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85 lines
4.0 KiB
ReStructuredText
========
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The |qp|
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========
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.. image:: /_static/qp2.png
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:align: center
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:width: 200px
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:alt: Quantum Package
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What it is
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==========
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The |qp| is an open-source **programming environment** for quantum chemistry.
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It has been built from the **developper** point of view in order to help
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the design of new quantum chemistry methods,
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especially for `wave function theory <https://en.wikipedia.org/wiki/Ab_initio_quantum_chemistry_methods>`_ (|WFT|).
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From the **user** point of view, the |qp| proposes a stand-alone path
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to use optimized selected configuration interaction |sCI| based on the
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|CIPSI| algorithm that can efficiently reach near-full configuration interaction
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|FCI| quality for relatively large systems.
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To have a simple example of how to use the |CIPSI| program, go to the `users_guide/quickstart`.
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The main goal is the development of selected configuration interaction |sCI|
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methods and multi-reference perturbation theory |MRPT| in the
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determinant-driven paradigm. It also contains the very basics of Kohn-Sham `density functional theory <https://en.wikipedia.org/wiki/Density_functional_theory>`_ |KS-DFT| and `range-separated hybrids <https://aip.scitation.org/doi/10.1063/1.1383587>`_ |RSH|.
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The determinant-driven framework allows the programmer to include any arbitrary set of
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determinants in the variational space, and thus gives a complete freedom in the methodological
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development. The basic ingredients of |RSH| together with those of the |WFT| framework available in the |qp| library allows one to easily develop range-separated DFT (|RSDFT|) approaches (see for instance the plugins at `<https://gitlab.com/eginer/qp_plugins_eginer>`_).
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All the programs are developed with the `IRPF90`_ code generator, which considerably simplifies
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the collaborative development, and the development of new features.
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What it is not
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==============
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The |qp| is *not* a general purpose quantum chemistry program.
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First of all, it is a *library* to develop new theories and algorithms in quantum chemistry.
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Therefore, beside the use of the programs of the core modules, the users of the |qp| should develop their own programs.
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The |qp| has been designed specifically for |sCI|, so all the
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algorithms which are programmed are not adapted to run SCF or DFT calculations
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on thousands of atoms. Currently, the systems targeted have less than 600
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molecular orbitals. This limit is due to the memory bottleneck induced by the storring of the two-electron integrals (see ``mo_two_e_integrals`` and ``ao_two_e_integrals``).
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The |qp| is *not* a massive production code. For conventional
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methods such as Hartree-Fock, CISD or MP2, the users are recommended to use the
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existing standard production codes which are designed to make these methods run
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fast. Again, the role of the |qp| is to make life simple for the
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developer. Once a new method is developed and tested, the developer is encouraged
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to consider re-expressing it with an integral-driven formulation, and to
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implement the new method in open-source production codes, such as `NWChem`_
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or |GAMESS|.
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A few examples of applications
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==============================
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Multiple programs were developed with the |qp|, such as:
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- Selected Full-CI + Epstein-Nesbet PT2 (CIPSI) :cite:`Caffarel_2016,Caffarel_2016.2,Loos_2018,Scemama_2018,Dash_2018`
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- Hybrid stochastic/deterministic MR-PT2 :cite:`Garniron_2017.2,Loos_2018`
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- Orbital optimization for open-shell systems :cite:`Giner2016Mar,Giner_2017.3`
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- CIS, CISD, MP2
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- Selected CISD
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- Jeziorsky-Monkhorst MR-PT2 :cite:`Giner_2017`
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- Effective Hamiltonian for variational MR wave functions :cite:`Giner_2017.2`
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- Selected CAS+SD
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- Selected difference-dedicated CI (DD-CI)
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- Multi-Reference Coupled Cluster (MR-CCSD) :cite:`Giner_2016,Garniron_2017`
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- Shifted-Bk with CIPSI :cite:`Garniron_2018`
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- CIPSI with range-separated DFT (plugins at `<https://gitlab.com/eginer/qp_plugins_eginer>`_)
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- DFT for basis set corrections :cite:`Giner_2018`
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All these programs can generate ground and excited states, and spin pure wave
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functions (eigenstates of |S^2|).
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