Ab Initio Simulations of Activated Processes : Application to CO₂ Reactions and a Photovoltaic Material

This thesis is devoted to the study of activated processes at the nanoscale by means of state-of-the-art computer simulations. On the one hand, we are interested in addressing specific aspects of chemico-physical processes of great practical importance, namely the early steps of wet carbon dioxide capture, and the diffusion of an impurity in cadmium telluride. On the other hand, we assess the limitations of current methodologies and propose new tools and protocols to extend their range of applicability. The simulation of the dynamics of activated processes at the atomistic level is a delicate issue facing two major difficulties: (i) an accurate description of the changes happening in the electronic structure has to be obtained ; and (ii) a statistically meaningful portion of the configuration space has to be explored, including scarcely populated regions like transition states. By employing finite temperature ab initio molecular dynamics empowered by metadynamics, it is possible to successfully overcome these challenges. We apply these methods and develop general robust protocols for these simulations and for the analysis of the results. Applications refer to CO2 chemical reactions, which are relevant to the science of carbon capture, and sulfur diffusion in CdTe, a photovoltaic material of current great interest. An unprecedented characterization of these processes is obtained, leading to new understanding of the experimentally observed behavior, well beyond the reach of state-of-the-art calculations. For example, our simulations disclose the active role played by water in the capture of carbon dioxide, and reveal the easiness by which sulfur is trapped in a CdTe lattice.