Though already somewhat complex and feature-rich, the use of feffit() described so far really only shows the basic fitting capabilities of the feffit(). Scattering paths are defined, what to vary and what to keep fixed in the fit is described, and the paths are summed together until they match the data, and the results are inspected. It is by no means trivial or easy to come up with a realistic fitting model or assess whether a fit is meaningful, but feffit() as described so far gives you all the tools to do these tasks.
In the rest of this chapter, more advanced features of feffit() are described. The features include the ability to refine the background ((E)) parameters at the same time as the structural model, the ability to include additional knowledge about the physical parameters of the systems, and the ability to create and fit a model describing more than one data set at a time. I call these features ``advanced'', but most of these features are very easy to use, especially when compared to the rather large undertaking of building up a simple fitting model. That is to say that although these features may seem like ``advanced topics'', and so best left alone by the beginner, they can, in fact, help greatly in assessing the quality and reliability of many fits, and should be kept in mind for many analyses, even by fairly new users. These abilities are being built-in to the ARTEMIS GUI program, and I heartly recommend trying these features.