Central and Scattering Atom Phase Shift Tables

Matt Newville and I have developed a strange new database. It's strange because we are using the latest X-ray absorption theory to generate a replacement for the databases which that theory was itself developed to replace. See, we have undertaken to use FEFF8 to make a replacement for scattering tables, like those from McHale, Teo and Lee, or FEFF3.

Say what?

Well, sometimes it is not expedient to make a full FEFF calculation, not even with FEFF6. In some situations, you just need some quick 'n' dirty values for scattering phase shifts and amplitudes. A couple of examples might be semi-automated analysis performed in real time as data is collected or for allowing phase-corrected Fourier transforms without requiring a full FEFF calculation. Indeed, it is for the latter that I am writing these tables. I want to add phase-corrected transforms as an option in my program ATHENA. To that end, I need a database of central atom phase shifts for all atoms and all edges.

My solution is (unsurprisingly for me) to write a program in perl. In this program, I loop over all atoms and all edges. For each edge that is over 100eV, I generate a `feff.inp' file and run FEFF8, albeit in non-self-consistent mode. After each run, the program parses the `feff0001.dat' file and saves the columns containing the central atoms phase shift and the scattering atom phase and amplitude.

These data get saved into two different data bases. One is a pair of flat text files which can be read by any program written in any langue and so can be incorporated into other peoples projects. The other database is a set of Packed Ascii Data (PAD) files for use with IFEFFIT.

The `feff.inp' files all look like this:

 TITLE %n (%s %z) at the %e edge
 TITLE making central atoms phase shifts

 EDGE  %e   1.0
 CONTROL   1      1     1     1
 PRINT     1      0     0     3
 RMAX        2.51

 POTENTIALS
        0   %z   %s
        1   %z   %s

 ATOMS
    0.00000    0.00000    0.00000  0  %s    0.00000
    1.44338    1.44338    1.44338  1  %s_1  2.50000
   -1.44338    1.44338    1.44338  1  %s_1  2.50000
    1.44338   -1.44338    1.44338  1  %s_1  2.50000
   -1.44338   -1.44338    1.44338  1  %s_1  2.50000
    1.44338    1.44338   -1.44338  1  %s_1  2.50000
   -1.44338    1.44338   -1.44338  1  %s_1  2.50000
    1.44338   -1.44338   -1.44338  1  %s_1  2.50000
   -1.44338   -1.44338   -1.44338  1  %s_1  2.50000
    2.88675    0.00000    0.00000  1  %s_2  2.88675
   -2.88675    0.00000    0.00000  1  %s_2  2.88675
    0.00000    2.88675    0.00000  1  %s_2  2.88675
    0.00000   -2.88675    0.00000  1  %s_2  2.88675
    0.00000    0.00000    2.88675  1  %s_2  2.88675
    0.00000    0.00000   -2.88675  1  %s_2  2.88675
 END
	    

The tokens %n, %s, %z, %e get replaced by the values of the central atom name, symbol, and Z-number and the edge symbol appropriate to each calculation. This input file is the first two shells of a BCC structure with a near neighbor distance of 2.5Å.

This input file is obviously unphysical for most materials. That's ok. This excercise is not meant to generate ideal values for these phase shifts and amplitudes. They are meant to generate values which are good enough for the sort of problems I described above.