7.2 Creating $\chi$(k) data with ff2chi()

Once a single path or set of paths have been defined, it is often desirable to convert them to $\chi$(k) data so that they can be plotted and compared to one another or to data arrays. The ff2chi() command will do a simple sum-of-paths to give $\chi$(k) data. This essentially mimics the final step of FEFF, with the additional features like being able to alter any of the path parameters and include feffnnnn.dat files from different runs of FEFF in the sum.

ff2chi() is a fairly simple command, with most of its arguments describing optional output parameters that are usually not needed. The main argument will be a ``path list'', which is a list of path indices to sum. Building on the path definition above, the simple

  ff2chi(1, group=feff)

will generate arrays feff.k and feff.chi from the path #1 defined above. If we then add a second path, with

  path(2, file = feffcu02.dat, s02 = s02, 
    sigma2  = {sqrt(2) * ss2}, e0 = e0)

we can add these two paths together, like this,

  ff2chi(1,2,  group=two_paths)

which will generate two_paths.k and two_paths.chi.

The path list for the ff2chi() (and feffit()) command can be a simple list like ``1,2,3,4'', or use a dash like ``1-6'', or some combination of both, like ``1, 3-8, 10, 100-105''. Be warned though that a path listed twice will be used twice.