CROSSEC (CCP4: Supported Program)


crossec - interpolate X-ray cross sections and compute anomalous scattering factors


[Keyworded input]


CROSSEC interpolates X-ray cross sections and computes anomalous scattering factors. f' and f'' values are derived for a given atom type at given wavelengths. These are important for data collection and analysis with heavy atoms.

Three contributions to f' are listed separately in the output (VERBose option):

Photoelectric contribution
Term dependent on the total energy of the atom (reference [2])
Term dependent on the X-ray energy (reference [3])

The output is marked up for plotting with XLOGGRAPH.

The cross section file has a number of orbitals for each atom from atomic number 3-98. The first MX records (MX=5 for this cross-section file and is set in the program for each orbital) will have cross sections at MX values of energy from about 1 to 80 keV approximately equally spaced in log(energy). The next five records will be cross sections at energies selected by the Gauss integration scheme. If the function type is 0 (IF=0) (reference [2]) a sixth value is read in for an energy of 1.001*(binding energy). If the X-ray energy is less than the binding energy, FUNCTION SIGMA3 will be used (reference [4]).


If An X-ray energy is very close to one of the energies used in the Gauss integration an `anomalous' anomalous scattering factor may result. There is no easy way out of this problem. A suggested way is to compute several values at nearby energies and draw a smooth curve. This method should work provided the points do not pass through an edge.


The data file is read from logical name CROSSECDATA (defaulting to $CLIBD/crossec.lib).


The ATOM keyword is compulsory. You must also specify a set of wavelengths using the NWAV keyword and/or the CWAV keyword. Any number of NWAV and CWAV keywords may be given, but there is currently a maximum of 1000 on the total number of wavelengths allowed. All other keywords are optional. The possible keywords are:


ATOM <atom>

Atomic symbol for the element of interest (case-insensitive).

NWAV <nwav> <wav_1> .... <wav_nwav>

Number <nwav> of wavelengths to be given, followed by a list of the wavelengths themselves (in Angstroms).

CWAV <nwav> <centre> <step>

CWAV is a more compact way of specifying a series of regularly spaced wavelengths than NWAV. This keyword specifies <nwav> wavelengths centred on <centre> (in Angstroms) and separated by <step> (in Angstroms). This may be useful if you want to look at a series of wavelengths on either side of an edge.

NORD <nord>

(Default 2).
Interpolation in the data file is governed by the NORD value as follows:
Interpolation is by fitting three closest points to a quadratic.
1,2,3 etc.
Use Aitken's interpolation method with NORD the interpolation order. 2 is probably the best value to use.


Verbose output. The default is to only produce the final table (which can be viewed with XLOGGRAPH).


End input and run.


Unix example script found in $CEXAM/unix/runnable/

  • crossec.exam


    Don Cromer, LANL.


    1. Don T. Cromer. `Calculation of Anomalous Scattering Factors at Arbitrary Wavelengths', J. Applied Cryst. 16 437-8 (1983)
    2. Cromer and Liberman. J. Chem. Phys. 53 1891-1898 (1970)
    3. Jensen. Physics Letters 74A 41-44 (1979)
    4. Cromer and Liberman Acta Cryst. A37 267-268 (1981)
    5. Henke, Gullikson and Davis, Atomic Data and Nuclear Data Tables Vol. 54 No.2 181-342 (1993).
      Note however that they are not given in fine intervals. This is only important in the immediate region of an absorption edge where they change quickly. In any case near edge features mean that any values of f' or f'' for an isolated atom in this region could be unreliable as they depend on the environment of the atom.
    6. Alain Soyer, J.Appl.Cryst. 28 244 (1995).
      The program Fhkl can plot values from the Henke et al and Cromer et al methods, though it is designed for other purposes.
    7. D. Waasmaier and A. Kirfel, `New Analytical Scattering Factor Functions for Free Atoms and Ions', Acta Cryst. A51 (1995).
      FTP Uni Wuerzburg