xas
======

This option controls the details of simulating X-ray absorption spectroscopy with target state optimization (TSO) DFT method.

.. contents::
   :local:

.. option:: elements

   .. list-table::
      :stub-columns: 1
      :widths: 5 20
   
      * - Value
        - Element list
      * - Default
        - None

   A list of heavy elements whose XAS is to be calculated. For example,
  
   .. code-block:: bash
    :linenos:

    xas
      elements C N O
    end
         
   Then, Qbics will calculate 3 XAS for C, N, and O, respectively. 

.. option:: num_states

   .. list-table::
      :stub-columns: 1
      :widths: 5 20
   
      * - Value
        - An integer
      * - Default
        - ``1``
   
   The number of excited states for each core electron to be calculated.

   
.. option:: exclude

   .. list-table::
      :stub-columns: 1
      :widths: 5 20
   
      * - Value
        - Atom range
      * - Default
        - None
   
   By default, Qbics will calculate the core excitations of all atoms for a given element. For example:
     
   .. code-block:: bash
    :linenos:

     xas
       elements C 
       exclude 2-6
     end
     mol
       C                  1.20809735    0.69749533   0.
       C                  0.00000000    1.39499067   0.
       C                 -1.20809735    0.69749533   0.
       C                 -1.20809735   -0.69749533   0.
       C                 -0.00000000   -1.39499067   0.
       C                  1.20809735   -0.69749533   0.
       H                  2.16038781    1.24730049   0.
       H                  0.00000000    2.49460097   0.
       H                 -2.16038781    1.24730049   0.
       H                 -2.16038781   -1.24730049   0.
       H                  0.00000000   -2.49460097   0.
       H                  2.16038781   -1.24730049   0.
     end
    
   Without ``exclude``, Qbics will calculate core excitations all 6 C atoms. But since the 6 C atoms are equivalent, this is unnecessary. So, in ``exclude`` we assign atom 2,3,4,5,6, meaning that the calculation of core excitations of these 5 atoms are skipped.