qp2/docs/source/users_guide/quickstart.rst

=================
Quick-start guide
=================

This tutorial should teach you everything you need to get started with
the the basics of the |qp|. As an example, we will run a frozen core
|CIPSI| calculation on the HCN molecule in the 631-G basis set.


Demo video
==========

.. Include demo video here


Hands on
========

.. important::

   Before using the |qp|, it is required to source the file
   :file:`quantum_package.rc` if it has not been done already in the
   current shell.

Create the EZFIO database
-------------------------

The data relative to calculations are stored in an |EZFIO| database.
|EZFIO| is a hierarchical data format which uses the hierarchy of the
file system to organize the data, as files stored in a directory. The
data in the |EZFIO| directory are stored as plain text files, so it can
be opened with any text editor.
To access the data of the |EZFIO| database, the APIs (Fortran, |Python|,
|OCaml| or Bash) provided by |EZFIO| should be used, or tools using
these APIs such as :ref:`qp_edit` provided with the |qp|.

First, create an `xyz` file containing the coordinates of the molecule.
The file :file:`hcn.xyz` contains::

   3
   HCN molecule
   C    0.0    0.0    0.0
   H    0.0    0.0    1.064
   N    0.0    0.0    -1.156


This xyz file is now used with the :ref:`qp_create_ezfio` command to
create an |EZFIO| database with the 6-31G basis set:

.. code:: bash

  qp_create_ezfio -b "6-31G" hcn.xyz -o hcn

The EZFIO database now contains data relative to the nuclear coordinates
and the atomic basis set:

.. code:: bash

  $ ls hcn
  ao_basis/  electrons/  ezfio/  nuclei/  pseudo/


Run a Hartree-Fock calculation
------------------------------

The program :ref:`qp_run` is the driver program of the |qp|. To run a
|SCF| calculation, just run

.. code:: bash

    qp_run scf hcn 

The expected energy is ``-92.827856698`` au.

.. seealso:: 

    The documentation of the :ref:`hartree_fock` module and that of the :c:func:`scf` program.

This creates the |MOs| in the |EZFIO| database that will be used to
perform any other post-SCF method. The |qp| does not handle symmetry and
the |MOs| are stored by increasing order of Fock energies.

Choose the target |MO| space
----------------------------

Now, modify to |EZFIO| database to make |CIPSI| calculation in the
full set of valence |MOs|, keeping the core |MOs| frozen. The simple
command :ref:`qp_set_frozen_core` does this automatically:

.. code:: bash

    qp_set_frozen_core hcn


The general command to specify core and active orbitals is :ref:`qp_set_mo_class`. 
In the case of HCN molecule in the 631G basis, one has 20 |MOs| in total and the two first orbitals are frozen:

.. code::

    qp_set_mo_class -core "[1-2]" -act "[3-20]" hcn



Run the |CIPSI| calculation
----------------------------

We will now use the |CIPSI| algorithm to estimate the |FCI| energy.

.. code::

    qp_run fci hcn | tee hcn.fci.out 


The program will start with a single determinant and will iteratively:

* Select the most important determinants from the external space and add them to the
  internal space
* Add all the necessary determinants to allow the eigenvector of |H| to be
  also an eigenstate of |S^2|
* Diagonalize |H| in the enlarged internal space
* Compute (stochastically) the second-order perturbative contribution to the energy 
* Extrapolate the variational energy by fitting
  :math:`E=E_\text{FCI} - \alpha\, E_\text{PT2}`

By default, the program will stop when more than one million determinants have
entered in the internal space, or when the |PT2| energy is below :math:`10^{-4}`.

To have a pictural illustration of the convergence of the |CIPSI| algorithm, just run 

.. code::

    qp_e_conv_fci hcn.fci.out

This will create the files "hcn.fci.out.conv" containing the data of the convergence of the energy that can be plotted, together with the file "hcn.fci.out.conv.1.eps" which is obtained from the gnuplot plot file "hcn.fci.out.conv.plt". 


The estimated |FCI| energy of HCN is ``-93.0501`` au.

.. seealso:: 

    The documentation of the :ref:`fci` module and that of the :c:func:`fci` program.


---------------------------

TODO 


.. important:: TODO

  .. include:: /work.rst

  * Parameters for Hartree-Fock
  * Parameters for Davidson
  * Running in parallel