# X-ray Absorption Spectra (XAS)

How x-ray absorption spectra are calculated on the Materials Project (MP) website.
X-ray Absorption Spectra (XAS) is calculated using the code FEFF
$^{1,2,3}$
.Feff is an ab initio multiple-scattering code for calculating excitation spectra and electronic structure. It is based on a real space Green’s function approach including a screened core-hole, inelastic losses and self-energy shifts, and Debye-Waller factors. The spectra include extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES), and then both are stiched together to give a total XAS spectra. In addition the code can treat relativistic electron energy loss spectroscopy (EELS).
Multiple parameters are checked for convergence, including:
• Self-consistent field (SCF)
• Full multiple scattering (FMS)
• EXCHANGE: The EXCHANGE card specifies the exchange correlation potential model used for XANES calculation.
• COREHOLE: The COREHOLE card is used for specifying how the core is treated during XANES calculation.
For full details, please refer to publication: High-throughput computational X-ray absorption spectroscopy

#### References:

1. 1.
Parameter-free calculations of x-ray spectra with FEFF9, J.J. Rehr, J.J. Kas, F.D. Vila, M.P. Prange, K. Jorissen, Phys. Chem. Chem. Phys., 12, 5503-5513 (2010)
2. 2.
Ab initio theory and calculations of X-ray spectra, J.J. Rehr, J.J. Kas, M.P. Prange, A.P. Sorini, Y. Takimoto, F.D. Vila, Comptes Rendus Physique 10 (6) 548-559 (2009)
3. 3.
Theoretical Approaches to X-ray Absorption Fine Structure, J. J. Rehr and R. C. Albers, Rev. Mod. Phys. 72, 621, (2000)