From crystal structure prediction to polymorph prediction: interpreting the crystal energy landscape

Many organic molecules are emerging as having many crystalline forms, including polymorphs and solvates, as more techniques are being used to generate and characterise the organic solid state. The fundamental scientific and industrial interest in controlling crystallisation is inspiring the development of computational methods of predicting which crystal structures are thermodynamically feasible. Sometimes, computing this crystal energy landscape will reveal that a molecule has one way of packing with itself that is sufficiently more favourable than any other so only this crystal structure will be observed. More frequently, there will be many energy minima that are energetically feasible, showing approximately equi-energetic compromises between the various intermolecular interactions allowed by the conformational flexibility. Such cases generally lead to multiple solid forms. At the moment, we usually calculate the lattice energy landscape, an approximation to the real crystal energy landscape at 0 K. Despite its limitations, many studies show that this is a valuable complement to solid form screening, which helps in discovering new structures as well as rationalising the solid forms that are found in experimental searches. The range of factors that can determine which of the thermodynamically feasible crystal structures are observed polymorphs, shows the many further challenges in developing crystal energy landscapes as a tool for control of the organic solid state.