document: Although the basic mechanisms of the AD process are reasonably well understood, it has not proved simple to apply existing theories to the interpretation of experimental data. What is needed is a combination of the AD theory and the electronic structure of realistic systems, including surface defects and adsorbed species. Such electronic structure calculations are still complex and time-consuming. In many cases, especially for insulating surfaces, attempts to model MIES spectra use simple or intuitive models. In Refs. [4,6,23] it is assumed that the main transition mechanism is Auger de-excitation, and the MIES spectra have been simulated by the surface density of states (DOS) projected on the surface oxygen ions of the uppermost surface layer using a Hartree–Fock method (the crystal code [24,25]) and a density functional theory (DFT) method (the cetep code [26]). The effect of the overlap between the surface and He(1s) wavefunctions was taken into account only approximately by applying an additional z-dependent exponential factor to the surface DOS. Other workers [5,6] estimated the AD transition probability using a DOS projected on to the projectile 1s atomic orbital. However, they were not able to use state-of-the-art methods for the surface electronic structure. Yet the success of the simplified treatments [4–6], especially for MIES features such as relative energies of the different peaks, suggests that real spectra are indeed related to the projection of the surface DOS on to the projectile orbital.
doc_bio_tags: O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B I I B O O O B I I O O O O O O O B I O B I O O O B I I B O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O O
extractive_keyphrases: ad adsorbed species cetep code crystal code density functional theory density of states dft dos hartree–fock method mies surface defects surface oxygen ions
abstractive_keyphrases: ad transition auger de-excitation mies spectra