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qp2/src/cisd/cisd.irp.f

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program cisd
implicit none
BEGIN_DOC
! Configuration Interaction with Single and Double excitations.
!
! This program takes a reference Slater determinant of ROHF-like occupancy,
!
! 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`).
!
! This program can be useful in many cases:
!
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! * **Ground state calculation**: if even after a :c:func:`cis` calculation, natural
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! 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
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! be dominanted by single- or double-excitations.
! Therefore, running a :c:func:`cisd` will save the "n_states" lowest states within
! the CISD space
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! in the |EZFIO| directory, which can afterward be used as guess wave functions
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! 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.
!
!
!
! Note: if you would like to discard some orbitals, use
! :ref:`qp_set_mo_class` to specify:
!
! * "core" orbitals which will be always doubly occupied
!
! * "act" orbitals where an electron can be either excited from or to
!
! * "del" orbitals which will be never occupied
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!
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END_DOC
read_wf = .False.
SOFT_TOUCH read_wf
call run
end
subroutine run
implicit none
integer :: i
call H_apply_cisd
print *, 'N_det = ', N_det
print*,'******************************'
print *, 'Energies of the states:'
do i = 1,N_states
print *, i, CI_energy(i)
enddo
if (N_states > 1) then
print*,'******************************'
print*,'Excitation energies '
do i = 2, N_states
print*, i ,CI_energy(i) - CI_energy(1)
enddo
endif
psi_coef = ci_eigenvectors
SOFT_TOUCH psi_coef
call save_wavefunction
call ezfio_set_cisd_energy(CI_energy)
end