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40 lines
1.5 KiB
ReStructuredText
.. _excited_states:
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==============
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Excited states
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==============
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It is possible to run excited states calculations with the quantum
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package. To do this, set :option:`determinants n_states` to the number
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of requested states. The selection criterion will be the maximum of the
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selection criteria for each state. If the Davidson diagonalization has
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difficulties to converge, increase the :option:`davidson n_states_diag`
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value.
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When computing multiple states, it is good to have the
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:option:`determinants s2_eig` flag |true|. This will force the Davidson
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algorithm to choose only vectors with a value of |S^2| equal to
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:option:`determinants expected_s2`. Otherwise, different spin states
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will come out in the diagonalization.
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The |qp| doesn't take account of the symmetry. Due to numerical noise,
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excited states of different symmetries may enter in the calculation.
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Note that it is possible to make state-average calculation of states
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with different symmetries and/or different spin multiplicities.
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To include excited state of all possible symmetries, a simple trick is
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to run a preliminary multi-state |CIS| calculation using the :ref:`CIS`
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program, and then running the selected |FCI| restarting from the |CIS|
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states, setting :option:`determinants read_wf` to |true|.
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Usually, it is good practice to use state-averaged natural |MOs| so that
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all states have |MOs| of comparable quality. This allows for a faster
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convergence of excitation energies.
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.. seealso::
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The documentation of the :c:func:`scf`, :c:func:`cis` and
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:c:func:`fci` programs.
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