ABSTRACT
Computational chemistry gives the most stable structure of a single molecule at standstill, which means an isolated molecule in vacuum at absolute zero temperature. In order to assess the capability of theoretical models, they must first be judged against data provided from crystallography. In most cases the calculated structure with the global minimum energy does correspond to the stable conformation observed in crystals, whether of ab initio, semiempirical, or molecular mechanics variety. However, when strong directional forces are present such as temperature, pressure, solvent, gravitational, electromagnetic field, etc., the structure of the grown crystal may be different from that of theoretical model, because the external (field) force overcomes the internal (molecular) force. In such cases, certainly the fact (by X-ray diffraction) overrules the theory (by calculation).
