1. Excitations and quasi-particles

A surface is an N-body system with interacting ionic cores and valence electrons. The ground state of the electronic subsystem can be described as a system of independent particles, each moving in an effective potential (the "one-electron" approach). The properties of the system's ground state can be calculated from a minimum of information (at least the nature of the atoms that make up the system) by so-called ab initio techniques, generally based on charge density functional (DFT) theory, combined with a suitable approximation for calculating the exchange and correlation potential V _xc . In practice, a system of Kohn-Sham equations is solved in a self-consistent manner. An independent electron in a Kohn-Sham state has a fixed energy and a certain, calculable probability of being in a given region of space. However, the excitations of the electron...