Schistosomiasis is a parasitic disease widely extended worldwide, mainly in the tropics and subtropics. Its pharmacological treatment is approached with praziquantel (PZQ), chemically named (RS)-2-(cyclohexylcarbonyl)-1,2,3,6,7,11b-hexahydro-4H-pyrazin-[2,1a]-isoquinolin-4-one. The PZQ commercial preparation is actually a racemic mixture, in which only the levo-enantiomer possesses anthelmintic activity. The knowledge of its spectroscopic and other chemical-physical properties is important to improve its applications. Therefore, the molecular and crystal structure of praziquantel have been investigated by means of calculations with classical mechanics and quantum mechanics methods based on Density Functional Theory (DFT), reproducing the experimental crystallographic and spectroscopic properties. Several crystal lattices of PZQ have been studied. Most of the intramolecular and intermolecular interactions of PZQ molecules in the crystal structure have been discussed. The vibration frequencies of the PZQ molecule were calculated with different molecular simulations and the assignments of some bands have been confirmed, such as, those of carbonyl groups.