Studying the polymorphism of pharmaceutical compounds is a growing field of activity. This is due, on one hand, to the economic pressure of the pharmaceutical industry, and on the other hand, to the more important awareness of the consequences of polymorphism on the properties of a medicine. A good knowledge of molecular interactions and crystal energies is essential for the determination of relative polymorph stability. We have developed a fundamental approach to study polymorphism, which combines high-resolution X-ray diffraction and ab-initio calculations. This approach would enable us to study the consequences of drug polymorphism on the physical and chemical properties of drugs. Piracetam is a small and flexible molecule, used against several neurodegenerative disorders such as Alzheimer's disease. We will present the results we have obtained for Piracetam (figure). In particular, we will compare electrostatic energies obtained from X ray diffraction experiments [1] with crystal structure prediction from ab-initio calculations [2]. We will show also thermodynamic aspect of the relative stability for piracetam.