National Synchrotron Light Source Seminar

First-principles calculations of optical properties across the electromagnetic spectrum can involve vibrational excitation in the terahertz region, valence excitations in the near-ir/visible/near-uv and core excitations in the soft x-ray and hard x-ray regions. In this talk, we will survey some of the results obtained at NIST use Bethe-Salpeter calculations for core and valence excitations, as well as modern perturbation theory calculations that predict vibration spectra in the one- and two-phonon region in semiconductors. Finally, we will look briefly at heuristic “orbital-less” methods to predict complicated screening effects in materials that may help calculations be more feasible in complex systems such as materials with large unit cells or aperiodic structures of interest in nanoscience. The screening effects are relevant to stopping powers for charged particles and screening of the electron-electron interactions in polyatomic systems in general.