The crystal packing of molecular crystals can be rationalized using quantum mechanical ab initio methods. This work describes the principles in which this approach is based, applying the basic principles to the 2-hydro nitronyl nitroxides. Two forms of packing analysis using ab initio methods are described. The first one is based on the use of the molecular electrostatic maps for the molecules. The second one uses the information about the strength of the molecular interactions present in the crystal provided by accurate ab initio computations from model systems. Using this information it is possible to rationalize the packing in terms of primary, secondary and so on structures. For the 2-hydro nitronyl nitroxide radical the C(sp2)-H ... O contacts should be the driving force behind the primary structure of the packing, while the C(sp3)-H ... O contacts should be responsible of the secondary structure. The N-O ... O-N contacts are found to be repulsive.