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