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4. Chapter 4: Input file and FEFF commands

The main program FEFF reads one file created directly by the user, the file feff.inp. This section describes feff.inp and the commands that tell FEFF what to do. It may be helpful to look at one or more of the sample input files in Appendix D while reading this section. An auxilliary program ATOMS can generate the feff.inp file. See the ATOMS card and the document for ATOMS for additional details. The input file for FEFF6 is identical to FEFF5, except that additional options have been added to permit XANES calculations and add polarization dependence.

4.1 File format

feff.inp is a free format line oriented file. Blank lines and lines beginning with an asterisk (comment lines) are ignored wherever the occur. End of line comments are also ignored. Each type of input read by the program must contain a card with a "keyword" in CAPITAL LETTERS, and in some cases, followed by data cards. The sequence of keyword cards is arbitrary. If any card is omitted, default values are used; an exception is the POTENTIALS card, which is required. Numeric values are listed in free format, separated by blanks. Tab characters are not allowed (due to portability constraints) and may cause confusing error messages. Characters appearing after the expected data on a given line are ignored by feff and can be used as end-of-line comments. All distances are in Angstroms and energies in eV.

4.2 Complete list of feff.inp keywords

The list of feff.inp options fall into four categories, standard options frequently and easily used, useful options that are often used, special options that are seldom necessary or helpful, and obscure options used during development that are included for completeness.


The standard data types are

ATOMS, CONTROL, DEBYE, HOLE, POTENTIALS, PRINT, SIG2, TITLE

Useful options include

CORRECTIONS, CRITERIA, ELLIPTICITY, END, NEMAX, NLEG, NOGEOM, PCRITERIA, POLARIZATION, RMAX, RMULTIPLIER, SS, XANES,

Special data types are recommended only for advanced users, awkward situations or diagnostic purposes

AFOLP, FOLP, EXCHANGE, ION, OVERLAP,

And some (e.g. IORDER) are so obscure that we

considered not mentioning them.


These data types are listed below, alphabetically within each category. Each DATATYPE is followed by a brief explanation and an example.

Basic data types

ATOMS

Cartesian coordinates and unique potential indices of each atom (in Angstroms) in the cluster are entered following the ATOMS card, one per line. Please see the discussion of unique potentials in section 4.2.1.5. An auxilliary code ATOMS written by B. Ravel (U. Washington) is supplied with FEFF 6 to generate the ATOMS list from given crystallographic data. See the document file to ATOMS for more information.

  ATOMS
  * x      y      z     ipot     SF6 molecule
    0.0    0.0    0.0     0      S K-shell hole

    3.61   0.00   0.00    1      F 1st shell atoms
    0.00   3.61   0.00    1
    0.00   0.00   3.61    1
   -3.61   0.00   0.00    1
    0.00  -3.61   0.00    1
    0.00   0.00  -3.61    1

CONTROL

The CONTROL card lets you run one or more of the modules separately. There is a switch for each module, 0 meaning do not run that module, 1 meaning do run it. The default is CONTROL 1 1 1 1, run all modules.

The syntax is: CONTROL potph paths genfmt ff2chi

  * run genfmt and ff2chi, do not run potph or paths
  CONTROL  0 0 1 1    potph  paths  genfmt  ff2chi

DEBYE

The Debye card is used to calculate Debye-Waller factors for each path using the correlated Debye Model. The model is best suited for homogeneous systems, where it is quite accurate. CAUTION: in heterogeneous systems the model only gives approximate values which can easily be off by factors of two or more. Temperatures are in K. Default: Use Debye-Waller factors from files.dat. If this card is present, explicit Debye-Waller factors in files.dat are ignored and the correlated Debye model is used.

The syntax is: DEBYE temperature Debye-temperature

  DEBYE  190 315   Calculate Debye-Waller factors for Cu at 190K

HOLE

The HOLE card includes the hole-code ihole and the amplitude reduction factor S0**2. An entry of 0.0 for S0**2 gives the default value s02=1. To match experiment, values of s02 are typically between 0.8 and 1.0. Defaults if HOLE card is omitted: ihole=1 (K shell), S02=1. Hole-codes presently available are as follows (if your favorite is missing, complain to the authors): 0 = no hole, 1 = K-shell, 2 = L1-shell, 3 = L2-shell, 4 = L3-shell

The syntax is: HOLE ihole S0**2

  HOLE  1   1.0     K-shell core hole, S02 = 1

POTENTIALS

The data following the POTENTIALS card assigns a unique potential index to each distinguishable atom. The potential index ipot is the index of the potential to be used for the phase shift calculation.

