First principles calculations of X-ray absorption in an ultrasoft pseudopotentials scheme: from α-quartz to high-T_c compounds

We develop a first-principles scheme based on the continued fraction approach an d ultrasoft pseudopotentials to calculate K-edge X-ray absorption spectra in solids. The method allows for calculations of K-edge X-ray absorption spectra in transition metal and rare-earths compounds with substantially reduced cutoffs 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 La$_2$CuO$_4$. For the case of Si and O edges in $\alpha$ quartz and in Copper we obtain a good agreement with experimental data. In the Cu K-edge spectra of La$_2$CuO$_4$, a material considered a real challeng e for density functional theory we attribute all the near-edge and far-edge peaks to single particle excitations.