Crystal structure optimization and gibbs free energy comparison of five sulfathiazole polymorphs by the embedded fragment QM method at the DFT level

Molecular crystal plays an important role in many fields of science and technology but it often crystallizes in different polymorphs with different physical properties. To guide the experimental synthesis of candidate materials the atomic-scale model is frequently used to predict the most stable polymorph and its structural properties. Here we show how an ab initio method can be used to achieve a rapid and accurate prediction of sulfathiazole crystal polymorphs (an antibiotic drug), based on the Gibbs free energy calculation and Raman spectra analysis. At the atmospheric pressure and the temperature of 300 K we demonstrate that form III (FIII) is the most stable structure of sulfathiazole. The agreement between the predicted and experimental crystal structures corresponds to the order of stability for five sulfathiazole polymorphs as FI < FV < FIV < FII < FIII which is achieved by employing the density functional theory (DFT) calculations.