The keyword POTENTIALS begins a unique potential list. Each following line (until the next keyword card) is a unique potential index, the atomic number and optional tag (max 6 characters) for that unique potential. The absorbing atom must be unique potential 0. These unique potential indices are simply labels, so the order is not important, except that the absorbing atom is ipot 0, and you may not have missing indices (ie, if you use ipot 3, you must also have defined unique potentials 1 and 2).

To save time the code calculates the overlapped atom potential for each unique potential only once, using as a sample geometry the first atom in the atom list with a given unique potential index. Thus it is essential that the neighborhood of that sample atom be representative. Failure to do so may cause the code to perform poorly (See Appendix G).

Because the phase shift calculation is the most time-consuming part of FEFF, it may be useful to assume that the potential for a given shell of atoms is the same as that of a previously calculated shell. For example, in Cu it is a good approximation to determine potentials only for the central atom and the first shell and to use the first shell potential (ipot=1) for all higher shells. Such approximations should be checked in each case.

  * molecular SF6
  POTENTIALS
  *   potential-index    Z   tag
            0           16   S         Sulfur K hole, absorbing atom
            1            9   F

PRINT

The PRINT card determines how much output is printed from each of the four modules. Default is print level 0 for each module. See section 6 for details of contents of these files.

The syntax is: PRINT potph paths genfmt ff2chi

potph

0 = phase.bin xsect.bin only; 1 = add misc.dat; 2 = add pot.dat, phase.dat; 5 = add atom.dat

paths

0 = paths.dat only; 1 = add crit.dat; 2 = add geom.dat; 3 = add f(beta) files (plane wave |f(beta)| approximations); 5 = Write only crit.dat and save geom.dat. This is very useful when exploring importance of paths for large runs. Does not write paths.dat.

genfmt

0 = files.dat, all feffNNNN.dat files with importance greater than or equal to two thirds of the curved wave importance criterion; 1 = keep all feffNNNN.dat files

ff2chi

0 = chi.dat; 1 = add sig2.dat with Debye-Waller factors; 2 = add chiNNNN.dat (chi from each path individually) This can quickly fill up your disk if you're doing a large run.

  * add crit.dat and small feffNNNN.dat files to minimum output
  PRINT  0  1  1  0

SIG2

Global Debye-Waller factor (sig2) written to files.dat The module ff2chi, which does the final calculation of chi, reads the Debye-Waller factors from files.dat unless over-ridden by the correlated Debye model (see the DEBYE card above.). This global Debye-Waller factor is written to files.dat. You can edit files.dat before running module ff2chi to change one or more of the Debye-Waller factors.

The syntax is: SIG2 sig2

TITLE

Keyword TITLE followed by a title line. You may have up to 10 of these. Titles may have up to 75 characters, leading blanks in the titles will be removed.

The syntax is: TITLE title line...

TITLE  Andradite  (Novak and Gibbs, Am.Mineral 56,791 1971)
TITLE  K-shell 300K

Useful Options

CORRECTIONS

The real energy shift will shift E0 in the final chi and the imaginary energy shift adds broadening to the result. The energy shift is useful to correct for the error in FEFF's fermi level, which is typically too high by about 3 eV, and the the broadening is typically used to correct for instrument broadening, or as a correction to the mean free path calculated by FEFF. This affects only the module FF2CHI, which combines the results in all of the feff.dat files. It is useful because you can simply make these energy corrections and see the results without redoing the entire XAFS parameter calculation. Both energies are in eV. See also the EXCHANGE card.

