General Comments

From FEFF

General Cards

Any FEFF calculation starts with two basic tasks: calculation of the potentials, followed by calculation of the scattering phase shifts. Using these phase shifts, scattering paths are found and their scattering amplitudes calculated. There are two ways to do this: either using an explicit enumeration and summation, most appropriate for extended absorption spectra; or using the implicit summation of full multiple scattering, appropriate for near-edge absorption spectra. Finally, these scattering amplitudes are combined and processed in the right way to produce a particular type of spectrum - currently EXAFS, XANES, ELNES, EXELFS, XES, NRIXS, DANES, or XMCD.

Each of these basic steps corresponds to a handful of feff program modules controlled by a set of ‘feff.inp’ input cards. In later chapters we discuss these aspects in a technical way by listing the details of program modules, input cards, and files. The current chapter takes a different, more user-oriented approach. We will discuss each type of spectroscopy individually. We will indicate which cards and program modules are relevant for that type of spectrum, and give general recommendations on strategies, as well as examples.

We encourage users to spend some time exploring the ‘feff90/test’ folder that came with their feff distribution. This folder contains several testcases, including one for each major type of spectroscopy, along with the corresponding ‘xmu.dat’ spectrum file.

We endeavour to keep feff accessible to the novice user: you need only one file ‘feff.inp’ and only one command, feff (or the Run button in the GUI). Nevertheless, we encourage you to gain insight in the underlying structure of feff. For example, many parameters can be changed without requiring the entire calculation to be redone. If the model (i.e., the atom positions) doesn’t change significantly, it is usually not necessary to recalculate the potentials. E.g., if one wants to add more paths to the path expansion, the calculation can be restarted with module path, saving much time. If one wants to change the beam orientation in an EELS calculation, only the eels module needs to run again. And so on. Generally, pot, fms, and genfmt take much time, while all other program modules are very fast.  However, significant changes to the model require rerunning the entire calculation. There are usually parameters to converge, for example the cutoff radius of the full multiple scattering problem. Debye-Waller factors may require much attention in the EXAFS regime but the XANES regime may be less sensitive.

The information in this chapter aims to illuminate such matters; some hands-on experience will do the rest.


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