generate_sing_doub_guga Subroutine

    subroutine generate_sing_doub_guga(ilut_list, space_size, only_keep_conn)
        ! routine to generate the singles and doubles core space from the
        ! HF (or current reference determinant) used in the semi-stochastic
        ! code when GUGA is in use
        integer(n_int), intent(inout) :: ilut_list(0:, :)
        integer, intent(inout) :: space_size
        logical, intent(in) :: only_keep_conn

        integer(n_int) :: ilutG(0:nifguga)
        integer(n_int), allocatable :: excitations(:, :)
        integer :: nexcit, i
        integer(n_int) :: temp_ilut(0:niftot)
        HElement_t(dp) :: temp_hel
        integer :: temp_nI(nel)

        ! essentially use the acthamiltonian on the HFdet and since i
        ! only keep non-zero matrix elements anyway as output, the
        ! only_keep_conn is kind of implicitly .true. always..

        call add_state_to_space(ilutHF, ilut_list, space_size)

        ! to the exact guga excitation to the HF det
        call convert_ilut_toGUGA(ilutHF, ilutG)

        call actHamiltonian(ilutG, CSF_Info_t(ilutG), excitations, nexcit)

        do i = 1, nexcit
            ! check if matrix element is zero if we only want to keep the
            ! connected determinants(not sure if thats implicitly done in the
            ! actHamiltonian routine(i think it only keeps non-zero excitations
            call convert_ilut_toNECI(excitations(:, i), temp_ilut, temp_hel)

            if (only_keep_conn .and. near_zero(temp_hel)) cycle

            call decode_bit_det(temp_nI, temp_ilut)

            call add_state_to_space(temp_ilut, ilut_list, space_size, temp_nI)
        end do

    end subroutine generate_sing_doub_guga