Study of polymorph prediction for L-ascorbic acid

Possible polymorphs of L-ascorbic acid were investigated, considering eight space groups and assuming one molecule in the asymmetric unit. The grid-search method was compared with a Monte Carlo approach as performed in the Biosym/MSI polymorph Predictor. A number of possible crystal structures were found, including the experimental structure. Energy minimizations were performed with a united-atom force field. In all cases, the experimental structure had a low lattice energy. The number of hypothetical crystal structures was reduced considerably by removing space-group symmetry constraints, or by a primitive molecular dynamic shake-up. Nevertheless, sufficient structures of equal or lower energy compared with the experimental structure remained to suggest that other factors need to be considered for polymorph predictions of materials.