The syntax is: CORRECTIONS real-energy-shift imaginary-energy-shift

* Reduce E0 by 3.0 eV and add 1 eV of broadening
* This will only affect module 4, ff2chi
CORRECTIONS   3.0   1.0       real shift, imag shift

CRITERIA

Since the number of multiple scattering paths gets large very quickly, it is necessary to eliminate as many paths as possible. Fortunately, we have found that most multiple scattering paths have small amplitudes and can be neglected. Various cutoff "criteria" are used in FEFF6 to limit the number of paths to consider. These criteria are based on the importance of the path, defined as the integral over the full energy range of |chi(k)|*dk.

critcw is the result of the full curved wave calculation. A calculation of critcw requires a complete spherical wave calculation, which takes about 10 seconds on a SUN4. The default value of critcw is 4%, meaning that any path with mean amplitude exceeding 4% of largest path will be used in calculation of chi. The criterion critcw is used by GENFMT; since the XAFS parameter calculation is already done, the savings is not in computer time, but in disk space and ease of analysis. The values of critcw for each path are written in the file files.dat written by module GENFMT.

critpw is a plane-wave approximation to chi. This is extremely fast to calculate, and is used in the pathfinder. The default value of critpw is 2.5, meaning that any path with mean amplitude 2.5% of largest path, including degeneracy factors, (in plane wave approximation) will be kept. Any path that does not meet this criterion will not be written to paths.dat, and there is no need to calculate the XAFS parameters for this path. The default for critpw is less than that for critcw since some paths are more important when the full curved wave calculation is done than they appear in the plane wave approximation. Since the plane wave estimate is extremely fast, use this to filter out as many paths as you can. The file crit.dat (written by the module PATHS) tells you critpw for each path that passes the criterion.

The method of calculation of these importance factors has been improved for the current release, so don't worry if the values for some paths has changed slightly from previous versions. Default values critcw=4.% critpw=2.5%

The syntax is: CRITERIA critcw critpw


CRITERIA  6.0  3.0   critcw 6%, critpw 3%

CRITERIA  0  0       use all paths (cw and pw criteria turned off)

ELLIPTICITY

Ellipticity is the ratio of amplitudes of electric field in two orthogonal directions for elliptically polarized light. Only the absolute value of the ratio is important for nonmagnetic materials. The present code cannot distinguish left and right circular polarization. A zero value of the ellipticity corresponds to linear polarization, and unity to circular polarization. Default value is zero.

x, y, z are coordinates of any nonzero vector in the direction of incident beam. This vector should be approximately normal to the polarization vector.

The syntax is: ELLIPTICITY ellipticity x y z


ELLIPTICITY  1.0  0.0 0.0 -2.0   for circular polarization about z-axis

END

The END card marks the end of reading the feff.inp file; all data following the END card is ignored. This is optional, but useful when making short runs with only part of an input file.

END    ignore any cards in feff.inp that follow this

NEMAX

This limits the number of energy points. NEMAX is normally 49, which will get you to k=20 invA. Smaller values will avoid the high energy parts of the calculation and may save you some time. This parameter is used when the scattering phase shifts are calculated, so once you have limited the number of energy points, that's what you will get for the rest of the calculations you do (based on the phase shifts). To change the number of energy points, re-run the phase shift calculation.

The syntax is: NEMAX nemax

* use only first 40 energy grid points
NEMAX  40

NLEG

The NLEG card limits the number of scattering paths to nleg. If nleg is set to 2, only single scattering paths are found. The default is nleg = 8.

The syntax is: NLEG nleg

NLEG 2      find only single scattering paths (ie, 2 legged paths)

NOGEOM

When this card is present, the file geom.dat will not be produced. Use this option when you want to take advantage of the symmetry in a cluster to speed the path calculation by using the geom.dat file produced by the auxilliary code ATOM rather than letting feff produce geom.dat from the atom list in feff.inp.

PCRITERIA

These criteria, like those described in the CRITERIA card, also limit the number of paths. However, they are applied in the pathfinder and eliminate unimportant paths while the pathfinder is doing its search. The pathfinder criteria (pcrit's) do not know the degeneracy of a path and are therefore much less reliable than the curved wave and plane wave criteria in the CRITERIA card above. These path finder criteria (keep and heap) are turned off by default, and we recommend that they be used only with very large runs, and then with caution.

The keep-criterion looks at the amplitude of chi (in the plane wave approx) for the current path and compares it to a single scattering path of the same effective length. To set this value, consider the maximum degeneracy you expect and divide your plane wave criterion by this number. For example, in fcc Cu, typical degeneracies are 196 for paths with large r, and the minimum degeneracy is 6. So a keep criterion of 0.08% is appropriate for a pw criteria of 2.5%.

