EQUIVALENCE

From FEFF

EQUIVALENCE

ieq - Advanced

This optional card is only active in combination with the CIF card. It tells FEFF how to generate potential types from the list of atom positions in the ‘cif’ file.

If ieq = 1, the crystallographic equivalence as expressed in the ‘cif’ file is respected; that is, every separate line containing a generating atom position will lead to a separate potential type. This means that, e.g., in HOPG graphite, the two generating positions will give rise to two independent C potentials. This is also the default behavior if the EQUIVALENCE card is not specified.

If ieq = 2, unique potentials are assigned based on atomic number Z only. That is, all C atoms will share a C potential and so on. This is how most FEFF calculations are run. Whether it is sensible or not to do this depends on the system and on the property one wishes to calculate. Keep in mind that FEFF is a muffin tin code, and may therefore be indifferent to certain differences between crystallographically inequivalent sites. On the other hand, if an element occurs in the crystal with different oxidation states, it may be necessary to assign separate potentials to these different types in order to describe the crystal properly and get accurate spectra.

If ieq = 3, unique potentials are assigned based on atomic number Z and the first shell. This can be useful e.g. to treat larger systems with crystal defects, where only first neighbors of the defect need to be treated differently from all more distant atoms of a certain Z. (To be implemented.)

If ieq = 4, a hybrid of methods 1 and 2 is used. That is, if the number of unique crystallographic positions does not exceed a hard-coded limit (nphx=9 in the current version), they are treated with the correct crystallographic equivalence. If the number of unique crystallographically inequivalent sites is larger, they get combined by atomic number Z. This ad hoc approach is a practical way of simply limiting the number of unique potentials. This makes sense because, first of all, there are certain hardcoded limits that would require recompilation of the code, requiring more RAM memory and more work than a user may want to do. Secondly, our SCF algorithm tends to have a harder time reaching convergence as the number of potentials increases, leading to substantially longer calculation times or even convergence failure if the number of potentials becomes very large.

If ieq = 5, unique potentials are assigned based on a label in the ‘cif’ file. That is, the user can edit the ‘cif’ file in a text editor and mark different sites with labels such as ”Ti1” and ”Ti2”. FEFF will assign the same unique potential to all sites with the same label. This gives the user complete control over potential assignment. (To be implemented.) If you require one of the solutions marked as ”To be implemented”, please contact us for assistance.

* Example : Do a traditional FEFF calculation where all atoms with the same Z
*                 have the same potential
CIF graphite.cif
EQUIVALENCE 2
* This would be equivalent to a file using LATTICE and ATOMS card, and
* POTENTIALS
**     ipot     z          label       lmax1       lmax2
*      0         6          C            -1             -1          * for the core hole atom
*      1         6          C            -1             -1          *for all other C atoms

* Example 2 : Do a calculation with true crystallographic equivalence,
*                    as most bandstructure codes do:
CIF graphite.cif
EQUIVALENCE 1  * This is the default and could be omitted for the same results
* This would be equivalent to a file using LATTICE and ATOMS card, and
* POTENTIALS
** ipot       z       label       lmax1       lmax2
*   0           6      C            -1             -1          * for the core hole atom
*   1           6      C            -1             -1          *forhalfoftheCatoms
*   2           6      C            -1             -1          *fortheotherhalfoftheC atoms
developer's resources