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qp2/src/fci/README.rst

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===
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.