================= 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 ========== This tutorial can be directly watched at: ``_ Hands on ======== .. important:: Before using the |qp|, it is required to load the environment variables relatives to the |QP| or to be in the |qpsh| mode. Please execute in the current shell: .. code:: bash ${QP_ROOT}/bin/qpsh where :code:`${QP_ROOT}` is the path to the source files of the |QP| installed on your architecture. The |QPSH| mode: a bash-like experience for quantum chemistry ------------------------------------------------------------- The |QP| has been designed pretty much as an *interactive* environment for quantum-chemistry calculations, in order to facilitate the user experience. Just like in bash, there are many commands in the |QP| (see for instance :ref:`qp_edit` or :ref:`qp_run`) which help in handling useful data or running executables (see for instance :ref:`scf` or :ref:`fci`). All commands designed within the |qp| **begins** with `qp`, and are two ways of running a **command**: * the *executable* associated to the command: .. code:: bash qp_command or the *qp* command which calls the *executable* :code:`qp_command`: .. code:: bash qp command Usually, when using the :command:`qp` command, the name of the |EZFIO| database is omitted. The advantage or using :code:`qp command` is that you can, just like in bash, have: * the :kbd:`Tab` key for the auto-completion for basically any command of the |QP| * man pages with -h, --help or qp man Just try, for instance: .. code:: bash qp and then use the auto-completion. You will show appear all possible commands that you can run: .. code:: bash convert_output_to_ezfio -h plugins unset_file create_ezfio man set_file update Then, try, still with the auto-completion, .. code:: bash qp create You will see appear all the options for the :ref:`qp_create_ezfio` commands. 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 becke_numerical_grid dft_keywords mo_one_e_ints perturbation ao_one_e_ints davidson dressing mo_two_e_erf_ints pseudo ao_two_e_erf_ints density_for_dft electrons mo_two_e_ints scf_utils ao_two_e_ints determinants ezfio nuclei work 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 The expected energy is ``-92.827856698`` au. .. seealso:: The documentation of the :ref:`module_hartree_fock` module and that of the :ref:`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, we will modify the |EZFIO| database to make |CIPSI| calculation only 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 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]" Run the |CIPSI| calculation ---------------------------- We will now use the |CIPSI| algorithm to estimate the |FCI| energy. .. code:: qp run fci | 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 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:`module_fci` module and that of the :ref:`fci` program.