Static exchange calculations of X-ray absorption fine structures in polymers and surface adsorbates

We review a recently implemented method for static exchange calculations of photoabsorption spectra of large molecules and clusters, with particular attention to near-edge X-ray absorption fine structures (NEXAFS), and describe here its extension to open-shell species. The static exchange matrices are determined directly from one- and two-electron integrals computed in the atomic orbital basis. Expensive storage and retrieval of integrals is thereby avoided and applications on large species possible. Using this method we have calculated discrete and continuum near-edge X-ray absorption spectra for large oligomers and surface adsorbates. We have been interested in the general convergence of the NEXAFS spectra, in the validity of the building block picture and in excitonic features when going from small to large oligomers. Computations of NEXAFS spectra of surface-adsorbates are carried out by means of cluster modeling of the surface.