CROSSEC (CCP4: Supported Program)
NAMEcrossec - interpolate X-ray cross sections and compute anomalous scattering factors
DESCRIPTIONCROSSEC 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):
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 ) 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 ).
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.
INPUT AND OUTPUT FILESThe 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, CWAV, END, NORD, NWAV, VERB
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:
VERBVerbose output. The default is to only produce the final table (which can be viewed with XLOGGRAPH).
ENDEnd input and run.
Unix example script found in $CEXAM/unix/runnable/
VMS example script found in $CEXAM/vms/
AUTHORSDon Cromer, LANL.