qp2/src/kohn_sham

=========
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