Ab initio crystal structure predictions for flexible hydrogen-bonded molecules

Crystal structure predictions for glycol and glycerol are reported. A serie s of increasingly accurate energy calculations is applied. and the final pr edictions are based solely on ab initio derived energies. A recently develo ped transferable ab initio potential is used for intermolecular interaction s, augmented with ab initio derived conformational energies. The experiment al structure of glycol was predicted with a low energy, 1.1 kJ/mol above th e global minimum. For glycerol the experimental structure corresponded to t he global minimum. This latter result provides a proposal for the positions of the hydrogen atoms in the crystal structure of glycerol. A three-dimens ional hydrogen-bonded network is formed which consists only of intermolecul ar hydrogen bonds. Together with previous work, the ab initio intermolecula r potential has now been applied to predict the crystal structures of six d ifferent compounds. The energy difference between the observed crystal stru cture and the global energy minimum varied from 0 to 2 kJ/mol. Standard For ce fields fail to consistently produce such low values. This demonstrates t he importance of highly accurate force fields in crystal structure predicti on.