2016. 57, 6 UDC 541.6:548.737 AB INITIO PREDICTION OF PLAUSIBLE CONFORMERS OF A FLEXIBLE N-(3-CHLORO-4-FLUOROPHENYL)THIOUREA (CFT) MOLECULE: A VALIDATION STUDY A.D. Stephen1, P.V. Nidhin1, P. Srinivasan2 2Department of Physics, C. Kandaswami Naidu College for Men, Chennai, India Received May, 26, 2015 1Department of Physics, Sri Shakthi Institute of Engineering and Technology, Coimbatore, India E-mail: davidstephen_dav@yahoo.co.in – . 1147 – 1156 The current aim of the ab initio crystal structure prediction is to find the possible conformers of the flexible N-(3-chloro-4-fluorophenyl)thiourea (CFT) molecule using gas phase optimisation with an MP2/6-31G(d,p) basis set, and the lattice energy minimization in the presence of a repulsion-dispersion electrostatic potential. <...> If the molecule deviates from the gas phase conformation, suitable intermolecular interactions are added, and the molecule favours stable packing. <...> The crystal structure is said to be feasible if the intermolecular lattice energy compensates the intermolecular energy penalty associated with the suboptimal gas phase conformers. <...> The idea of the current research is to find the least energy hydrogen bonded crystal structure from a set of rigid conformers in a conformation region, with a significant similarity of packing, which may lead to the prediction of polymorphs associated with the considered CFT molecule. <...> DOI: 10.15372/JSC20160605 Keywords: ab initio crystal structure prediction, lattice energy minimisation, polymorphism. <...> INTRODUCTION The key importance of the ab initio crystal structure prediction method lies in the application area where a successful prediction method can give a good understanding of the crystallisation process [ 1 ], and also the prediction of highly energetic molecules might decrease the level of experimental risks. <...> The presence of a polymorph of an organic molecule in a crystal structure can differently affect the action of the organic molecule or alter the molecular geometry [ 2 ], which may lead to the development of a new drug or to opt out the negative effect of the concerned drug system. <...> The crystal structure prediction method allows the theorists to discover new polymorphs of such molecules. <...> Recently, in the report on the fifth blind test, Price et al. have analyzed the polymorphic hydrate of gallic acid mono hydrate [3 ] by reducing the search space to a more manageable <...>