Description of xxcom input parameters:
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NAME: A title.
SUBSTANCE: Element, compound or mixture.

DESCRIPTION: Give the description formula for your substance. In the case 
  SUBSTANCE=Element (Atomic Number) enter a integer with the atomic number. 
  For SUBSTANCE= Element (Symbol) just enter the chemical symbol. For 
  Compounds enter the formula (i.e H2O for water) and for a mixture enter the 
  formulas of its components separated by ":".

FRACTION: Only available when SUBSTANCE=Mixture. Enter here the fraction of 
  the total weight of each different constituents separated by ":"

GRID:	
  Standard (Log scale in the interval 1keV 100GeV) 
  Standard+points: standard + user defined points.
  Points only: Only user-defined points

GRID POINTS: If GRID=Standard+points or Points only then the input energy 
  values [in MeV] can either be entered from a file or from the keyboard.

GRID POINTS/FILE: If GRID POINTS=From keyboard, the energy values separated 
  by ":", else the file name.
  THE FILE NAME SHOULD CONTAIN IN THE FIRST LINE THE NUMBER OF POINTS, 
  THEN THE LIST OF POINTS, LIKE:
    322
    0.0010178123
    0.0010377131
    ...
OUTPUT QUANTITIES: If SUBSTANCE=Element, it is possible to choose between 
  different options for the output magnitudes and units.