Chapter 5: DFTB and Hybrid-DFTB Schemes: Application to Metal Nanosystems, Isolated and in Environments

This chapter provides a survey of the basic concepts of the density functional-based tight-binding formalism (DFTB), its relationship with traditional tight-binding schemes and density functional theory, as well as its most popular extensions. Various hybrid approaches are reviewed; they rely on the combination of DFTB with lower- or higher-level methods, either from the point of view of the electronic structure calculation of large systems or within strategies for extensive potential energy surface explorations and dynamical simulations. The efficiency of these approaches is illustrated in the particular context of metal nanosystem investigations. DFTB appears to be a method of choice to address the properties of isolated metal nanoparticles over a wide size range, in particular the relationship between morphology and quantum size effects as well as dynamical and thermodynamical properties. Due to their computational efficiency, DFTB and hybrid-DFTB schemes also allow for the modeling of metal nanoparticles within an environment (nanoparticles that are functionalized, deposited on a surface or surrounded by a solvent), the treatment of extended but non-regular systems being a challenging task. Finally, perspectives are given for future developments of DFTB-based approaches that would allow scale up with more complex systems involving metals as encountered in nano-biological sciences or nanosciences.