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QuantumPackage/docs/source/programs/cis.rst

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.. _cis:
.. program:: cis
===
cis
===
Configuration Interaction with Single excitations.
This program takes a reference Slater determinant of ROHF-like
occupancy, and performs all single excitations on top of it.
Disregarding spatial symmetry, it computes the `n_states` lowest
eigenstates of that CI matrix. (see :option:`determinants n_states`)
This program can be useful in many cases:
1. Ground state calculation
To be sure to have the lowest |SCF| solution, perform an :ref:`scf`
(see the :ref:`module_hartree_fock` module), then a :ref:`cis`, save the
natural orbitals (see :ref:`save_natorb`) and re-run an :ref:`scf`
optimization from this |MO| guess.
2. Excited states calculations
The lowest excited states are much likely to be dominated by
single-excitations. Therefore, running a :ref:`cis` will save the
`n_states` lowest states within the |CIS| space in the |EZFIO|
directory, which can afterwards be used as guess wave functions for
a further multi-state |FCI| calculation if :option:`determinants
read_wf` is set to |true| before running the :ref:`fci` executable.
If :option:`determinants s2_eig` is set to |true|, the |CIS|
will only retain states having the expected |S^2| value (see
:option:`determinants expected_s2`). Otherwise, the |CIS| will take
the lowest :option:`determinants n_states`, whatever multiplicity
they are.
.. note::
To discard some orbitals, use the :ref:`qp_set_mo_class`
command to specify:
* *core* orbitals which will be always doubly occupied
* *act* orbitals where an electron can be either excited from or to
* *del* orbitals which will be never occupied
Needs:
.. hlist::
:columns: 3
* :c:data:`read_wf`
Calls:
.. hlist::
:columns: 3
* :c:func:`run`
Touches:
.. hlist::
:columns: 3
* :c:data:`fock_matrix_ao_alpha`
* :c:data:`fock_matrix_ao_alpha`
* :c:data:`mo_coef`
* :c:data:`level_shift`
* :c:data:`mo_coef`
* :c:data:`read_wf`