.. _module_kohn_sham: 
 
.. program:: kohn_sham 
 
.. default-role:: option 
 
=========
kohn_sham
=========


The Kohn-Sham module performs *Restricted* Kohn-Sham calculations (the
spatial part of the |MOs| is common for alpha and beta spinorbitals).

The Kohn-Sham in an SCF and therefore is based on the ``scf_utils`` structure.
It performs the following actions:

#. Compute/Read all the one- and two-electron integrals, and store them in memory
#. Check in the |EZFIO| database if there is a set of |MOs|. If there is, it
   will read them as initial guess. Otherwise, it will create a guess.
#. Perform the |SCF| iterations

The definition of the Fock matrix is in :file:`kohn_sham fock_matrix_ks.irp.f`
For the keywords related to the |SCF| procedure, see the ``scf_utils`` directory where you will find all options.
The main are:

#. :option:`scf_utils thresh_scf`
#. :option:`scf_utils level_shift`

At each iteration, the |MOs| are saved in the |EZFIO| database. Hence, if the calculation
crashes for any unexpected reason, the calculation can be restarted by running again
the |SCF| with the same |EZFIO| database.

The `DIIS`_ algorithm is implemented, as well as the `level-shifting`_ method.
If the |SCF| does not converge, try again with a higher value of :option:`level_shift`.

To start a calculation from scratch, the simplest way is to remove the
``mo_basis`` directory from the |EZFIO| database, and run the |SCF| again.




.. _DIIS: https://en.wikipedia.org/w/index.php?title=DIIS
.. _level-shifting: https://doi.org/10.1002/qua.560070407



 
 
 
Programs 
-------- 
 
 * :ref:`ks_scf` 
 
Providers 
--------- 
 
.. c:var:: ks_energy


    File : :file:`ks_enery.irp.f`

    .. code:: fortran

        double precision	:: ks_energy	
        double precision	:: two_e_energy	
        double precision	:: one_e_energy	
        double precision	:: fock_matrix_energy	
        double precision	:: trace_potential_xc	


    Kohn-Sham energy containing the nuclear repulsion energy, and the various components of this quantity.

    Needs:

    .. hlist::
       :columns: 3

       * :c:data:`ao_num`
       * :c:data:`ao_one_e_integrals`
       * :c:data:`ao_potential_alpha_xc`
       * :c:data:`ao_two_e_integral_alpha`
       * :c:data:`e_correlation_dft`
       * :c:data:`e_exchange_dft`
       * :c:data:`fock_matrix_ao_alpha`
       * :c:data:`nuclear_repulsion`
       * :c:data:`scf_density_matrix_ao_alpha`
       * :c:data:`scf_density_matrix_ao_beta`

    Needed by:

    .. hlist::
       :columns: 3

       * :c:data:`extra_e_contrib_density`

 
 
Subroutines / functions 
-----------------------