# EXCHANGE

### From FEFF

* Useful *

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 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.

Defaults if the EXCHANGE card is omitted are: ixc = 0 (Hedin–Lundquist), vr0 = 0.0, vi0 = 0.0. For XANES, the ground state potential (ixc0 = 2) 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.