# FEFF Variables

## k

The wave number in units of $\AA\!^{-1}$. $k = \sqrt{E-E_f}$ where $E \,\!$ is energy and $E_f \,\!$ is the Fermi level computed from electron gas theory at the average interstitial charge density.

## χ(k)

$\chi(k) = S^2_0 \mathcal{R}\sum_{shells}\frac{N F_{eff}}{k R^2}\exp(-2r/\lambda)\sin(2kR+\phi_{eff}+\phi_c)\exp(-2k^2\sigma^2)$

## φc

The total central atom phase shift, $\phi_c=2\delta_{l,c}-l\pi \,\!$

## Feff

The effective curved-wave backscattering amplitude in the EXAFS formula for each shell.

## φeff

The phase shift for each shell

## $\mathcal{R}$

The total central atom loss factor, $\mathcal{R}=\exp(-2\mbox{Im}(\delta_c))$

## R

The distance to the central atom for each shell

## N

The mean number atoms in each shell

## σ2

The mean square fluctuation in R for each shell

## λ

The mean free path in Å, $\lambda = 1/|\mbox{Im} p| \,\!$

## kf

The Fermi momentum at the average interstitial charge density

## p(r)

The local momentum, $p^2(r)= k^2 + k^2_f(r)+\Sigma-\Sigma_f$

## Σ(E)

The energy dependent self energy at energy E, $\Sigma_f \,\!$ is the self energy at the Fermi energy.

## μ(E)

The total absorption cross-section

## μ0(E)

The embedded atomic background absorption

### FEFF Input and Output Files

1. Intermediate output files
2. Diagnostic files
3. Main output data
4. Variables in the EXAFS and XANES formulae