First principle calculations of periodic crystal structure were successfully combined with powder X-ray diffraction measures to determine the structure of orotaldehyde monohydrate. This approach was particularly helpful to overcome the inadequacy of powder X-ray diffraction to reliably locate the hydrogen atoms of the intermolecular bond network of the crystal molecules. Density functional calculations were accomplished for the free molecule and its cyclic dimers showing that the most stable centrosymmetric dimer is the building block of the molecular crystal.