The heap-criterion filters paths as the pathfinder puts all paths into a heap (a partially ordered data structure), then removes them in order of increasing total path length. Each path that is removed from the heap is modified and then considered again as part of the search algorithm. The heap filter is used to decide if a path has enough amplitude in it to be worth further consideration. If we can eliminate a path at this point, entire trees of derivative paths can be neglected, leading to enormous time savings. This test does not come into play until paths with at least 4 legs are being considered, so single scattering and triangular (2 and 3 legged) paths will always pass this test. Because only a small part of a path is used for this criterion, it is difficult to predict what appropriate values will be. To use this (it is only necessary if your heap is filling up, and if limiting rmax doesn't help), study the results in crit.dat from runs with shorter rmax and experiment with the heap criterion accordingly. In the future, we hope to improve this filter.

Before using these criterion, study the output in the file crit.dat (use print option 1 for paths, see PRINT card), which has the values of critpw, keep factor and heap factor for all paths which pass the critpw filter.

Default: If this card is omitted, the keep and heap criteria are set to zero, that is, no filtering will be done at this step in the calculation.

The syntax is: PCRITERIA keep-criterion heap-criterion


* fcc Cu had degeneracies from 6 to 196, so correct for this by
* dividing pw-crit of 2.5% by 30 to get 0.08 for keep crit.  Check this
* empirically by running with pcrits turned off and studying crit.dat.
* After studying crit.dat, choose 0.5 for heap crit.
PCRITERIA   0.08  0.5

POLARIZATION

This card specifies the direction of electric field in the incident beam. (Main axis of the ellipse in the case of elliptical polarization). x, y, z - coordinates of any nonzero vector. Only this card is necessary in the case of linear polarization. If the POLARIZATION card is omitted, polarization averaged xafs will be calculated.

Note that polarization reduces the degeneracy of the paths, increasing the calculation time. Choosing polarization in the directions of symmetry axes will yield faster results.

The syntax is: POLARIZATION x y z

POLARIZATION  1.0  2.5  0.0

RMAX

The RMAX card determines the maximum effective distance, rmax, of a given path. Note that rmax is one-half of the total path length in multiple-scattering paths. Setting this too large can cause the heap in the pathfinder to fill up. Default is RMAX = twice the near neighbor distance.

The syntax is: RMAX rmax

RMAX  5.10   only include paths with effective length up to 5.10 Ang

RMULTIPLIER

The use of the RMULTIPLIER card multiples all atomic coordinates by a constant factor, rmult. This is useful, for example, if unit cell coordinates are used. (Default value rmult=1.)

The syntax is: RMULTIPLIER rmult

RMULTIPLIER 1.01   (increase distances by 1%)

SS

The SS card is used when the structure of a given shell is unknown. No multiple scattering is produced but a single scattering path of degeneracy deg is produced. Overlap cards may be used to construct the potential for such a path. The parameters are a shell index, which is a label used for feffNNNN.dat file name, a unique potential index ipot, identifying the unique potential of atom from which to scatter, the degeneracy, i.e., the multiplicity of this single scattering path, and the distance to central atom, rss. This could be used to add single scattering beyond the limits of a cluster.

The syntax is: SS index ipot deg rss

*  index  ipot   deg  rss
SS   29     1     48  5.98       parameters for 19th shell of Cu

XANES

The XANES card is used when a calculation of the near edge structure including the atomic background and absolute energies are desired.

The XANES calculation is limited to the (extended) continuum spectrum beyond the Fermi level. Thus bound states are not generally included; however, in molecules weakly bound states that are below vacuum but above the muffin-tin zero show up as resonances. The absolute energies are based on atomic total energy calculations using the Desclaux code; the accuracy of this approximation varies from a few eV to a few hundred eV for very large Z. No parameters are needed. Default: XANES not calculated.

Sometimes useful options for expert users

AFOLP

This automatically overlaps all muffin-tins to reduce the effects of potential discontinuities at the muffin-tins. It is useful in highly inhomogeneous materials. It works fairly well, but may fail in some cases. See FOLP for non-automatic version.

