in all known phases (О±, ОІ, Оі).

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