# 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:

1. Compute/Read all the one- and two-electron integrals, and store them in memory
2. 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.
3. Perform the SCF iterations

The definition of the Fock matrix is in `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:

1. `scf_utils thresh_scf`
2. `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 `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.

## Programs

> - ks_scf

## Providers

`ks_energy`

  File : `ks_enery.irp.f`

      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:

  `ao_num` `ao_one_e_integrals` `ao_potential_alpha_xc` `ao_two_e_integral_alpha` | `e_correlation_dft` `e_exchange_dft` `fock_matrix_ao_alpha` | `nuclear_repulsion` `scf_density_matrix_ao_alpha` `scf_density_matrix_ao_beta`

  Needed by:

  `extra_e_contrib_density` |  |

## Subroutines / functions