DFT and TDDFT for core spectroscopies

 The Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) of molecules adsorbed on surfaces is widely used to investigate the orientation and geometry of the molecular adsorbate as well as the extent of the adsorbate-substrate interaction. The simulation of NEXAFS spectra of these systems represents a computational challenge for both a proper modelling of the adsorbate/surface system as well as for its size which can prevent the application of very accurate theoretical methods. We calculate the NEXAFS spectra in terms of excitation energies and oscillator strength values calculated at DFT or TDDFT level employing a cluster approach to mimic the adsorbate systems. Finite size cluster model for surface adsorption has proven very efficient and accurate to simulate NEXAFS spectra due to the localized nature of the core excitation process. Total spectra as well as angle resolved spectra can be simulated. Previous optimization of the surface and of the molecules adsorbed on it is performed by a periodic slab methodology in the frame of density functional theory (DFT). (Giovanna Fronzoni and Daniele Toffoli)