AFOLP     use automatic maximum overlapping

FOLP

The FOLP card sets a parameter which determines by what factor muffin-tin radii are overlapped. We recommend that the AFOLP card be used In cases with severe anisotropy, and FOLP only used for diagnostic purposes.

The syntax is: FOLP ipot folp

FOLP 1  1.1  (10% overlap of muffin tin of unique potential 1)

EXCHANGE

The EXCHANGE card specifies the energy dependent exchange correlation potential to be used. (See also the CORRECTIONS card which is similar but allows the user to refine values of vr0 and vi0 after a calculation is completed.) The EXCHANGE card contains the index of the xc pot (ixc) and the constant imaginary part of the self-energy (vi0). The Hedin-Lundqvist self-energy (default) appears to be the best choice for all applications we have tested in detail. Optionally one may use the Dirac-Hara exchange correlation potential and an appropriate imaginary potential vi0. This may be useful to correct FEFF's typical error of 2 eV (high) in the location of the Fermi level and to add instrumental broadening. Defaults if EXCHANGE card is ommitted: ixc=0 (Hedin-Lundquist), vr0=0.0 vi0=0.0.

The syntax is: EXCHANGE ixc vr0 vi0

ixc values

ixc=0: Hedin-Lundqvist + const imag part; ixc=1: Dirac-Hara + const imag part; ixc=2: ground state + const imag part; ixc=3: Dirac-Hara + HL imag part + const imag part

vi values

vi0 is (the negative of the) imaginary const shift

*Hedin-Lundqvist -2eV edge shift and 1eV expt broadening
EXCHANGE 0 2. 1. 

*Dirac-Hara exchange -3 eV edge shift and 5 eV inner potential
EXCHANGE 1 3. 5.

ION

The ION card ionizes all atoms with atom type ipot. Negative values and non-integers are not permitted (however, we may allow these in future versions of FEFF). The ION card is used to ionize all atoms of a particular potential index. For example, for diatomics like Br2, the fully relaxed configuration has ionization=1 on the scattering atom. The ionization card should be used with caution; because of charge transfer, the actual degree of ionization is not directly related to the chemical valency. The default (non-ionized) scattering potentials are often superior to those empirically ionized, and the results should be checked both ways. Defaults if ION cards are ommitted: atoms are not ionized.

The syntax is: ION ipot ionization

ION  1  1   ipot, ionization

OVERLAP

The OVERLAP card contains information needed to construct the overlapped atom potentials when atomic coordinates are not known or specified. If the atomic positions are listed following the ATOMS cards, the OVERLAP cards are probably not needed. The OVERLAP card contains the potential index of the atom being overlapped and must be followed by cards specifying the potential index, number of atoms of a given type to be overlapped and and their distance to the atom being overlapped. This option can be useful in crystals, especially for distant shells -- see sample input files for an example. It can also be useful for calculating single scattering XAFS in very complex materials where very little is known about the structure.

You should verify that the coordination chemistry built in using the OVERLAP cards is realistic; it is important to specify all the close neighbors of a typical atom in the shell to be overlapped. The most important factor in determining the scattering amplitudes is the atomic number of the scatterer, but the coordination chemistry must be approximately correct to ensure good scattering potentials. Thus it is important to specify as accurately as possible the coordination environment of the scatterer.

The syntax is: OVERLAP iph

OVERLAP 4         determine overlap for 3rd shell of Cu
  0  1 2.55266    ipot, number in shell, distance
  1  4 2.55266
  2  7 2.55266
  2  6 3.61000
  2 24 4.42133

Obscure Options

IORDER

Order of the approximation to use in module GENFMT. We use order 2, which is correct to terms of order (1/pR)**2, and corresponds to 6x6 matrices. However, we do single scattering exactly. This approximation is accurate to within a few percent in every case we have tried (that is, higher order doesn't change the result more than a few percent). Changing the default values requires some familiarity with the Rehr-Albers paper and the structure of the module GENFMT. To do so, follow the instructions in subroutine SETLAM. Iord is passed to setlam for processing. You may need to change the code parameter lamtot if you want to do higher order calculations. This is another of the arcane cards that we have used for testing. For details of the algorithm used by GENFMT, see the paper by J.J.Rehr and R.C.Albers (see Appendix C, references).

The syntax is: IORDER iord


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