ipot Z [tag lmax1 lmax2 xnatph spinph]

The POTENTIALS card is followed by a list which assigns a unique potential index to each distinguishable atom. The potential index ipot is the index of the potential to be used for the phase shift calculation.

The required list entries are the unique potential index ipot and the atomic number Z. The tag is at most 6 characters and is used to identify the unique potential in the `paths.dat' output file.

The optional list entries lmax1 and lmax2 are used to limit the angular momentum bases of the self-consistent potentials (SCF) and full multiple scattering calculations (FMS). If a negative number (e.g., $\mathtt{lmax1}=-1$) is specified for either lmax1 or lmax2, FEFF will automatically use a default based upon the atomic number of the species normal lmax(atomic).

The next optional entry, xnatph, can be used to specify the stoichiometric number of each unique potential in the unit cell of a crystalline material. This helps in the calculation of the Fermi level. In the case of an infinite solid, $\mathtt{xnatph}=0.01$ (default value) is a suitable value for the absorbing atom. For materials finite in extent, see the example below.

The last optional entry spinph is used to specify the spin amplitude and relative spin alignment for spin-dependent calculations (see the SPIN card in this section for more information on spin-dependent calculations).

The absorbing atom must be given unique potential index 0 (there is an exception to this rule when the CFAVERAGE card is used). These unique potential indices are simply labels, so the order is not important, except that the absorbing atom is index 0, and you may not have missing indices (i.e., if you use index 3, you must also have defined unique potentials 1 and 2).

To save time, the code calculates the overlapped atom potential for each unique potential only once, using the atom with a given unique potential index. Thus it is essential that the neighborhood of that sample atom be representative. If it is not, the code may generate inaccurate potentials and phase shifts, as well as poor XAS results.

It is often useful to assume that the potential for a given shell of atoms is the same as that of a previously calculated shell in order to save calculation time. For example, in Cu it is a good approximation to determine potentials only for the central atom and the first shell and to use the first shell potential ( $\mathtt{ipot}=1$) for all higher shells. Such approximations should be checked in each case, however.

  * molecular SF6  Sulfur K edge, lamx1=default, lmax2=3 (spdf basis)
  *   ipot     Z  tag  lmax1 lmax2  xnatph
       0      16   S    -1     3     1
       1       9   F    -1     3     6
An example of spin-dependent usage can be found in the SPIN card description in the next section.