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3. Program and file structure

This section describes the files and data used by the various modules. The main program FEFF reads one file created directly by the user, the file feff.inp; it is line oriented with a KEYWORD followed by data in free format. Case is not significant; ABCDE is equivalent to abcde or AbcDe. Tab characters are treated as if they were blanks. The routine that reads the input assumes that your machine uses the ASCII character set.

Comments may be added at the end of any line after any required data, and spaces between lines are ignored. Any line beginning with an asterisk (*) is a comment and is ignored. An auxiliary program (ATOMS) is provided which generates the feff.inp file from crystallographic input parameters.

Other files required by the various modules are created by FEFF from feff.inp. Some of these other files may be edited by the user as a way to modify the input data to the modules, see section 5. See the PRINT card in section 4 to obtain various diagnostic files. Table 3.1 summarizes this structure, the rest of this section describes the structure in more detail.

3.1 Table 3.1

module  input files     output files    other useful output
------  -----------     ------------    -------------------
POTPH   feff.inp        phase.bin       diagnostic files
        potph.inp       xsect.bin       (see PRINT card)

PATHS   feff.inp        paths.dat       crit.dat

GENFMT  feff.inp        feff.bin
        phase.bin       list.dat

FF2CHI  feff.inp        chi.dat         chipNNNN.dat
        list.dat        xmu.dat         feffNNNN.dat
Note that geom.dat is written when feff.inp is read.  If you are
using a hand-made geom.dat (possibly to take advantage of some
symmetry), use the NOGEOM card to prevent your geom.dat from
being overwritten.

3.2 Scattering potential and phase shift code, POTPH

Reads potph.inp, which is created by FEFF's input routine from feff.inp. potph.inp is deleted by FEFF after use. POTPH writes the binary file phase.bin, which contains the scattering phase shifts and other information needed by PATHS and GENFMT. Also, POTPH prints binary file rkk.bin, which contains dipole matrix elements, normalized to cross-section data in xsect.bin. The last file is written only if XANES card is used. Optionally, POTPH will write other diagnostic files with information about the potentials and phase shift calculations.

3.3 Path enumeration, PATHS

Reads geom.dat, which is created by FEFF from the information in feff.inp, and phase.bin. geom.dat is deleted after use unless explicitly saved (see PRINT card). PATHS writes paths.dat for use by GENFMT and as a complete description of each path for use of the user. PATHS will optionally write other diagnostic files. The file crit.dat is particularly useful when studying large numbers of paths. When studying large numbers of paths, this module will optionally write only crit.dat and save geom.dat, not writing paths.dat.

3.4 Scattering amplitude and other XAFS parameter calculation, GENFMT

GENFMT reads phase.bin, rkk.bin and paths.dat, and writes a file feff.bin which contains all the EXAFS information for the paths, and list.dat which tells you some basic information about them. These files are the main output of FEFF. To read feff.bin into your own program, use the subroutine feffdt as an example.

For compatibility with older versions, the program can read feff.bin and write the feffnnnn.dat files and files.dat in module ff2chi, if appropriate PRINT option was specified. They are exactly like the old ones.

3.5 XAFS spectrum calculation, FF2CHI

FF2CHI reads list.dat and feff.bin, and writes chi.dat with the total XAFS from the paths specified in list.dat Additional instructions are passed to FF2CHI from feff.inp, so you can change S02, Debye temperature and some other parameters without re-doing the whole calculation. The file list.dat can be edited by hand to change the paths being considered, and individual chipNNNN.dat files with chi from each path are optionally written. If the XANES card is specified and xsect.bin exists, FF2CHI will write the XANES data in xmu.dat. Various filtering options and corrections are possible at this point -- see input cards below.

There is an internal limit on the number of paths (set to 1000) that will be read from feff.bin. This limit was chosen to handle any reasonable problem without using an excessive amount of memory. If you must use more paths, change the parameter "npx" is subroutine ff2chi to whatever you need. It will need more memory. We have not had a case where the filter criteria were not able to solve the problem with fewer than 1000 paths.

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