Transferable ab initio intermolecular potentials. 2. Validation and application to crystal structure prediction

We have tested the performance of a previously developed ab initio potentia l in the simulation of crystalline phases. First, the model was validated b y performing energy minimizations for the experimental crystal structures o f several small organic molecules such as hydrocarbons, ethers, alcohols, a nd carbohydrates. Generally, the experimental structures were maintained ve ry well. Calculated packing energies were in good agreement with experiment al heats of sublimation. For flexible molecules, change of the molecular ch arge distribution with conformation was seen to be important. Secondly, cry stal structure predictions were performed for methanol, ethanol, 1,4-dioxan e, and propane. For methanol the experimental structure corresponded to the most favorable structure with one independent molecule, although a few str uctures with two independent molecules had a marginally lower energy. In th e case of ethanol the experimental structure, which contains two independen t molecules, was among the best ones, but in this case three structures wit h one independent molecule were slightly more favorable. For dioxane the hi gh- and the low-temperature phase were predicted with low energy, but in th e wrong order. The experimental structure of propane was predicted correctl y: it corresponded to the most favorable structure in our ab initio potenti al. In all cases the predictions using the ab initio potentials were superi or to predictions based on standard force fields.