Final Report Summary - AB INITIO AND QMC (Phase transition and polymerization of molecular solids by<br/>ab initio calculations and quantum Monte Carlo simulations) 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 are 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.Since the beginning of the project, we have studied the polymerization of simple molecules, such as N2, CO and HCl, under pressure. We first perform ab initio random structure searching at the pressure points of interest and find a series of good candidates for the high pressure phase. Then we perform calculations with more accurate criteria to get the enthalpy vs. pressure relation for the system. After we find the best candidate for the high pressure phase, we perform phonon calculations to see whether the structure we find is dynamically stable or not. We also study the electronic structure properties and other properties we are interested in, such as mechanical and superconducting properties, etc.In the nitrogen system, we predict several stable phases at multi-TPa pressures, including a P4/nbm structure consisting of partially charged N2 pairs and N5 tetrahedra, which is stable in the range 2.5–6.8 TPa. This is followed by a modulated layered structure between 6.8 and 12.6 TPa, which also exhibits a significant charge transfer. The P4/nbm metallic nitrogen salt and the modulated structure are stable at high pressures and temperatures, and they exhibit strongly ionic features and charge density distortions, which is unexpected in an element under such extreme conditions and could represent a new class of nitrogen materials. This work has led to a publication in the most prestigious physics journal – Physical Review Letters [J. Sun, M. Martinez-Canales, D. D. Klug, C. J. Pickard, R. J. Needs, “Stable all nitrogen metallic salt at terapascal pressures”, Phys. Rev. Lett. 111, 175502 (2013).] Results for CO and HCl have also been obtained and will be analysed more fully soon.Another point worth to mentioning is that the fellow, Dr. Jian Sun, obtained a professorship in Nanjing University in China during the period of his stay in Cambridge. The training in Cambridge in terms of the Marie Curie fellowship accelerated his scientific career and helped him to reach a position of professional maturity.