2019-01-25 11:39:31 +01:00
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.. _cis:
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.. program:: cis
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===
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cis
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===
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2019-01-25 14:54:22 +01:00
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Configuration Interaction with Single excitations.
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2019-01-25 11:39:31 +01:00
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This program takes a reference Slater determinant of ROHF-like
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occupancy, and performs all single excitations on top of it.
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Disregarding spatial symmetry, it computes the `n_states` lowest
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eigenstates of that CI matrix. (see :option:`determinants n_states`)
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This program can be useful in many cases:
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Ground state calculation
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2019-01-25 14:54:22 +01:00
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------------------------
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2019-01-25 11:39:31 +01:00
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2019-01-25 14:54:22 +01:00
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To be sure to have the lowest |SCF| solution, perform an :ref:`scf`
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(see the :ref:`hartree_fock` module), then a :ref:`cis`, save
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the natural orbitals (see :ref:`save_natorb`) and re-run an
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:ref:`scf` optimization from this |MO| guess.
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2019-01-25 11:39:31 +01:00
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Excited states calculations
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2019-01-25 14:54:22 +01:00
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---------------------------
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The lowest excited states are much likely to be dominated by
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single-excitations. Therefore, running a :ref:`cis` will save
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the `n_states` lowest states within the |CIS| space in the |EZFIO|
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directory, which can afterwards be used as guess wave functions for
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a further multi-state |FCI| calculation if :option:`determinants read_wf`
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is set to |true| before running the :ref:`fci`
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executable.
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2019-01-25 11:39:31 +01:00
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If :option:`determinants s2_eig` is set to |true|, the |CIS|
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will only retain states having the expected |S^2| value (see
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:option:`determinants expected_s2`). Otherwise, the |CIS| will take
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the lowest :option:`determinants n_states`, whatever multiplicity
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they are.
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2019-01-25 14:54:22 +01:00
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.. note::
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2019-01-25 11:39:31 +01:00
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To discard some orbitals, use the :ref:`qp_set_mo_class`
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command to specify:
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* *core* orbitals which will be always doubly occupied
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* *act* orbitals where an electron can be either excited from or to
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* *del* orbitals which will be never occupied
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Needs:
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.. hlist::
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:columns: 3
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* :c:data:`read_wf`
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Calls:
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.. hlist::
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:columns: 3
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* :c:func:`run`
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Touches:
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.. hlist::
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:columns: 3
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* :c:data:`fock_matrix_ao_alpha`
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* :c:data:`fock_matrix_ao_alpha`
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* :c:data:`mo_coef`
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* :c:data:`level_shift`
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* :c:data:`mo_coef`
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* :c:data:`read_wf`
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