First-principles calculations of x-ray absorption in a scheme based on ultrasoft pseudopotentials: From α-quartz to high-Tc compounds

We develop a first-principles scheme based on the continued-fraction approach and ultrasoft pseudopotentials to calculate K-edge x-ray absorption spectra in solids, allowing such calculations in transition-metal and rare-earth compounds with substantially reduced cutoffs with respect to the norm-conserving case. We validate the method by calculating Si and O K edges in alpha-quartz, Cu K edge in copper and in La2CuO4. For the case of Si and O edges in alpha-quartz and in copper, we obtain good agreement with experimental data. In the Cu K-edge spectra of La2CuO4, a material considered a real challenge for density-functional theory, we attribute all the near-edge and far-edge peaks to single-particle excitations.