Quantum-chemical modelling of clay mineral surfaces and clay mineral–surface–adsorbate interactions

Atomistic simulations provide insight into the crystal structure of minerals, surfaces, and mineral-fluid interaction mechanisms. Such modelling has been successfully applied to better understand structural, thermodynamic, and transport properties of clay minerals, the thermodynamics of ions adsorption, and clay mineral surface reactivity at an atomic scale. In principle, quantum mechanics- based modelling allows system description without use of any empirical system-dependent parameters. In practice, however, the complete quantum mechanical description of the condensed matter is only feasible for small systems containing few atoms, due to the limitations of currently available computational resources. Therefore, the simulations of complex reactive processes rely on a number of approximations at different levels of theory. These approximations are chosen as a compromise between the computational accuracy and the ability to include the relevant chemical processes. This chapter starts with a short overview of the methods of quantum chemistry currently applied to the simulations of clay minerals. Theoretical equations are intentionally excluded. The focus is on the physical rationale behind the methods, assumptions applied, and their consequences for the interpretation of the results. For the theoretical details of the methods, the reader is directed to the specialized text books and review articles provided as references. The second part of the chapter deals with the applications. The chapter starts with the description of the bulk crystal structure, continues with the structural properties of the surfaces and surface-fluid interfaces, and concludes with thermodynamic and structural aspects of adsorption