The main subject of this thesis is the resolution of crystal structures by X-ray and neutrons powder diffraction using global optimisation methods. The first subject is the order-disorder phenomena observed in some globular organic molecular solids. The second is the opiate family of neuropeptides. These neurotransmitters regulate sensory functions including pain and control of respiration in the central nervous system. The aim of our study was to try to determine the crystal structure of Leu-enkephalin and some of its sub-fragments. The determination of the crystal structures has been done performing Monte Carlo simulations. The third is the location of benzene in a sodium-X zeolite. The zeolite framework was already known and the benzene has been localized by simulated annaeling and by the use of maximum entropy maps.
