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38 lines
1.8 KiB
ReStructuredText
38 lines
1.8 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 spatial symmetry and compute the "n_states" lowest 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: if even after a :c:func:`cis` calculation, natural orbitals (see :c:func:`save_natorb`) and then :c:func:`scf` optimization, you are not sure to have the lowest scf solution, do the same strategy with the :c:func:`cisd` executable instead of the :c:func:`cis` exectuable to generate the natural orbitals as a guess for the :c:func:`scf`.
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* EXCITED STATES CALCULATIONS: the lowest excited states are much likely to be dominanted by single- or double-excitations. Therefore, running a :c:func:`cisd` will save the "n_states" lowest states within the CISD space in the EZFIO folder, which can afterward be used as guess wave functions for a further multi-state fci calculation if you specify "read_wf" = True before running the fci executable (see :option:`determinants read_wf`). Also, if you specify "s2_eig" = True, the cisd will only retain states having the good value :math:`S^2` value (see :option:`determinants expected_s2` and :option:`determinants s2_eig`). If "s2_eig" = False, it will take the lowest n_states, whatever multiplicity they are.
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Note: if you would like to discard some orbitals, use :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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File: :file:`cisd/cisd.irp.f` |