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FF2CHI: XAFS spectrum


 

The module FF2CHI constructs the XAS spectrum tex2html_wrap_inline3343 or tex2html_wrap_inline3345 using the XAFS parameters described in Section 3.2.4 from one or more paths, including any FMS contributions. Single and multiple scattering Debye-Waller factors are calculated using, for example, the correlated Debye model. Output from this module is the total XAFS spectrum and optionally, the contribution to the XAFS from each path individually. Numerous options for filtering, Debye-Waller factors, and other corrections are available.

 DEBYE    temperature Debye-temperature [idwopt]    (Standard)

The Debye card is used to calculate Debye-Waller factors for each path using the correlated Debye Model. The model is best suited for homogeneous systems, where it is quite accurate. CAUTION: in heterogeneous systems the model only gives approximate values which can easily be off by factors of two or more. Temperatures are in kelvin. If this card is present, the correlated Debye model Debye-Waller factors will be summed with the DW factors from the SIG2 card and from the 'list.dat' file, if present.


*Debye-Waller factors for Cu at 190K with correlated Debye Model
  DEBYE  190 315

By default, idwopt=0 specifies that the correlated Debye model is used to calculate EXAFS Debye-Waller factors. Two additional models for calculating DW factors are available in FEFF8 based on the information about the harmonic force constants in the material. idwopt=1 means the equation of motion (EM) method is used to get Debye-Waller factors and idwopt=2 means the recursion method (RM) which is an improved correlated Einstein model. Both methods are faster than molecular dynamics simulations, and the recursion method is much faster than the equation of motion method. However, the equation of motion method leads to somewhat more accurate results than the recursion. These additional methods seem to be superior to correlated Debye model in cases with tetrahedral coordination, such as solid Ge or many biological materials. Both EM and RM methods need additional input (the force constants) and a complete description of both is given in Anna Poiarkova's thesis (see the FEFF Project web site (http://feff.phys.washington.edu) and in the associated documentation.
* Calculate Debye-Waller factors for Cu at 190K with equation of motion
  DEBYE  190 0 1

 CORRECTIONS    real-energy-shift imaginary-energy-shift    (Useful)


The real energy shift moves tex2html_wrap_inline3347 in the final tex2html_wrap_inline3343 and the imaginary energy shift adds broadening to the result. The real energy shift is useful to correct the error in FEFF's Fermi level estimate and the imaginary part can be used to correct for experimental resolution or errors in the core-hole lifetime. This error in the Fermi level is typically about 1 eV with self-consistent calculations and about 3 eV with overlapped atom potentials. The imaginary energy is typically used to correct for instrument broadening or as a correction to the mean free path calculated by FEFF. This affects only the module FF2CHI, which combines the results in all of the 'feffNNNN.dat' files. This card is useful in fitting loops because you can simply make such energy corrections and see the results without redoing the entire XAFS parameter calculation. CAUTION: the results are not as accurate as those obtained with the EXCHANGE card. Both energies are in eV. (See also the EXCHANGE card in Section 2.3).
* Reduce E0 by 3.0 eV and add 1 eV of broadening (full width)
* This will only affect module 4, ff2chi
CORRECTIONS   3.0   1.0       real shift, imag shift

 SIG2    sig2    (Useful)

Specify a global Debye-Waller factor to be used or added to Debye-Waller calculations (see DEBYE card) for all paths. This value will be summed with the correlated Debye model value (if the DEBYE card is present) and any value added to 'list.dat'. Units are Åtex2html_wrap_inline3351. This card can be used, for example to add Debye-Waller factors from structural disorder.


SIG2 0.001    add 0.001 globally to all DW factors

next up previous contents
Next: Output Files Up: Input File Controls Previous: GENFMT: XAFS parameters
John J. Rehr
Thu Jul 1 14:31:17 PDT 1999