
                       CROSSEC (CCP4: Supported Program)

NAME

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

SYNOPSIS

   crossec
   [Keyworded input]

DESCRIPTION

   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):

   P.E.
          Photoelectric contribution

   Eterm
          Term dependent on the total energy of the atom (reference [2])

   Jensen
          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]).

   Warning

   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 FILES

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

KEYWORDED INPUT

   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

  ATOM <atom>

   (Compulsory).
   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:

   0
          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.

  VERB

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

  END

   End input and run.

EXAMPLE

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

     crossec.exam

AUTHORS

   Don Cromer, LANL.

REFERENCES

    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).
       (See http://xray.uu.se/hypertext/henke.html)
       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

SEE ALSO

   xloggraph
