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 OK edges in α -quartz, CuK edge in copper and in La2 CuO4. For the case of Si and O edges in α -quartz and in copper, we obtain good agreement with experimental data. In the CuK -edge spectra of La2 CuO4, a material considered a real challenge for density-functional theory, we attribute all the near-edge and far-edge peaks to single-particle excitations. © 2009 The American Physical Society.