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8f89e3d239
* Documentation * Improved qp_set_frozen_core * Changed all ishft * New install script OK * Variable level shift in HF
34 lines
1.5 KiB
ReStructuredText
34 lines
1.5 KiB
ReStructuredText
Excited states
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==============
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.. TODO
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It is possible to run excited states calculations with the quantum package. To
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do this, set the ``n_states`` variable in the ``Determinants`` section to the
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number 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 ``n_states_diag`` variable in the
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``Davidson`` section.
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When computing multiple states, it is good to have the ``s2_eig`` flag of the
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``Determinants`` section set to ``true``. This will force the Davidson algorithm to
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choose only vectors with a value of S^2 equal to the ``expected_s2``.
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Otherwise, different spin states will come out in the diagonalization.
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The Quantum Package doesn't take account of the symmetry. Due to numerical
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noise, 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 with
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different symmetries 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, and then running the selected
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FCI restarting from the CIS states, setting the ``read_wf`` flag of the
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``Determinants`` section to ``true``.
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Usually, it is good practice to use state-averaged MOs so that all states have
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MOs of comparable quality. For example, when searching for a singly excited
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state, one can use state-average natural orbitals of a preliminary CIS
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calculation.
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