qp2/src/fci/README.rst

===
fci
===


|CIPSI| algorithm in the full configuration interaction space.


The user point of view
----------------------

* :ref:`fci` performs |CIPSI| calculations using a stochastic scheme for both
  the selection and the |PT2| contribution,

* :ref:`pt2` computes the |PT2| contribution using the wave function stored in
  the |EZFIO| database.


The main keywords/options for this module are:

* :option:`determinants n_det_max` : maximum number of Slater determinants in
  the |CIPSI| wave function. The :ref:`fci` program will stop when the size of
  the |CIPSI| wave function will exceed :option:`determinants n_det_max`.

* :option:`perturbation pt2_max` : absolute value of the |PT2| to stop the
  |CIPSI| calculation. Once the abs(|PT2|) :math:`<` :option:`perturbation pt2_max`,
  the |CIPSI| calculation stops.

* :option:`determinants n_states` : number of states to consider in the |CIPSI|
  calculation.

* :option:`determinants read_wf` : if |false|, starts with a |ROHF|-like
  determinant, if |true|, starts with the current wave function(s) stored in
  the |EZFIO| directory.

.. note::
   For a multi-state calculation, it is recommended to start with :ref:`cis`
   or :ref:`cisd` wave functions as a guess.

* :option:`determinants expected_s2` : expected value of |S^2| for the
  desired spin multiplicity.

* :option:`determinants s2_eig` : if |true|, systematically add all the
  determinants needed to have a pure value of |S^2|. Also, if |true|, it
  tracks only the states having the good :option:`determinants expected_s2`.




The programmer's point of view
------------------------------

This module was created with the :ref:`module_cipsi` module.

.. seealso::

    The documentation of the :ref:`module_cipsi` module.