Application of CASTEP Code to the Simulation of Soft X-ray Emission Spectra of Light Element Materials

The Cambridge sequential total energy package (CASTEP) is a first-principles code based on density functional theory, which has been used for calculating spectra of soft X-ray absorption spectroscopy, for example. A post-processing code, OptaDOS, was recently developed, which can calculate the density of states (DOS) and soft X-ray emission spectroscopy (XES) spectra of materials. In this study, we investigate a methodology to calculate the XES of light element materials within the scope of the conventional CASTEP code without using OptaDOS. We investigated two pseudopotential approaches, with and without a core-hole effect and we compared the results with the measured spectra. We found that the simulation without a core-hole effect showed a better agreement with the experiment among the other calculation conditions. In addition, the method successfully deconvoluted the contributions of σ and π orbital components to XES spectra from the takeoff-angle dependent measurement of oriented samples, such as graphite and hexagonal boron nitride. Our study shows that the spectral shape and energy distribution of the π orbital component can also be accurately reproduced using this methodology.