CRYSTAL-STRUCTURE PREDICTIONS FOR ACETIC-ACID

Possible crystal structures of acetic acid were generated, considering eight space groups and assuming one molecule in the asymmetric unit. Our grid-search method was compared with a Monte Carlo approach as imp lemented in the Biosym/MSI Polymorph Predictor. This revealed no sampl ing deficiencies. A large number of possible crystal structures were f ound (similar to 100 within only 5 kJ/mol), including the experimental structure. Energy minimizations were done with a united-atoms force f ield (GROMOS), an all-atoms force field (AMBER), and a potential that describes the electrostatic interactions with distributed multipoles ( DMA). In all cases, the experimental structure had a low lattice energ y. The number of hypothetical crystal structures was reduced considera bly by removing space-group symmetry constraints, or by a primitive mo lecular dynamics shake-up. Nevertheless, sufficient structures of equa l or lower energy compared with the experimental structure remained to suggest that other factors need to be considered for genuine structur e prediction. (C) 1998 John Wiley & Sons, Inc.