X-ray absorption near-edge structure calculations with the pseudopotentials: Application to theKedge in diamond andα-quartz

Abstract

We present a reciprocal-space pseudopotential scheme for calculating X-ray absorption near-edge structure (XANES) spectra. The scheme incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole interaction through large supercells (hundreds of atoms). The method is compared with recently developed Bethe-Salpeter approach. The method is applied to the carbon K-edge in diamond and to the silicon and oxygen K-edges in α-quartz for which polarized XANES spectra were measured. Core-hole effects are investigated by varying the size of the supercell, thus leading to information similar to that obtained from cluster size analysis usually performed within multiple scattering calculations.

Structural relaxations around Ti, Cr and Fe impurities in α-Al2O3probed by x-ray absorption near-edge structure combined with first-principles calculations

“…13 The XANES calculations are then carried out within the scheme of the PARATEC code, 14 using the formalism described in Ref. 5 Both codes are based on the density functional theory (DFT) within the spin polarized local density approximation (LSDA), and use planewave basis set and norm-conserving Troullier-Martins pseudopotentials 15 in the Kleinman-Bylander form. 16 In the following, we give all the parameters used in both kinds of calculation.…”

“…5,21 Therefore its main aspects are recalled here. The method uses periodic boundary conditions, plane-wave basis-set, pseudopotentials and reconstructs all-electron wave functions within the projector augmented wave (PAW) framework.…”

“…5 Here we apply this method to the K-edge of substitutional paramagnetic impurities in aluminum oxide. More precisely, we investigate the structural modifications of the corundum crystal structure induced by the presence of substitutional Ti 3+ , Cr 3+ or Fe 3+ .…”

Half or full core hole in density functional theory X-ray absorption spectrum calculations of water?

“…One way to estimate the effect of the final state is to use the Z+1 approximation, which implies that the core-excited atom is replaced by the Z+1 atom. In XA spectrum calculations this full core-hole, final-state-dominated potential is obtained by removing a core electron (in all-electron calculations) or by specially designed pseudopotentials with occupation 1s 1 (in e.g plane-waves-based spectrum calculations) 4,9,16 . A more wellestablished technique is the transition potential approach by Slater in which excitation energies are obtained as orbital energy differences between initial and final orbitals with half an electron excited 17 ; in the context of XA spectrum calculations this implies that half a core-electron is excited in the calculations [17][18][19] .…”

“…On the other hand, Hetényi and coworkers claim agreement with experiment from summing spectra computed using the FCH potential of all 64 molecules in a typical CPMD snapshot in spite of only 19% H-bonds being broken 4 . However, because of the lack of the 1s orbital in the pseudo-potential plane-waves approach in the CPMD framework it is not possible to obtain the absolute energy scale of the spectra and a shift of the excitation energies by an arbitrary 4,16 or a computed 21 , based on all-electron calculations, constant is necessary to allow direct comparison with experiment. The use of the all-electron model-cluster approach is in this case advantageous since absolute excitation energies are available and the Kohn-Sham method makes it possible to compare HCH and FCH calculations without arbitrary energy shifts.…”