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39 lines
1.3 KiB
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====


cisd


====




This module contains a CI of single and double excitations.




The user point of view







The :command:`cisd` program performs the CI of the ROHFlike + all single and double excitations on top of it.


This program can be very useful to :




* **Ground state calculations**: generate a guess for the ground state wave function if one is not sure that the :c:func:`scf` program gave the lowest SCF solution. In combination with :c:func:`save_natorb` it can produce new MOs in order to reperform an :c:func:`scf` optimization.




* **Excited states calculations**: generate guess for all the :option:`determinants n_states` wave functions, that will be used by the :c:func:`fci` program.






The main keywords/options to be used are:




* :option:`determinants n_states` : number of states to consider for the cisd calculation




* :option:`determinants s2_eig` : force all states to have the desired value of :math:`S^2`




* :option:`determinants expected_s2` : desired value of :math:`S^2`




The programmer point of view







This module have been built by setting the following rules:






* The only generator determinant is the HartreeFock (singlereference method)


* All generated determinants are included in the wave function (no perturbative


selection)




These rules are set in the ``H_apply.irp.f`` file.




