program lccsd
  implicit none
  BEGIN_DOC
! Linerarized CCSD 
!
  ! 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:
  !
  ! * **Ground state calculation**: if even after a :c:func:`cis` calculation, natural
  !   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`.
  !
  !
  !
  ! * **Excited states calculations**: the lowest excited states are much likely to
  !   be dominanted by single- or double-excitations.
  !   Therefore, running a :c:func:`cisd` will save the "n_states" lowest states within
  !   the CISD space
  !   in the |EZFIO| directory, which can afterward be used as guess wave functions
  !   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
  !
  END_DOC
  PROVIDE N_states
  read_wf = .False.
  TOUCH read_wf
  call run
end

subroutine run
  implicit none

  if(pseudo_sym)then
   call H_apply_cisd_sym
  else
   call H_apply_cisd
  endif
  call get_lccsd_2
end

subroutine get_lccsd_2
  implicit none
  integer                        :: i,k
  double precision               :: cisdq(N_states), delta_e
  double precision,external      :: diag_h_mat_elem
  psi_coef = lccsd_coef
  SOFT_TOUCH psi_coef
  call save_wavefunction_truncated(save_threshold)
  call ezfio_set_cisd_lcc_energy(lccsd_energies)

  print *,  'N_det = ', N_det
  print*,''
  print*,'******************************'
  print *,  'LCCSD Energies'
  do i = 1,N_states
    print *,  i, lccsd_energies(i)
  enddo
  if (N_states > 1) then
      print*,'******************************'
      print*,'Excitation energies (au)    (LCCSD)'
      do i = 2, N_states
        print*, i ,lccsd_energies(i) - lccsd_energies(1)
      enddo
      print*,''
      print*,'******************************'
      print*,'Excitation energies (eV)    (LCCSD)'
      do i = 2, N_states
        print*, i ,(lccsd_energies(i) - lccsd_energies(1)) * ha_to_ev
      enddo
  endif

end