The chief goal of the proposal is to study the phase transitions and polymerization of molecular solids under pressure, which is of fundamental importance for the physics and chemistry of geological/planetary processes and in searching for new materials with advanced properties. The calculations and simulations will be performed within the framework of ab initio methods based on density functional theory (DFT) and quantum Monte Carlo (QMC), together with other computational schemes including ab initio random structure searching (AIRSS) and metadynamics simulations. Two systems, hydrogen chloride (HCl) and carbon monoxide (CO) have been chosen because of the scientific importance and potential applications, for instance, the possible superconductivity of high pressure HCl and the high energy density of polymeric CO. We already have some preliminary AIRSS results for CO, so we can proceed with more accurate QMC simulations immediately. Other interesting properties of high pressure phases of HCl and CO, e.g., metallization, hardness and superconductivity will also be studied. Based on the high quality of the researcher and the host, promising results are expected, which will definitely accelerate the professional career of the researcher and enhance the scientific excellence of the EU.