For many years, the goal of chemists has been finding relations between the molecular structure and the molecular properties. This includes understanding the nature of chemical bonds, predicting the molecular reactivity and the response toward external stimulus or otherwise the spontaneous behaviors of materials. Many empirical or semi-empirical relations have been proposed, leading sometimes to interesting and widely adopted interpretative frameworks (for example, the structure correlation analysis, by Dunitz and Bürgi). While the molecular structures can be nowadays determined with very high accuracy, many intriguing connections between structure and properties are not yet solved. The interest of scientists has now moved toward the understanding and rationalization of supramolecular structures. For this reason chemical bonding and material properties that depend on soft interactions are currently much exploited. This project intends to investigate how molecular and supramolecular changes induced by external stress modify the chemical bonding and the aggregation of molecules in the solid state. Combination of experimental X-ray diffraction and theoretical calculations with periodic boundary conditions will be adopted to investigate structural changes in molecular crystals, at high pressure. This will provide more information on the potential energy associated with the intermolecular interactions. In this study, various kinds of hydrogen bridged aggregates will be investigated, with the aim to challenge the current theories on the strong hydrogen bond. This project will also investigate the possibility to obtain higher resolution structural determinations. This will enable observing structural changes occurring in the molecular skeleton. For example, in metal organic complexes the metal-ligand linkage and the bonding within the organic ligands will be accurately investigated. One goal is understanding the relative role of the different resonant structures of a molecule and see how the bonding mechanism change as a function of the external compression.Some recent observations demonstrate the possibility to obtain at high pressure a segregation of ionic moieties in the solid state structures of elemental (covalent) solids or hydrogen bonded molecules. These findings are very interesting because they allow to investigate an entirely new chemistry. In facts, the energy supplied to the system through pressure is such that when the system contains different electronic configurations, their relative contributions can be significantly modified. The possibility to map the electron density from diffraction at high pressure will be also investigated.