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81 lines
2.3 KiB
ReStructuredText
81 lines
2.3 KiB
ReStructuredText
.. _cisd:
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.. program:: cisd
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====
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cisd
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====
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Configuration Interaction with Single and Double excitations.
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This program takes a reference Slater determinant of ROHF-like occupancy,
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and performs all single and double excitations on top of it, disregarding
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spatial symmetry and compute the "n_states" lowest eigenstates of that CI
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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**: if even after a :c:func:`cis` calculation, natural
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orbitals (see :c:func:`save_natorb`) and then :c:func:`scf` optimization, you are not sure to have the lowest scf
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solution,
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do the same strategy with the :c:func:`cisd` executable instead of the :c:func:`cis` exectuable to generate the natural
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orbitals as a guess for the :c:func:`scf`.
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* **Excited states calculations**: the lowest excited states are much likely to
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be dominanted by single- or double-excitations.
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Therefore, running a :c:func:`cisd` will save the "n_states" lowest states within
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the CISD space
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in the |EZFIO| directory, which can afterward be used as guess wave functions
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for a further multi-state fci calculation if you specify "read_wf" = True
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before running the fci executable (see :option:`determinants read_wf`).
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Also, if you specify "s2_eig" = True, the cisd will only retain states
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having the good value :math:`S^2` value
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(see :option:`determinants expected_s2` and :option:`determinants s2_eig`).
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If "s2_eig" = False, it will take the lowest n_states, whatever
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multiplicity they are.
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Note: if you would like to discard some orbitals, use
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:ref:`qp_set_mo_class` 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:`n_states`
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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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