EXCHANGE card question

[ALNemir@aol.com]

Dear FEFF Users,

 

I am making use of the EXCHANGE for my XANES calculations, and I found the following line in the description of the EXCHANGE card (that also appears below) somewhat confusing: 

"For XANES, the ground state potential (ixc0=0) is used for the background function and for EXAFS the Hedin–Lundqvist (ixc0=0) is used.." 

Should not it say instead 

"For XANES, the ground state potential (ixc0=2) is used for the background function and for EXAFS the Hedin–Lundqvist (ixc0=0) is used?" 

 

And a related question. While, as evidenced from the above quote, different models are used for the background function calculations for XANES and EXAFS, is the same true about the models used for the fine structure for the XANES and EXAFS calculations? And if so, which model is considered to be best for each calculation?

 

Thank you very muich in advance,

Lydia Nemirovsky
Research Assistant
EXAFS Laboratory
Department of Physics and Astronomy
Hunter College


 

 

EXCHANGE ixc vr0 vi0 [ixc0]

The EXCHANGE card specifies the energy dependent exchange correlation potential to be

used for the fine structure and for the atomic background. ixc is an index specifying the

potential model to use for the fine structure and the optional ixc0 is the index of the model

to use for the background function. The calculated potential can be corrected by adding a

constant shift to the Fermi level given by vr0 and to a pure imaginary “optical” potential (i.e.,

uniform decay) given by vi0. Typical errors in feff’s self-consistent Fermi level estimate are

about 1 eV. (The CORRECTIONS card in Section 2.8 is similar but allows the user to make

small changes in vi0 and vr0 after the rest of the calculation is completed, for example in a

fitting process.) The Hedin–Lundqvist self-energy is used by default and appears to be the best

choice for most applications we have tested in detail. The partially nonlocal model (ixc=5)

gives slightly better results in some cases, but has not been tested extensively.

Another useful exchange model is the Dirac-Hara exchange correlation potential with a

specified imaginary potential vi0. This may be useful to correct the typical error in non-selfconsistent

estimates of the Fermi level of about +3 eV and to add final state and instrumental

broadening.

2.3. POT: SCATTERING POTENTIALS 15

Defaults if EXCHANGE card is omitted are: ixc=0 (Hedin–Lundquist), vr0=0.0, vi0=0.0.

For XANES, the ground state potential (ixc0=0) is used for the background function and for

EXAFS the Hedin–Lundqvist (ixc0=0) is used.

Indices for the available exchange models:

0 Hedin–Lundqvist + a constant imaginary part

1 Dirac–Hara + a constant imaginary part

2 ground state + a constant imaginary part

3 Dirac–Hara + HL imag part + a constant imaginary part

5 Partially nonlocal: Dirac–Fock for core + HL for valence electrons + a constant imaginary

part

*Hedin-Lundqvist -2eV edge shift and 1eV expt broadening

EXCHANGE 0 2. 1.

*Dirac-Hara exchange -3 eV edge shift and 5 eV optical potential

EXCHANGE 1 3. 5.