The OVERLAP card can be used to construct approximate overlapped atom potentials when atomic coordinates are not known or specified. If the atomic positions are listed following the ATOMS card, the OVERLAP card is not needed. FEFF8 will stop if both the ATOMS and OVERLAP cards are used. The OVERLAP card contains the potential index of the atom being overlapped and is followed by a list specifying the potential index, number of atoms of a given type to be overlapped and their distance to the atom being overlapped. The examples below demonstrate the use of an OVERLAP list. This option can be useful for initial single scattering XAFS calculations in complex materials where very little is known about the structure.

You should verify that the coordination chemistry built in using the OVERLAP cards is realistic. It is particularly important to specify all the nearest neighbors of a typical atom in the shell to be overlapped. The most important factor in determining the scattering amplitudes is the atomic number of the scatterer, but the coordination chemistry should be approximately correct to ensure good scattering potentials. Thus it is important to specify as accurately as possible the coordination environment of the scatterer. Note: If you use the OVERLAP card, then you cannot use the FMS or SCF cards. Also the pathfinder won't be called and you must explicitly specify single scattering paths using the SS card .

  * Example 1. Simple usage
  * Determine approximate overlap for central and 1st nearest neighbor in Cu
  OVERLAP 0         determine overlap for central atom of Cu
    *iphovr   novr   rovr       * ipot, number in shell, distance
     1        12     2.55266
  OVERLAP 1         determine approximate overlap for 1st shell atoms
    *iphovr   novr   rovr       * ipot, number in shell, distance
     0        12     2.55266

  * Example 2. More precise usage
  * Determine approximate overlap for 3rd shell atoms of Cu
    0  1 2.55266    ipot, number in shell, distance
    1  4 2.55266
    2  7 2.55266
    2  6 3.61000
    2 24 4.42133