Order of the approximation used in module GENFMT. feff uses order 2 by default, which is correct to terms of order and corresponds to 6x6 scattering matrices in the Rehr–Albers formalism. Single scattering is calculated exactly to this order. The 6x6 approximation is accurate to within a few percent in every case we have tried (that is, higher order doesn’t change the result more than a few percent). However MIV shells and higher shells may require increased iorder for coupling the matrix elements. Changing the default values requires some familiarity with the Rehr–Albers paper and the structure of the module GENFMT. To do so, follow the instructions in the feff source code in subroutine setlam. The key iord is passed to setlam for processing. You may need to change the code parameter lamtot if you want to do higher order calculations. For details of the algorithm used by GENFMT, see the paper by J.J. Rehr and R.C. Albers (see the references in Appendix C). For the MIV and higher edges, you may receive an error message like: Lambda array overfilled. In that case the calculations should be repeated with IORDER -70202 (10x10 matrices).
* change iorder for M4 calculations IORDER -70202